[Radio Fundamentals]
[From the U.S. Government Publishing Office, www.gpo.gov]

WAR DEPARTMENT
TECHNICAL MANUAL
RADIO FUNDAMENTALS
July 17, 1941
gift
2 2 zz) ( 3
U OF NT
libraries
*	'	™ H-455
Document
Reserve	NON-CIRCULATING
TM 11-455
1
TECHNICAL MANUAL
No. 11-455
WAR DEPARTMENT, Washington, July 17, 1941.
RADIO FUNDAMENTALS
Prepared under direction of the Chief Signal Officer
Paragraphs
Section I. General____________________________________________ 1-5
II.	Resonant circuits________________________________ 6-12
III.	Filters__________________________________________13-15
IV.	Vacuum tubes_____________________________________16-30
V.	. Vacuum tube amplifiers_______________________  31-42
VI.	Vacuum tube oscillators__________________________43-48
VII.	Continuous wave transmitters_____________________49-53
VIII.	Modulated transmitters___________________________54-60
IX.	Vacuum tube detectors____________________________61-65
X.	Receivers____________________________,__________66-74
XI.	Power supplies___________________________________75-77
XII.	Frequency modulation_____________________________78-84
XIII.	Antennas_________________________________________85-90
XIV.	Major component parts of radio circuits_________91-104
Page
Appendix I. Abbreviations_____________________________________ 129
II. Bibliography____________________________________ 134
Index_________________________________________________________ 135
Section I
GENERAL
Paragraph
Introduction_______________________________________________________________ 1
Communication frequencies__________________________________________________ 2
Distributed inductance and capacitance_____________________________________ 3
Effective a. c. resistance------------------------------------------------- 4
Insulators_________________________________________________________________ 5
1.	Introduction.—The basic laws which govern electrical phenomena in radio communication systems are the same as in power systems. A discussion of these basic principles of electricity is presented in TM 1-455, and it is presumed that the student is acquainted with the material contained therein. TM 1-455 includes a study of
476847 0 - 42 -1
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1-3
SIGNAL CORPS
the current and voltage relations in elementary direct current (d. c.) and alternating current (a. c.) circuits with applications to power equipment and to measuring instruments. This manual presents a discussion of applications of these basic principles to radio transmitters and receivers. A knowledge of the fundamentals of radio enables a radio operator or radio technician to understand the equipment he handles and to obtain the best results in its employment.
2.	Communication frequencies.—Communication frequencies are divided generally into two broad groups, audio frequencies and radio frequencies.
a.	Audio frequencies are those between, roughly, 20 and 15,000 cycles per second. Sound waves with frequencies in this range are those to which the human ear normally responds. Sounds which occur at frequencies below 20 cycles per second, such as the staccato tappings of a woodpecker, are recognizable more as individual impulses than as tones. The frequencies that are most important in rendering human speech intelligible fall approximately between 200 and 2,500 cycles per second; that is, vibrations per second. The fundamental range of a pipe organ is from about 16 to 5,000 cycles, and the highest fundamental note of the flute is about 4,000 cycles. Speech and music actually consist of very complicated combinations of vibration frequencies of irregular and changing shape; harmonics, or overtones, which are multiples of the fundamental tones, give individual characteristics to sounds of the same fundamental frequency from different sources. It has been determined by experiment that the human ear responds best to sounds of about 2,000 cycles. Sound waves around 15,000 cycles per second, such as those due to very high pitched whistles, are likely to be inaudible to the average ear.
b.	Frequencies from, roughly, 50 kilocycles per second to 500 megacycles per second are referred to as radio frequencies. These are the frequencies employed in the propagation of radio waves. Frequencies below 500 kilocycles (per second) are employed for some army and marine services; frequencies between 500 and 1,500 kilocycles are used for standard broadcasting; and frequencies above 1,500 kilocycles are used for .many types of operation, including amateur, police, general commercial, and army radio communication.
3.	Distributed inductance and capacitance.—a. In addition to the inductance and capacitance included in inductors and capacitors, there are distributed, or stray, inductance and capacitance effects present in miscellaneous components of radio instruments, as in connecting wires, switches, and sockets. These become of considerable concern at radio frequencies.
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RADIO FUNDAMENTALS	3-4
b.	Capacitive reactance is inversely proportional to the frequency (Xc=-^—\ This means that as the frequency of an applied volt-X Ar/G/
age is increased, the capacitance of the circuit offers less opposition to the flow of current, so that at high frequencies undesirably large currents may appear where negligible currents would flow at low frequencies. The inherent capacitance which occurs between adjacent elements of a vacuum tube or between adjacent turns of a coil presents a large capacitive reactance at the lower frequencies. However, at radio frequencies the reactance may become sufficiently small that the increased magnitude of the current flowing across it determines the upper frequency limit for the usefulness of the associated circuit.
c.	Inductive reactance increases proportionally to frequency (Xl=2tt/L), which .means that as the frequency of an applied voltage is increased, the inductance of the circuit offers more opposition to the flow of current. A simple connecting wire, the inductive reactance of which may be insignificant at low frequencies, may have a sufficiently large inductive reactance at higher frequencies to render an instrument inoperative.
4.	Effective a. c. resistance.—Fundamentally, a measure of the resistance of a circuit is given by the power dissipated as heat when unit current is flowing in the circuit. In its broadest sense, the term “resistance” is taken to mean all effects leading to a dissipation of energy in any form such that the energy is not recoverable for any useful purpose within the immediate system. Thus a radio antenna for transmitting is said to have a radiation resistance associated with radiative “losses,” that is, with the energy which is radiated into space; and a particular transmitter or receiver circuit may be said to exhibit certain “reflected” resistance because of the power consumed by other circuits which it directly or indirectly supplies. With alternating current, for a given current magnitude, considerably more electrical power may be consumed than is required by the same circuit with direct current. The resistance which is indicated by a. c. power consumption is called “effective” a. c. resistance. Part of this additional power is required to maintain the heat losses accompanying parasitic circulating currents (eddy currents) which are induced in conductors of the circuit (in particular, in transformer cores) by the varying magnetic field. Another source of a. c. electrical power dissipation is represented by dielectric and hysteresis losses. In the presence of an electric field a dielectric polarizes, that is, the constituent atoms of the dielectric are alined in the direction of the field, being reversed as the
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4—5
SIGNAL CORPS
field reverses. With rapidly changing fields such as are encountered in radio, the energy expended during the polarization is often appreciable. This energy appears as heat and is not recoverable in useful form, so it constitutes a definite loss. A similar effect that occurs in a magnetic material which finds itself in a varying magnetic field, for example in transformer cores, is referred to as hysteresis loss. A further factor which makes for more required power for a given magnitude alternating current is the “skin effect,” the tendency of alternating currents to travel with greater density near the surface of the conductor than at the center. This tendency increases with frequency. The magnetic field about a currentcarrying conductor is more intense at the center of the conductor than it is near the surface of the conductor. Thus the back voltage set up by the rising and falling magnetic field (Lenz’s Law) is greater at the center than near the surface, and practically all of the current through a wire at high frequencies is confined to the outer surface of the conductor. The result is increased heating for the same current, that is, higher resistance. The nonuniform distribution of current throughout the cross section of a conductor at High frequencies is more pronounced if the conductor is wound into the form of a coil than it is if it is used as a straight wire. At radio frequencies the effective a. c. resistance of a coil may be 10 or 100 times its true d. c. resistance. Wherever alternating cir-rents are studied, it is generally understood, if not specifically stated, that “resistance” means effective a. c. resistance.
5.	Insulators.—Insulators which are satisfactory for power purposes may not be suitable for radio work. In radio circuits which operate with microwatts of energy, dielectric losses, which appear, for example, in the dielectric bars which insulate the stator plates from the frame of a variable air capacitor, are of definite concern. Also of interest are the minute leakage currents on insulator surfaces, for example, tube bases and sockets. It is well to keep radio insulators away from strong electric fields, and to maintain all insulators dry and clean.
4
TM 11-455
RADIO FUNDAMENTALS	6
Section II
RESONANT CIRCUITS
Paragraph
Vector representation of voltage and current_________________________ 6
Circuit containing inductance and capacitance in series__________ 7
Resonance_______________________________________________________  g
Circuit containing inductance, capacitance, and resistance in series_	9
Circuit containing inductance and capacitance in parallel_ _ ________ 10
-----------------------------------------------------------   u
Coupled circuits; voltage	gain__________________________________ 12
6.	Vector representation of voltage and current.—a. The simplest type of recurrent voltage or cun-ent is a sine wave type, that is, one whose instantaneous magnitude may be graphically represented as varying with time in accordance with the sine curve of figure 1. A current or voltage varying in exactly this manner is rarely, if ever,
cc /	\	/	\
O ui /	\	/	\
1-0	/	\	/	\	,
Sb --------------\---------------T-------------\-----------------/time
\ / \ /
TL 2601
Figure 1.—Sine current or voltage.
attained in practice. However, the sine wave is a convenient simplification for analysis, and its use in this connection is justified by the fact that any regularly recurrent voltage or current may be regarded as a composite of individual sine waves. For all purposes of circuit analysis only sine wave currents and voltages will be considered, the actual resultant effect in any case being a composite of the individual effects so considered. (See TM 1-455.)
Z>. To facilitate representation, a sine curve such as that of figure 1 may be indicated as in figure 2 by an arrow of unit length which rotates at a uniform rate. The vertical projection, 8, of the arrow represents instantaneous magnitude of the sine action. Consider that the arrow begins rotation when it is pointing horizontally to the right (figure 3(T)). After it has rotated 30° the arrow is at B. and the vertical projection is at B' as shown in figure 3®. Rotating another 30° brings the arrow to C and the projection to Cf. A complete rotation of the arrow is accompanied by the corresponding curve traced
5
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6
SIGNAL CORPS
out by the projection as shown in figure 3@. Continuous rotation produces the repeated sine wave of figure 3@.
c.	Had the arrow started rotating when it was pointing up, figure 3@, that is, 90° ahead of its former starting position, the accompany-
TL-i602
Figure 2.—Uniformly rotating vector. Projection £? develops a sine action.
ing sine curve would have appeared as in figure 3@. The sine curves of @ and of @ differ in phase by 90° ; the former is said to be “leading” the latter by 90°, the latter “lagging” the former by 90°.
______________c'
/0\b___________/	\
/^30° )a_______A' I	/________\_____________
(D	©	@ V ~7
________La \	/
©	v /©
A_A TL-26O3
Figure 3.—Development of sine curves.
The curves of © and of © might represent sine waves of current of the same amplitude and frequency which differ in phase by 90°.
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6
RADIO FUNDAMENTALS
Or, if the curve of @ is taken to represent the wave of alternating voltage across a capacitor, ® may be employed to represent the wave of (leading) current. More simply, the current and voltage could be represented by their corresponding vectors, i and e of figure 4®. The vectors are “photographs” of the rotating arrows at any particular instant. The particular instant represented by figure 4® is that at the beginning of the rotation. Figure 4® shows the same arrows a quarter cycle later. Either the sine curve or the vector representation presents an adequate picture of the phase relationship
/e
1 ./ ex \.	/
__________
® © © TL-2604
Figure 4.—Vector representation of currents and voltages.
between the current and voltage. In a vector picture the magnitudes of the current and the voltage are indicated by the relative lengths of the corresponding vectors. Let us assume that figures 4® and ® represent a voltage of 1 volt maximum and a current of 1 ampere maximum. On this scale, figure 4® would then represent a voltage of 3 volts maximum and a 90° leading current of 2 amperes maximum.
Figure 5 is a vectorial representation of the current («) and voltage (e) in a resistor (both “in phase”). The lengths of the vectors are independent of each other and depend upon the scales selected for each.
.--------4----------------£
TL-2605
Figure 5.—Current and voltage in phase.
e. For the capacitor-resistor combination of figure 6®, figure 6® gives the vectorial representation of the current through the circuit and of the voltage across the circuit (voltage lagging the current by less than 90°). The current is uniform throughout the circuit. The voltage at any instant between points 1 and 3 of figure 6® is
TM 11-455
6
SIGNAL CORPS
the algebraic sum of the voltages existing between 1 and 2 and between 2 and 3. The voltage between 1 and 2 is represented in sine form by the curve marked ec of figure 7, that between 2 and 3 by the curve
----------1' x-----------------
C--	\
A	2	\
R >	\
S	\e
L---------J 3	@
(T)	TL-260 6
® A. c. generator with capacitance-resistance load.
® Current and voltage vectors for circuit of ®
Figure 6.
marked eR, and the additive resultant, which is the voltage at the generator terminals, by the curve marked e. It is left as an exercise for the student to demonstrate graphically that the vectorial addition
e
A-xA ...
/ w
/A	\	\	TL-2607
/ / ee	\	\
Figure 7.—Voltage across capacitor and resistor in series combination in circuit of figure 6®.
of the voltages as shown in figure 8 is equivalent to the detailed addition pictured in figure 7. In general it may be shown that the sum resultant of any two voltage vectors (or of any two current vectors) is represented correctly, both in magnitude and in phase, by the diagonal of the parallelogram formed from the component vectors as in figure 9(T), (2), and (3).
8
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7
RADIO FUNDAMENTALS
7. Circuit containing inductance and capacitance in series.— Among the most important radio circuits is one containing induct-
Sr i ---->
ec e
TL-2608
Figure 8.—Vector representation of addition corresponding to figure 7.
ance (Z) and capacitance (<7) in series (fig. IO®). The voltage and current relations which exist in such a circuit are represented
©a	e
/ / /	_ e	/
/ / / \/
// / \ / \ /Z /
Si	Gi	/
a	TL-2609
Figure 9.—Vector addition of voltages.
vectorially in figure 10@. In magnitude eL is equal to iXL, and e0 in magnitude is equal to iXc. The conditions represented in figure
f T' ---------------------------------->i
&	k=iXe Tt‘“'°
® A. c. generator with inductance-capacitance load.
(5) Current and voltage vectors for circuit of @ under condition of II greater than Xc.
Figure 10.
9
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7
SIGNAL CORPS
10 are such that Xjr, is greater than hence 6l is greater than ec and the net effect in this case is that the series inductancecapacitance combination acts as an inductance alone. This is
Ae
---------------->i TL-2611
Figure 11.	—Over-all current and voltage in circuit of figure 10®.
evident on compounding the two voltages to obtain the resultant leading voltage, as in figure 11. If the capacitance is decreased so that the capacitive reactance is increased and the consequent voltage
iec
TL-2612
Figure 12.	—Current and voltage vectors for circuit of figure 10® under condition of Xc greater than X .
across the capacitor increased as in figure 12, then the net effect of the circuit is that of a capacitance alone. In this manner, the react-
i Ji v 1 yi v
—	~	---- wi	~~~—
tE	or i
-------------'_______________ _________________________________
CAPACITANCE	FREQUENCY
Q	@	*>--2613
® Capacitance varied, inductance and frequency constant. ® Frequency varied, capacitance and inductance constant.
Figure 13.—Resonance in series circuit.
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7-9
RADIO FUNDAMENTALS
ance of the circuit can be varied from inductive to capacitive. When the capacitive and inductive reactances are of equal magnitude, the net reactance of the circuit is zero, a condition described as resonance.
8. Resonance.—a. At resonance the current in the circuit of figure 10® becomes infinitely great. This ideal is never attained in practice on account of the presence of resistance. However, even with resistance in the circuit, the current at resonance may reach very large values. As the capacitance (or inductance) is varied either way from resonance, the current falls off as illustrated in figure 13®. Or, if the capacitance and the inductance are fixed, a variation of the frequency of the generator results in a similar variation of current (fig. 13®) with the maximum occurring at that frequency, fr, for which XL equals Xc, that is,
This equation gives for the frequency at resonance f=-l_________________________________
^LC
b. It should he noted that at resonance, although the resultant potential drop across the complete circuit is relatively low, the voltages across the individual inductive and capacitive branches may be very large, often as much as several hundred times the voltage developed by the generator. This feature of a tuned circuit makes it possible to obtain considerable voltage amplification of radio signals of that particular frequency to which the circuit is resonant. In transmitters, the circuit components, in particular the capacitors, must be chosen to withstand high voltages at resonance.
L
--------------->i
__________Tc
ej	TL-2614
© ®
® A. c. generator with inductance-resistance-capacitance load.
© Current and voltage vectors for circuit of © under condition of Xc greater than &,
Figure 14.
9. Circuit containing inductance, capacitance, and resistance in series.—a. When a series resistor is included in the circuit (fig. 14®)), the same considerations as in paragraph 7 hold, with such
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SIGNAL CORPS
modification as is incurred by the addition of a voltage drop across the resistor in phase with the current (fig. 14@). The resultant voltage of figure 14(2) is obtained by first compounding and ec (fig. 15(T)) and then adding this resultant to eR (fig. 15(2)).
I-----*—*’	*^7—"
ec-eL	e
® TL-26IS
Figure 15.—Vectorial addition of voltages of figure 14®.
Z>. It is convenient to represent reactances in vector form: inductive reactance by an arrow pointing up, capacitive reactance by an arrow pointing down, and resistance by an arrow pointing to the right. Then the net reactance, X. is obtained as the difference between the inductive reactance, XL-> and the capacitive reactance, Xc; the impedance vector, Z (Z==7^2+-^2),is given by the diagonal of the rectangle formed from the vectors X and R. The magnitudes of the individual reactances in the circuit of figure 14(1) are obtained on dividing the corresponding voltage drops by i:
c i L i i
The associated vectors are shown in figure 16. The resistance, R, above is the effective a. c. resistance, and is attributed almost entirely to the coil, scarcely at all to the capacitor.
~r : \
x’
V	TL-2616
Figure 16.—Vectorial addition of reactances to obtain total impedance of circuit of figure 14®.
c. The variation of the net impedance with frequency (Z and C fixed) is shown in figure 17. For frequencies below resonance the capacitive reactance is greater than the inductive reactance, and the net effect is that of a capacitance and a resistance in series. At resonance, where the reactances balance, the circuit acts as a pure resistance. For frequencies above resonance the inductive reactance prevails, and the circuit behaves as a simple inductance and a resistance in series.
12
RADIO FUNDAMENTALS
TM 11-455
9
Xi> i
si
S L
I 3 XlM
© J r ------------------------------------->R	----->R
1
________J	xj TL-2617 x/
—~1	—I	—I
FREQUENCY	RESONANT	FREQUENCY
BELOW RESONANCE	FREQUENCY	ABOVE RESONANCE
Figure 17.—Series circuit. Actual circuit at left. Equivalent circuits for various frequencies below corresponding vector diagrams.
d. It is apparent from figure 18 that if the resistance 7? is very small, the effects of the capacitance and inductance are predominant in determining the net effect of the circuit. At resonance with small cir-
XL|
Xlm
Xla
—>R	—>R	—
XJ z x /
xc*	A	/
/	/	TL-2616
zM	/	/
RESONANCE	SLIGHTLY OFF	WELL OFF
RESONANCE	RESONANCE
Figure 18.—Effect of small resistance on sharpness of resonance for circuit of figure 17. Individual reactances above; net impedance below.
13
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9
SIGNAL CORPS
cuit resistance the current is quite large, while slightly off resonance the current drops sharply. On the other hand, if the resistance is large, the resistance predominantly affects the current in the neighborhood of resonance; and the current is only slightly greater at resonance, where the resistance alone is effective, than either side of resonance, where the inductive and capacitive reactances also come into play (fig. 19) :
e. Figure 20 illustrates resonance curves for three different values of resistance. These resonance curves demonstrate the practicability of a tuned circuit as a selective device. If voltages of many frequencies
XtA ---------->R	---------->R	---------->R
x /
x / .s'	/	TL-2619
,z(=R)	/
RESONANCE	SLIGHTLY OFF	WELL OFF
RESONANCE	RESONANCE
Figure 19.—Effect of large resistance on sharpness of resonance for circuit of figure 17. Individual reactances above; net impedance below.
are applied to the tuned circuit, the resulting current is principally of frequencies which are approximately equal to the resonant frequency. As resistance is added to the circuit, the current is attenuated in such a manner that a more nearly uniform but reduced response is obtained over an extended range of frequencies in the neighborhood of resonance. The property of a tuned circuit to accept a limited range of frequencies with essential rejection of all others is called selectivity of the circuit. As shown in figure 20, resistance in the circuit acts to reduce the selectivity. It may be shown that the effect of shunt resistance across either the inductor or the capacitor will likewise reduce the selectivity; the lower the resistance, the poorer the selectivity. Occasionally resistance is deliberately introduced into radio circuits for the purpose of broadening the range of frequencies to which they
14
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RADIO FUNDAMENTALS
are responsive, although generally their inherent resistance is more than enough for this purpose.
10. Circuit containing inductance and capacitance in parallel.—a. Whereas in a series circuit the current is uniform and the voltages across the circuit elements are added to yield the total potential drop across the circuit, in a parallel circuit the voltage across each branch is the same, and the separate branch currents are added to yield the total current through the circuit. Consider the parallel circuit of figure 21. At low frequencies, the reactance in the capacitive branch is high, and consequently the current through that branch is low; at high frequencies the reactance is low, and the current is high. In the inductive branch the opposite relations are true.
J7\<-R=IO OHMS
]\<--Vr=20 OHMS
1 \\ XX R=3O OHMS
। 1
।	^^___L_AND C CONSTANT
____________________i_____________________
fr FRE®UENCY TWM()
Figure 20.—Resonance curves showing broadening effect of series resistance.
b, There are three commonly used conditions for resonance in a parallel circuit. Probably the most common, from a transmitter tuning standpoint, is that resonance is obtained when the line current is a minimum. Another condition is that which makes the impedance of the circuit equivalent to pure resistance. The third condition is that resonance occurs at the frequency for which XL equals Xc. These three conditions are not identical, but the frequencies obtained by them differ by much less than 1 percent in well proportioned parallel circuits. Therefore, for all practical purposes the resonant frequency of a parallel circuit can be taken as the frequency that satisfies the relation—
XL=XC or
-f =__I__
T 2tt4LC
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TM 11-455
10
SIGNAL CORPS
It then follows that the parallel resonant frequency of a tuned circuit is exactly the same as the series resonant frequency of a circuit
LINE CURRENT—}	IcA
Q) i| --C------------------------------------->e ----------->e
।------ k”
iui ,	TL-2621
£j Q
frequency	resonant	frequency
BELOW RESONANCE	FREQUENCY	ABOVE RESONANCE
Figure 21—Ideal (no resistance) parallel circuit. Actual circuit at left. Equivalent circuits for various frequencies below corresponding vector diagrams.
composed of the same values of Z, R, and C in series. The effective a. c. resistance of the capacitor is frequently negligible, but the resistance of the inductor must be taken into account. Diagrams for
ic
LINE CURRENT —,
f -Q 7" XT
__________ \iUR	huR TL-2622
D £j Q
FREQUENCY	RESONANT	FREQUENCY
BELOW RESONANCE FREQUENCY	ABOVE RESONANCE
Figure 22.—Parallel circuit. Actual circuit at left. Equivalent circuits for various frequencies below corresponding vector diagrams.
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RADIO FUNDAMENTALS
this case are shown in figure 22. At frequencies below resonance the net line current may be resolved into an in-phase component and a lagging component; that is, the circuit is equivalent to a simple inductance-resistance series combination. At frequencies above resonance the net current may be resolved into an in-phase component and a leading component; that is, the circuit is equivalent to a simple capacitance-resistance combination. At resonance, that is, for XL equal to Xc, the net line current, at least in the practical case of R very much smaller than X L, is essentially in phase with the applied voltage, and the circuit acts substantially as a pure resistance. Due to the presence of resistance the actual lagging current at resonance is very slightly less than the leading current. However, for most practical purposes it is adequate to consider the lagging and leading components as equal at the frequency of resonance.
uj	/	'	\
G	/	1	\
ui	/	1	\
|
FREQUENCY
TL-2623
Figure 23.—Resonance curve for a parallel tuned circuit.
c. Resonance in a parallel circuit is often referred to as antiresonance because of the inverse relations as compared with the series case. At resonance the series circuit presents a very low resistance, the parallel circuit a very high resistance; at frequencies below the resonant frequency the series circuit behaves as a capacitance, the parallel circuit as an inductance; and at frequencies above the resonant frequency the series circuit behaves as an inductance, the parallel circuit as a capacitance. The resonance curve of a series circuit in which current is plotted against frequency (fig. 13©) resembles in shape the resonance curve of a parallel circuit in which impedance is plotted against frequency (fig. 23).
It will be found that the selectivity of the parallel circuit is inversely related to the resistance in either individual branch of the circuit. Further, the selectivity is adversely affected by a resistance shunted across the entire circuit; the lower the resistance, the broader the response.
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SIGNAL CORPS
e. In a circuit as in figure 24, for a fixed frequency of the generator potential, a variation of the capacitor C is accompanied by a variation of the ammeter reading as the over-all impedance of the circuit changes. Minimum current in the line indicates anti-resonance and maximum circulating current within the LC circuit. A parallel resonant circuit in a radio transmitter is tuned in this
—(2>_n—i
TL-ZW4
Figure 24.—Minimum ammeter reading indicates resonance.
manner by watching for a dip in the line current or ammeter reading.
f. For the parallel circuit in the practical case of R relatively small, a detailed study yields for the over-all impedance (resistance) at resonance
7? - A
that is, the net impedance at resonance is equivalent to a resistance, the magnitude of which is directly proportional to the ratio of L to
Al	i
Rj	|
]	--C at RESONANCE = I Ro
L K	I
Figure 25.—Equivalent circuits at resonance.
C and inversely proportional to R. The reasonableness of this equation may be attested to, at least, by an examination of the current-voltage relations for special cases. Resonance implies merely XL= Xc- Table I lists some of the possible corribinations of L and C which correspond to resonance at a frequency of 10,000 kilocycles. For R equal to zero, Ro is infinite for any L to C ratio. This is apparent on examination of figure 26, which shows vector diagrams for resonance, corresponding (T) to XL=X0 large and (2) to XL=X0
18
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10-11
RADIO FUNDAMENTALS
small. In either case no current flows from the generator into the tuned circuit regardless of the applied potential; that is, in either case the resistance of the tuned circuit at resonance is infinite. When
R is not zero and XL is large large > the net current i and consequently the resistive component iR are small, so that the effective resistance of the tuned circuit is large. When R is not zero and XL is small small^ i and iR may be large, indicating a small effective resistance of the tuned circuit.
Mc= £
AG
------->e
'Mu
©
tl-2626
2
© High^-
® Lowg
Figuke 26.—Antiresonance in ideal (no resistance) circuit.
Table I
L (micro-henrys)	c (micromicrofarads)	Xl = 2rfrL (ohms)	XC^2VfrC (ohms)
0. 1 1 10 100	2, 533 253 25. 3 2. 53	6. 28 62. 83 628. 3 6, 283	6. 28 62. 83 628. 3 6, 283
11. “$•”—a. The merit of an inductor (coil) in a circuit is most conveniently expressed as the ratio of the inductive reactance to the
19
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11-12
SIGNAL CORPS
effective a. c. resistance of the coil. This ratio is so important in the theory of resonant circuits that it is considered as a fundamental property and is usually referred to by the symbol Q. Since the total inductance and resistance of a circuit are almost entirely concentrated in the coil, Q may be represented as follows:
2irfL_XL
V- R ~ R
The effective resistance R of the coil includes any dielectric loss which the coil might have; however, in a well-designed coil the R is due almost entirely to skin effect. Two coils of identical shape can have different (>’s if their resistances differ; or two coils of the same resistance can have different if their inductances differ. The Q of any given coil remains practically constant over a wide range of frequencies, because the effective a. c. resistance of a coil is roughly proportional to the frequency, while the inductive reactance is exactly proportional to the frequency. Typical radio inductors have (?'s of the order of 100 to 800, depending upon the nature of the service for which 'they are designed.
Z>. At resonance in a parallel tuned circuit the net resistance is Q times the reactance of either of the branches. This follows from the equation—
7? —	V/1	V/1
° RC RC^2rjT R~~^-^
Thus the current through either reactor at resonance is Q times the net line current. In the series circuit at resonance the potential across each reactor is Q times the net potential across the complete circuit. This is apparent from the fact that the ratio of the voltages in a series circuit is equal to the ratio of the reactances. Since the net impedence offered by the series circuit at resonance is equal to that of the resistance alone, the potential across either reactor is Q times the net applied line potential. For either parallel or series tuned circuits a high Q. that is, relatively low R. implies good selectivity.
12. Coupled circuits; voltage gain.—a. A common form of coupled circuit arrangement for radio receivers is that of figure 27. The voltage gain at the resonant frequency of this circuit, that is, the ratio of output voltage to input voltage, is the product of the individual voltage gains in the transformer and in the tuned circuit. If the voltage gain of the transformer is unity, so that the induced voltage in the secondary is equal in magnitude to the applied voltage in the primary, then the gain of the circuit is equal to as explained
20
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12
RADIO FUNDAMENTALS
in paragraph 11&. The voltage gain of this circuit is no violation of the principle of the conservation of energy. Energy is alternately exchanged between the inductor and the capacitor, and the only power dissipated is that which is converted into heat by the inherent resistance of the circuit. This latter is the only power which the primary circuit is called upon to supply. If energy is drawn from the tuned circuit, for example, to supply a third circuit, the potential across the secondary output' will drop to a value consistent with the available input power, which in radio receiver circuits is apt to be quite small.
ein© §' jo ''' e° ilJL-Xj tsJ-J-
© Common form of coupled circuit arrangement for radio receivers. ® Equivalent circuit for unity coupling.
Figure 27.
b. One tuned circuit is frequently coupled to another, as in figure 28. The over-all frequency characteristic—that is, secondary current versus frequency for a given magnitude of applied primary voltage— depends upon the degree of coupling employed. If each circuit is independently tuned to the same frequency and then the circuits are
—0—	t——
TL-2620
Figure 28.—Inductively coupled tuned circuits.
loosely coupled together, the over-all frequency characteristic is similar to the resonance curve for an isolated series circuit (figure 13®). However, if the coupling is sufficiently increased, for instance, by winding the secondary coil directly over the primary coil, the over-all reactance and the effective resistance are so altered that a double-humped frequency characteristic results, one peak occurring on either side of the frequency to which the circuits were individually tuned. A compromise (fig. 29®) is often struck be
21
TM 11-455
12-13
SIGNAL CORPS
tween very loose (fig. 20®) and very tight (fig. 29@) coupling to permit nearly uniform energy transfer over a particular restricted range of frequencies. The selectivity of a coupled circuit is adversely affected by the presence of resistance across or in series with any part of the circuit. The effect is similar to that for an individual tuned circuit (fig. 20), a high series resistance or a low shunt resistance producing the broadest tuning.
/-------
/ ' / \? \ \
z	/	/	/	\	\	\
or	/	/	/	\	\	\
cr	/	/	/	\	\	\
U	//	\	\	\
fre«uency
® Loose coupling. ® Intermediate coupling. ® Tight coupling.
Figure 29.—Frequency characteristics of coupled circuits.
Section III
FILTERS
Paragraph
Filter action of individual capacitors, inductors, and resistors____________ 13
Filter action of resonant circuits__________________________________________ 14
General filter networks____________________________________ 15
13. Filter action of individual capacitors, inductors, and resistors.—The analysis in section II indicated the feasibility of using resonant circuits for selecting energy at desired frequencies and for rejecting energy at undesired frequencies. Certain other inductor-capacitor arrangements are better adapted to passing or rejecting more or less uniformly a wide band of frequencies.
a. In the circuit of figure 30 the applied potential consists of three components: direct current, edc; audio frequency alternating current, eafand radio frequency alternating current, erf. The current which flows is given by
/__ e
*r’O
TL-2,633
Figure 33.	—Low pass filter.
find it worth while to demonstrate for himself that the inductor and the series resistor in the circuit of figure 34 act as a high pass filter.
e. Figure 35 presents a pictorial concept of currents which flow in series circuits corresponding to various applied potentials.
•	R
edU=-
^dc	r-f
e.,Q K L I R (L0AD)
Ch
TL-2634
Figure 34.	—High pass filter.
14.	Filter action of resonant circuits.—Resonant circuits can be made to serve as filters in a manner similar to the individual inductors and capacitors of paragraph 13. The series resonant
25
TM 11-455
14
SIGNAL CORPS
circuit offers a very low impedance to currents of the particular frequency to which it is tuned, and a relatively high impedance to currents of other frequencies. A series resonant circuit replacing the inductor of figure 31 would act as a band pass filter, passing
R.F.
A.F+R.E
AX+RX ♦ D.C.
applied	CURRENT
voltage	small	LARGE	SMALL	LARGE
ca PACITANCE	CAPACITANCE	INDUCTANCE	INDUCTANCE
	\ A / A A
	V V / V \
	
vuvuvvvvv ypiilililjpli	■ "■
A..A.b iWilM					 /11V Li	,,M.
t 1	\| A r p	I ) A	n	
AS . ... .	IJ Li >1
		 . iiMm	1 V
WW	
Figure 35.—Filter action of individual series capacitances and inductances
currents of frequencies in the neighborhood of its natural frequency and attenuating all others. A series resonant circuit shunted across the load to replace the inductor of figure 34 would bypass currents of its natural frequency. The parallel tuned circuit, on the other hand, offers a very high impedance to currents of its natural fre
26
TM 11-455
14-15
RADIO FUNDAMENTALS
quency and a relatively low impedance to others. In series with a load it acts as a band stop filter; in parallel with a load as a band pass filter.
15.	General filter networks.—a. Combinations of capacitances, inductances, and resistances are frequently used in networks. The arrangement of figure 36 is commonly employed to “smooth” the output of rectifier tubes supplying plate current for a transmitter or receiver. The capacitors shown are each of 10 microfarads
20 HENRYS 100 OHMS D.C. 20 HENRYS 100 0HM5 D C.
.		r	W	 A	,	6	——rOOO(P	*	WV— '<~2ooo ^wmTax^ - 10 MICROFARADS	C	G = 10 MICRO- < FARADS <	>10,000 OHMS > DC. AND A.C.
2,000 OHMS A.C.			
£	D	H	Tt-2636
Figure 36.—Low pass filter for power supply.
capacitance; and each choke is of 20 henrys inductance with inherent resistance of 100 ohms to direct current and total impedance of 2,000 ohms to 120-cycle alternating current. The load is a 10,000-ohm (d. c. or a. c.) resistance. A semi-quantitive analysis of the action of such a filter may be made as follows. The impressed voltage at AjE* (voltage output of a full-wave rectifier), which is actually shown in figure 37'eac--i,R
Tl-2639 Gco~i|Xj,
Figure 39.	—Alternating voltages in first section of low pass filter.
The voltage output of the first section is shown in figure 40(1). md the output of the second section in @.
@	©	11*2640
® First section.	@ Second section.
Figure 40.	—Voltage output of low pass filter sections.
28
TM 11-455
15
RADIO FUNDAMENTALS
b. A high pass filter is shown in figure 41. Band pass and band stop filters employing resonant circuits are shown in figures 42 and 43 respectively.
----------11-------------------11---------------------
>o	i°
I	।
TL-Z64I
Figure 41.	—High pass filter.
-----------------qw---------------1|-----------------
TL-Z642
Figure 42.	—Band pass filter.
TL-2643
Figure 43.	—Band stop filter.
29
TM 11-455
16	SIGNAL CORPS
Section IV
VACUUM TUBES
Paragraph
General_____________________________________________________________ 16
Diode rectifier___________________________________________________   17
Characteristic curves_______________________________________________ 18
Triode amplifier____________________________________________________ 19
Distortion__________________________________________________________ 20
Static and dynamic characteristics__________________________________ 21
Miscellaneous characteristics_______________________________________ 22
Families of characteristics used in analysis________________________ 23
Resistance, amplification factor, and transconductance______________ 24
Interelectrode capacitance__________________________________________ 25
Tetrodes____________________________________________________________ 26
Pentodes and beam tubes_____________________________________________ 27
Variable a tube_____________________________________________________ 28
Multipurpose tubes__________________________________________________ 29
Directly and indirectly heated cathodes_____________________________ 30
16. General.—a. Tungsten and certain	other metals and metallic
oxides yield electrons freely when heated to a high temperature in a low-pressure atmosphere. Any isolated positively charged body in the vicinity of an electron emitter attracts the electrons, which then ultimately neutralize the positive charge. The positive charge can be
\ /z^--©..........—X
-e-	--------e-------
U	e- y
--------------'I 'I *1------------1
TL-264-4
Figure 44.—Emitted electrons attracted by positively charged body.
maintained, however, if electrons are removed just as fast as they strike, as, for example, by connecting a source of constant voltage between the positively charged body and the emitter (fig. 44). This is the general arrangement in a two-element vacuum tube (diode). The emitter may resemble the familiar incandescent lamp filament heated by passing a current through it. The charged body usually surrounds the emitter and is called the plate. The whole assembly is enclosed in an evacuated glass or metal container, called the “envelope.” The plate, being the terminal at which current normally enters the tube, is sometimes called the anode; the emitter, being the terminal at which current normally leaves the tube, is commonly called the cathode.
30
TM 11-455
16-18
RADIO FUNDAMENTALS
(The conventional direction of current flow is opposite to the direction of electron motion.)
b. The pressure inside a vacuum tube is reduced to such an extent that only about Koo,ooo,ooo of the original air in the tube remains. For pressures much above this amount spurious effects are introduced by the presence of gas molecules. The electrons are deflected from their normal paths by collisions with gas molecules; the positive ions formed from the molecules as the result of such collisions make for background noise in a radio receiver; these positive ions further serve to lower the internal resistance and amplifying power of a vacuum tube, and they may lower the effectiveness of an emitter or even destroy it. In general, tubes are evacuated to the highest extent consistent with economical commercial manufacture.
______________J"
TL-Z645
Figure 45.—Diode with resistive load.
17. Diode rectifier.—The symbolic representation of a diode with a resistive load is as shown in figure 45. The battery potential controls the magnitude of the current through the load resistance. Increasing the potential increases the rate at which the emitted electrons move to the plate and the rate at which current flows through the load; decreasing the potential decreases the current through the load. Reversing the potential, which makes the plate negative with respect to the emitter, causes repulsion of the electrons from the plate with consequently no current through the load. One immediate application of the vacuum tube is obvious: its use as a rectifier. Thus with an alternating potential applied (fig. 46), current flows through the load only during alternate half cycles; that is, when the plate is positive with respect to the emitter—and only in one direction, from plate to emitter (fig. 46).
18. Characteristic curves.—The load current in the above circuit is not proportional to the applied voltage, as is the case with pre
31
TM 11-455
18
SIGNAL CORPS
viously studied circuits. Ohm's Law is strictly applicable only to small increments of currents and voltages, and current-voltage relations in general in vacuum tube circuits are studied by means of experimentally obtained characteristic curves. A plot of the direct current in the load of figure 45 against the plate-to-emitter potential is shown in figure 47. At the lower values of plate potential the
©2 ui J> O	Z
LD __	_ ______ ID /	\	\	\
>	>- Z\ z\ /\ cc /	\	/	\	/	\
r < —A /—X /—\ I \ / \
S	TIME g	time
o	<
J	tl-2646
Figure 46.—The diode as a rectifier.
accumulated emitted electrons in the neighborhood of the cathode are eff ective in repelling the electrons nearest the cathode back toward the cathode, and only those electrons which are nearest the plate are attracted to the plate. For intermediate values of plate potential the space charge in the vicinity of the cathode is reduced, owing to
h	/
z	/
ui	/
oc	/
tr	/
/  y y y* 1 y y
CL Al	I	_____
O	50
PLATE POTENTIAL IN VOLTS tl-2655
Figure 55.—Plate current vs. plate potential curves for triode.
pressed signal voltage will cause variations along this load line in both directions from P. Corresponding to an instantaneous grid potential of 10 volts, the plate current, plate voltage, and voltage across the load can be found by following the 10-volt characteristic to where it intersects the load line. From the curves of figure 56® this yields 16 milliamperes plate current, 25 volts plate potential, and
37
TM 11-455
23-24
SIGNAL CORPS
90 — 25 = 65 volts drop across the load. The family of plate currentplate potential curves is thus useful in determining the limitations of a particular tube under various operating conditions. A particular tube can be selected to fit certain circuit constants, or vice versa, with the aid of the information contained in the vacuum tube characteristics.
24.	Resistance, amplification factor, and transconductance.—The a. c. internal resistance, Rv, of a vacuum tube is measured by the increase in plate voltage required to produce given increase in plate current. A tube for which a 10-volt plate potential increase is required to produce a 10-microampere increase in plate current has a relatively high internal resistance, q qqqqjq = 1,000,000 ohms. A tube which requires a 10-millivolt plate potential increment to
^0/	0 a /	/
/ ',q/ / / <30 '	/	/ o/	/
< / / / / * ^ / z	-/// 7 y
“2.0/	/	/	7	/
5	y
«	1	/ /i	/	/	/
“ nL	°	/	/ '	/\P /	/	/
o(0--------- U.IO/ / I /	/	/
in	‘	<	/ ; /	/	/
«	;
Ol__________:______^^9 a y I -f i y K
o	50	90 O io 20 30 40 50 60 70 60 90
PLATE POTENTIAL IN VOLTS
©	®	TL-2656
® Construction.	® Application.
Figure 56.—Load line.
produce a 10-microampere plate current change has a relatively low internal resistance, q qqqoiq = 1,000 ohms. Low internal resistance is manifested by a steep iv-ev characteristic corresponding to large increases in plate current for small increases in plate voltage. The amplification factor, /z, (a Greek letter, “mu”) of a tube is a measure of the relative effectiveness of grid and plate potential increments in changing the plate current, and is an indication of the suitability of the tube for voltage amplification purposes. If a 1 milliampere increase of plate current is accomplished either by a 10-volt increase in plate potential or by a 0.1 volt increase in grid potential, the amplification factor isn~i= 100. Transconductance, Gm, is defined as the ratio of a small change in plate current to the change in grid potential
38
TM 11-455
24-26
RADIO FUNDAMENTALS
producing it, all other voltages remaining constant. Transconductance is a criterion of the suitability of a tube for power amplification purposes, a tube with a high transconductance yielding large plate current variations corresponding to small variations in grid potential. The transconductance of a tube is dimensionally a ratio of amperes to volts and is thus measured in reciprocal ohms or mhos. (“Mho” is “ohm” spelled backwards.) A tube with a high transconductance, perhaps 10,000 micromhos, is evidenced by a steep ijr-eg characteristic; a tube with a low transconductance, perhaps 10 micromhos is shown by an ip-eg characteristic with a small slope. Internal resistance, amplification factor, and transconductance are interrelated, amplification factor being essentially a product of the other two. A tube which has both a high transconductance and a high internal resistance accomplishes the same increase in plate current with a small increase in grid potential alone (due to the high transconductance) as with a large increase in plate potential alone (due to the high internal resistance). In other words, such a tube also has a high amplification factor.
25.	Inter electro de capacitance.—The inherent capacitance between grid and plate elements of a triode is of sufficient importance at high frequencies to require special consideration in radio circuits. Where this capacitance is undesirable, it can be counteracted by introducing a neutralizing circuit which presents r. f. potentials equal in magnitude but opposite in phase to those occurring across the interelectrode capacitance, with the result that the effects of the interelectrode capacitance are nullified. The extra circuit complications can generally be avoided by the use of tetrodes or pentodes, four and five element tubes respectively, which are particularly designed to have low interelectrode capacitance. The grid to plate capacitance of an ordinary receiving triode runs about 3 micromicrofarads. This represents a capacitive reactance of 53,000 ohms at 1 megacycle and only 530 ohms at 100 megacycles. Tetrodes and pentodes offer corresponding reactances of about 16,000,000 ohms at 1 megacycle and 160,000 ohms at 100 megacycles.
26.	Tetrodes.—The tetrode includes a second grid, called a screen grid, between the regular control grid and the plate. The screen grid is operated at a potential which is positive with respect to the cathode but less positive than the plate. By connecting the screen to the cathode through a capacitance, the screen is at approximately the same potential as the cathode as regards r. f. currents. The screen acts as an electrostatic shield between the cathode and the plate. The effect of the screen thus connected is twofold: the grid to plate capacitance of the tube is considerably reduced (see par. 25) and the
89
TM 11-455
26-28	SIGNAL CORPS
amplification factor of the tube is considerably increased. The control grid potential regulates the plate current in much the same manner as in a triode; however, in the screen grid tube the plate potential has very little effect on the plate current. Because of the screening action of the second grid, the same change in plate current which requires a very large change in plate potential is accomplished by a small increment of control grid potential; that is, the amplification factor of the screen grid tube is high. An incidental and generally unwanted effect in tetrodes is the extent of secondary emission, that is, release of electrons from the plate on bombardment by the electrons in the current stream. In a triode these secondary electrons are eventually attracted back to the plate. In a tetrode, however, the positively charged screen competes with the plate for the attraction of these electrons, with the result that when the potential of the plate approaches, or falls below, that of the screen, the screen draws large currents, the plate current is lowered, and the amplification of the tube is reduced.
27.	Pentodes and beam tubes.—Pentodes and beam tubes effectively cope with the problem of secondary emission. In the pentode a suppressor grid inserted between the screen grid and the plate, and electrically connected to the cathode, serves to prevent the secondary electrons from moving to the screen grid without otherwise appreciably altering the characteristics of the tetrode. In the beam tube suppression is achieved by a particular design arrangement which provides for a very low intensity electric field midway between the screen grid and the plate. In this region of the low intensity field, electrons are slowed down to such an extent as to accumulate and to present a space charge effect similar to that which surrounds the cathode. The negative charge cloud repels secondary electrons in . the same manner as would a suppressor grid. Screen current is minimized in the beam tube by the design of the screen grid, which is such that the constituent wires are in the shadow formed by the control grid of the electron stream from the cathode. The beam tube has a high power sensitivity, that is, high power output for a given signal voltage input.
28.	Variable p. tube.—The variable p tube is a modified tetrode or pentode receiving tube in which the control grid is constructed with some of the grid elements widely separated and others closely spaced. For large negative values of grid bias the closely spaced elements present a high density negative charge to prevent the flow of electrons through them; and the only electrons from the cathode to reach the plate are those passing through the openings between
40
TM 11-455
28-30
RADIO FUNDAMENTALS
the widely spaced elements. It is possible by means of an auxiliary rectifier circuit to vary the grid bias of the variable p tube in accordance with the intensity of the incoming signals and thus to provide automatic volume control to compensate for fading.
29.	Multipurpose tubes.—Certain types of receiving tubes have two or more complete sets of elements within one envelope which perform various associated functions. For example, the duplex diode has two cathodes and two plates, and it may be used as a full wave rectifier. The possible combinations are numerous. Some of them will be encountered in section X on radio receivers.
30.	Directly and indirectly heated cathodes.—a. A cathode which is in the form of a filament directly heated by passing a local current through it has the disadvantage of introducing a ripple in the
I
5 VOLTS AC $
i <	iii
^ \ > -/ b	y
WWMMA-
,	. 2,000 OHMS
k- ioo -U VOLTS
TL-4657
Figure 57.—Directly heated cathode.
plate current when alternating current is used for heating. The ripple is most objectionable if the plate and grid returns are made to one end of the filament. In figure 57 the resistor AB represents a filament which is heated by applying 5 volts of alternating current across it. For no current flowing through the tube the plate is maintained at a potential of 100 volts above that of point B. For a 5-mil-liampere steady plate current the potential across the tube from B to 5
the plate is always 100—2000X =90 volts; whereas the potential
from A to the plate varies from 85 to 95 volts depending upon the potential of point A relative to point B, and the total plate current rises and falls at the frequency of the filament current. This condition is remedied to a large extent by connecting the grid and plate
41
TM 11-455
30
SIGNAL CORPS
returns to the electrical center of the filament as in figure 58® or ®. But even with a center return arrangement, for a 60-cycle filament current, there is still present 120-cycle modulation of the plate current. This double frequency ripple arises from the effects on the plate current provided by the intermittent rise and fall of the filament temperature, the voltage drop in the filament, and the alternating magnetic field set up by the filament current. Temperature fluctuations in the filament are ordinarily negligible. The latter two effects, how-
H—v-——'H'l'H	'—'I—”——H'l'l-
O „ O © ®
l| --------------- Tl-2656
®
@ Center-tapped resistor.
@ Center-tapped transformer. ® Indirectly heated cathode.
Figure 58.—Methods of utilizing a. c. filament supply.
ever, may be troublesome. The magnetic field about the filament serves to deflect the electrons from their normal paths and so in effect to reduce the plate current. The resulting plate current is largest when the heating current is zero, that is, at intervals which occur at double the heating current frequency. With a voltage drop in the filament, the space current from the negative half of the filament exceeds that from the positive half because of the manner in which space current varies with the electrostatic field across the tube (space current varies as the three-halves power of the plate potential). The result is that
42
TM 11-455
30-32
RADIO FUNDAMENTALS
each time the current is a maximum in either direction in the filament, that is, at a frequency which is double the heating current frequency, the space current is increased slightly from the value which obtains during those instants when the current through the filament is zero and the potential of the filament is uniform.
b. In transmitting tubes and in the power stages of a receiver, where the signal currents are large, the double frequency ripple current is neglible in comparison. However, in all other receiver tubes, indirectly heated cathodes (fig. 58®) are necessary wherever a. c. filament operation is desired. An indirectly heated cathode is formed by a metallic sleeve closely surrounding a heated filament and electrically insulated from the filament. The cathode is heated by radiation from the filament. Such an emitter is sometimes referred to as an equipotential cathode, since all parts of it are at the same potential. In general throughout this manual, for simplicity tube heater elements and heater power circuits are not shown in circuit diagrams.
Section V
VACUUM TUBE AMPLIFIERS
Paragraph
Classification of amplifiers_____________________________________________ 31
Class A operation----------------------------------------------------------- 32
Class B operation___________________________________________________________ 33
Class C operation------- ----------.------------------------------------- 34
Voltage gain________________________________________________________________ 35
Methods of coupling amplifier stages---------------------------------------- 36
Bias________________________________________________________________________ 37
Distortion-----------------------------------------------------------------  38
Maximum power transfer------------------------------------------------------ 39
Feedback____________________________________________________________________ 40
Regeneration-----------.------------------------------------------------- 41
Degeneration---------------------------------------------------------------- 42
31. Classification of amplifiers.—Amplifiers are classified according to their general usage as radio or audio frequency amplifiers, and as voltage or power amplifiers; according to the type of coupling between stages as resistance coupled, impedance coupled, or transformer coupled amplifiers; and according to the method of operation as class A, class B, or class C amplifiers. The A, B, and C classifications are based on the following general considerations: class A, high fidelity reproduction; class B, plate circuit rectification; and class C, high efficiency operation.
32. Class A operation.—Class A operation is such that with a single tube in an amplifier stage it is possible to obtain an output plate
43
TM 11-455
32
SIGNAL CORPS
current wave shape that is a good replica of the input signal voltage wave shape. -This requires that the grid and plate potentials applied to the tube confine the operation to within the substantially straight portion of the ip-eg* dynamic characteristic as in figure 59. It is generally desirable to prevent positive swings of grid potential because of the accompanying grid current. A tube which does not draw grid current presents an infinite input resistance. On the other hand, a tube which does draw grid current is equivalent to a shunt resistance
»- /
'	z /
Ul	/
a / cc /
OUTPUT CURRENT
/• । !
] ।________________________________
7Z	' (T O GRID POTENTIAL	-f.
di'jA x	TL-265S
-iW: m । I
1 1 I 1 !
Figure 59.—Class A operation.
across the input circuit ; the higher the current, the lower the resistance. In r. f. receiver amplifiers, where the grid-to-cathode portion of the tube shunts the preceding tuned circuit, it is in the interest of good selectivity to keep the grid-to-cathode resistance high by operating the tube so as not to draw grid current. A tube which draws grid current suffers distortion of the signal voltage, as indicated in para-
*The subscript g and references to grid, unless otherwise specified, will be taken to indicate control grid.
44
TM 11-455
32-33
RADIO FUNDAMENTALS
graph 20, and it requires grid power to excite it. In power amplifiers where economy of grid circuit excitation is generally secondary, distortion of the input wave form with a limited amount of grid current is sometimes tolerated in exchange for the higher plate circuit . a.c. plate power output ...	„
efficiency—□—	—4 —which results. Negative feedback
J d.c. plate power input
(par. 42) may be employed to counteract the distortion introduced by the grid current.
✓OUTPUT CURRENT
IX	/I
iZJii
I	o	GRID POTENTIAL +
-—TL-2660
tH	—----
o	--!--—,
o	a
i*=——
O e	7~-
3 e	__
Z RP + ipRd
e’=—;
The output voltage is
Co ipRfj
Thus the gain of the amplifier stage is
g0__ P-Ro
6 g Ro Rp
This shows that for the gain of a resistance coupled amplifier to approach the amplification factor of the tube, Ro must be very large so that Rp is negligible in comparison. However, a practical upper limit to Ro is set by the fact that the potential required to maintain the plate current becomes increasingly large as Ro is increased. The value of Ro is usually compromised on as about the same order of magnitude of Rp.
36. Methods of coupling amplifier stages.—a. The resistance coupled amplifier is used extensively for audio frequency applications because of its low cost and relative freedom from distortion. It is occasionally used for certain applications in radio frequency work where an untuned circuit is satisfactory.
b. Figure 64 shows a typical two-stage resistance coupled amplifier. The capacitors C couple the output of each stage to the input of
48
TM 11-455
36
RADIO FUNDAMENTALS
the following circuit. Each capacitor serves to block the d. c. plate voltage of one tube from the grid of the next, while at the same time permitting ready transfer of the a. c. signal voltages. The function of the resistor R is the same in each stage, in conjunction with the cathode series resistor Rr, to maintain the grid of the tube at the proper bias for class A operation. Normally no current flows through R. Thus the potential drop across R (potential drop= IXR) is zero; that is, the potential of the grid relative to the cathode is entirely determined by the drop in potential occurring with the flow of plate current through Rt. The coupling capacitor C should be large enough to offer a low reactance to the frequencies to be amplified, while the grid leak R should have a very large value so that the shunting effect of the grid leak and coupling capacitor upon the coupling resistor Ro is small. This requirement is manifested by an
—li—,--------—0—।-----------------------
INPUT r .	Ro| output
[‘fp <43 I cTr ‘4],___________________________,
——	I Rj.
O.C. PLATE POTENTIAL	TL-2.664
Figure 64.—Two-stage resistance coupled amplifier.
examination of figure 65, which is an equivalent circuit of one stage of resistance coupled amplification. It will be recalled from paragraph 35 that the requirement for good voltage gain of the resistance coupled amplifier is a load circuit resistance which is as high as practicable. The capacitors Ch across the biasing resistors Rt provide low impedance paths for the a. c. components of plate current, so that grid bias is not varied in accordance with variations in plate current. The reason for the resistors R2 and capacitors C2 in the plate circuit of each tube can be seen by a study of the simple circuit of figure 66. The plate current of the second tube, including a. c. and d. c. components, flows through the common plate source. If the plate source contains an internal resistance, the potential across AB fluctuates in accordance with the a. c. output of the second tube. Thus the potential applied to the first tube is modulated by the action of the second tube. This interaction, termed feedback, is
476847 0 - 42 -4
49
TM 11-455
36
SIGNAL CORPS
avoided by the use of a decouplings filter—C2 and R2 (fig. 64)_______in
each plate lead which serves to bypass the a. c. components around the plate potential source.
c.	The response of the resistance coupled audio frequency amplifier falls off at low frequencies (below roughly 50 cycles) because
ftp	c
--------W--------------------1|----
Ro |	|R
TL-2665
Figure 65. Equivalent circuit of stage of resistance coupled amplifier.
of the high reactance of the coupling capacitors. It falls off at high frequencies (above roughly 5,000 cycles) because of the low reactance of the tubes’ interelectrode capacitances, which shunt the load resistors. For intermediate frequencies the response is substantially uniform.
-------------ii—
L__________________A _______________________
B
Tl-2666
Iiglre 66. Two-stage amplifier with common plate supply.
d.	If large inductors (choke coils) are inserted in place of each plate resistor Ro in figure 64, an impedance coupled amplifier results. Each inductor offers a high impedance to alternating current, giving a high gain, while at the same time offering low d. c. resistance, thus requiring considerably less supply potential than is needed for the comparable resistance coupled amplifier. The frequency response
50
TM 11-455
36
RADIO FUNDAMENTALS
characteristic of the impedance coupled amplifier is similar to that of the resistance coupled amplifier.
e.	If transformers are used as coupling units between adjacent amplifier stages, the coupling capacitors and grid resistors can be omitted. Figure 67 shows a two-stage transformer coupled amplifier. Transformer coupled amplifiers can be made to give more gain
<-----r-----------\ c----------------------L g--------------------*
INPUT I I p । I |	| ]I0UTPUT
n if i i ------------------——------------—1---------
X-	—>o--------------------------
-t
D.C. PLATE POTENTIAL	TL-266T
Figure 67.—Two-stage transformer coupled audio amplifier.
than either resistance or impedance coupled amplifiers with the use of step-up transformers. The response falls off at the lower frequencies due to the fact that the reactance of the transformer primary decreases with the frequency. At the upper frequencies a decline in response is associated with the effect of the grid cathode capacitance of the following circuit.
CAPACITOR ROTOR SECTIONS ALL ON SAME SHAFT ----------------------rzzz~--------- yyy yr-
+	TL-Z668
D.C. PLATE POTENTIAL
Figure 68.—Two-stage r. f. transformer coupled amplifier.
/. An r. f. transformer coupled amplifier employing tuned secondaries and pentode tubes is shown in figure 68. The selectivity (as well as the gain) of such a tuned r. f. amplifier increases with the number of stages in the manner illustrated in figure 69.
51
TM 11-455
37
SIGNAL CORPS
37. Bias. a. The choice of a particular type of grid bias depends on the service to which the amplifier is subjected. Most receiver amplifiers use the cathode return resistor bias with shunt capacitor, as in the circuits of figures 64, 67, and 68. Omission of the shunt capacitor, or too small a value of the capacitor, incidentally produces
T___________' r___________________	_________
L- FIRST --------------- SECOND ------------- THIRD --------
TUNED R.F.	TUNED R.F.	TUNED RE
STAGE	STAGE	STAGE "
Lk 1 \ LA.
FREQUENCY	FREQUENCY	FREQUENCY
TL-2669
Figure 69.—Increase in selectivity with the number of tuned stages in an r. f. amplifier.
degeneration (par. 42) as a result of the variations of grid bias which then accompany the a. c. pulsations of plate current.
b. Grid leak bias (fig. 70) is suitable for use under conditions where grid current flows. This type of bias is economical of power and is thus frequently employed in transmitters. The bias results from the drop in potential across the grid leak (resistor) with the
—	i—11—i	—
g RFC.
®	(2)	TL-267Q
© Shunt arrangement.	@ Series arrangement.
Figure 70.—Grid leak bias.
flow of current on positive signal swings. The capacitor across the leak offers a low impedance to a. c., so that the bias is essentially steady in character and is a function of only the magnitude of the grid current. A disadvantage of grid leak bias is that if for any reason the excitation is removed, the bias is removed also, and the
52
TM 11-455
RADIO FUNDAMENTALS	‘	37-38
plate current may assume dangerous proportions, causing the liberation of gas from internal parts of the tube or even melting the plate.
c. Batteries, or a separate rectifier filter system distinct from the plate power supply, have the advantage of giving practically constant bias voltage under all conditions of excitation. This type of bias, further, offers protection to an amplifier tube in case of excitation’ failure. To combine the advantages of grid leak and battery bias, transmitter amplifiers often employ a combination of both types in series. Some amplifier tubes are conveniently designed, as regards bias supply, to operate with the grid at cathode potential (zero bias).
38. Distortion.—a. Distortion in an amplifier may be broadly classified under three different headings: Frequency distortion, nonlinear distortion, and delay (or phase) distortion. Frequency distortion arises because of the inability of an amplifier to amplify equally all frequencies. Nonlinear distortion is a consequence of operating over a curved (nonlinear) portion of a tube’s characteristic, so that harmonic or multiple frequencies are introduced. Delay distortion results from the effects of transmission of different frequencies at different speeds, giving a relative phase shift over the frequency spectrum in the output. Except at the ultrahigh frequencies or m transmission line work, the effects of delay distortion are usually insignificant. Frequency distortion in r.f. transmitter amplifiers is ordinarily of little concern, since these amplifiers operate over only a relatively narrow range of frequencies at any one time.
b. In r. f. receiver amplifiers, various compensating devices are sometimes employed to provide uniform response to a band of frequencies. Figure 71 illustrates one such compensating arrangement. A high inductance primary winding P, loosely coupled to the secondary N, resonates (due to self-capacitance) at a lower frequency than the lowest for which the amplifier is to operate. This gives high gain at the low end of the band because of the high plate load impedance at the lower frequencies. The small capacitance C, due to a loop of wire hooked around the top of the secondary, provides increased coupling at the higher frequencies to improve the response at the upper end of the band.
c. Distortion which arises from operating a vacuum tube over a nonlinear portion of its characteristic consists principally of multiple frequencies (harmonics) and of sum and difference frequencies corresponding to each frequency present in the original signal. Suppose, for instance, that the input signal to a nonlinear radio frequency amplifier is composed of three frequencies: 500,000, 501,000 and 501,025
53
TM 11-455
38	SIGNAL CORPS
cycles. The output then contains in addition to the three original frequencies mainly the following distortion frequencies:
(1)	Harmonics: 1,000,000, 1,500,000
1,002,000, 1,503,000
1,002,050, 1,006,075
(2)	Sum frequencies: 1,001,000, 1,001,025, 1,002,025
(3)	Difference frequencies: 1,000,25, 1,025.
The filtering action of a parallel resonant circuit in an amplifier plate circuit which is tuned to about 500,000 cycles minimizes the effects of all these distortion components. The extent of this suppression of the distortion frequency components may be controlled by proper design of the tuned circuit. At frequencies well off resonance the parallel circuit offers essentially the impedance of the lowest im-
-H:_________________________1_________d
_	Y	TL-2671
Figure 71.—Special circuit arrangement in r. f. amplifier to provide uniform response over a band of frequencies.
pedance branch. In a circuit tuned to 500,000 cycles the impedance offered to currents of frequency 1,000,000 cycles is practically that of the capacitor alone, and the impedance offered to currents of frequency 1,000 cycles is practically that of the inductor alone. Thus a low Z-to-Z ratio minimizes the voltages developed across the parallel circuit at the distortion frequencies. Two tuned circuits between which it is desired to transfer energy sometimes employ link coupling as shown in figure 72. In this manner incidental coupling between the two circuits due to the distributed capacitance of the turns is avoided, and the transfer of harmonics from one circuit to the other is avoided.
d. In an audio frequency amplifier the distortion frequencies corresponding to 
------Zd ICZ—F-----'—X	o	J 'F
INPUT	GAm-lOO	___|100MV OUTPUT
S g | riOMV.H i GAIN IUU	--->
k< QO-OJ?)---6	0-----f-------------
INPUT 1	\ |lOOMV. OUTPUT
gj jo i riQ mv*i	_q ? ।
--------------------------------- Tl-2676 ® No feedback. ® and ® Ten percent degenerative feedback. Figure 76.—Effect of degeneration.
that is, the over-all gain is substantially independent of the normal gain of the amplifier. For a specific example to illustrate this effect, consider an amplifier which without any feedback has a normal gain of 100. An input of 1 millivolt gives rise to an output of 100 millivolts (fig. 76®). Suppose degenerative coupling is now provided
61
TM 11-455
42
SIGNAL CORPS
such that 10 percent of the output is returned to the input. If the output under these circumstances is 100 millivolts, 10 millivolts will be introduced into the input of the amplifier in reversed phase from the signal. In order for the output to remain 100 millivolts, it is necessary that a net input of 1 millivolt in phase with the signal be maintained. A signal of 11 millivolts is necessary to maintain this 1 millivolt net input: 11 millivolt signal-10 millivolt feedback= 1 millivolt input (fig. 76@). It should be noted how under conditions of degeneration the over-all amplification is considerably re-
A-z'.
5	.——- ®
I
------------@
10	100	1,000	10,000
FREQUENCY IN C.P. S. 11*2677 ® Straight amplification.
© Same amplifier with degeneration. ® Same amplifier with increased degeneration.
Figure 77.—Response, characteristic of amplifier with loudspeaker load.
duced. Without degeneration the over-all gain is 100 to 1. With degeneration the over-all gain is now 100 to 11. Suppose now that under some particular circumstances, possibly for some particular frequency range, the normal gain of the amplifier is reduced to 50. It would then require a 2 millivolt net input to give an output of 100 millivolts. For 10 percent degeneration this means a 12 millivolt signal: 12 millivolt signal —10 millivolt feedback=2 millivolt input (fig. 76(§)). The over-all amplifier gain is now 100 to 12. Thus only a very small percentage reduction in over-all amplification, from 10%i to 10%2 results from a 50 percent reduction in the normal
62
TM 11-455
42-43
RADIO FUNDAMENTALS
amplifier gain. Actual experimental curves showing response characteristics corresponding to straight operation and to degeneration for the same receiver amplifier with a loudspeaker load are shown in figure 77. The independence of amplifier gain with frequency means an improvement in the quality of the amplifier reproduction. Whereas the amplifier normally might discriminate against certain frequencies and accentuate others, with degeneration all the desired frequencies are amplified nearly uniformly.
c. The second feature of degeneration—reduction of noise produced within the amplifier—depends on the fact that the signal, which is introduced in the grid circuit of the amplifier, receives greater relative magnification than those particular noises which are introduced in the plate circuit. The grid signal is amplified, whereas the plate noise is not; while that portion of the output which is returned out-of-phase to the input is amplified equally for both noise and signal components. The reamplification of this out-of-phase signal component reduces the amplifier gain; the reamplification of the out-of-phase noise component effectively reduces the noise current present. Hence, the cancellation effect of the degeneration combined with the differential effect of straight amplification results in a relative reduction of the noise produced within the tube, at the price of a general reduction in gain. If the feedback can be made into a preceding amplifier stage, where it is presumed that no distortion of the same type occurs, then the degeneration could be controlled so that the output of the first stage consists of the desired signal, and a distortion component sufficient to counterbalance the noise present in the output of the last stage, while at the same time the over-all gain of the two stages is reduced only slightly.
Section VI
VACUUM TUBE OSCILLATORS
Paragraph
Mechanical oscillations_________________________'---------------------------------- 43
Electrical oscillations------------------------------------------------------------- 44
Simple oscillator circuit_________________________________________________________   45
Practical oscillator circuit------------------------------------------------------   46
Oscillator circuits in general use-------------------------------------------------- 47
Oscillators for very high frequencies-------------------------------------_-------- 48
43. Mechanical oscillations.—Two fundamental requirements of any type of natural oscillation are an inertial element and a restoring force. Consider a coil spring lying horizontally on a table with one end of the spring clamped, as in figure 78®. If a force is applied
63
TM 11-455
43-44
SIGNAL CORPS
to the free end so as to compress the spring and then the compressing force is removed, the energy stored in the spring on compression is released to extend the spring back to its normal length. More than likely the spring may actually distend a small amount beyond its normal length and then compress again slightly; that is, the spring may alternately expand and contract so that the free end oscillates a few times about its equilibrium position before coming to rest. What causes the spring to continue beyond its normal length is described as the inertia of the spring and is attributed to its inherent
NORMAL POSITION, ONE END FIXED
SPRING COMPRESSED
‘-OJL2. SPRING RELEASED ©
® Oscillations of a coil spring.
® Spring with mass attached to free end.
Figure 78.
TL-2678
mass. If a large concentrated mass is attached to the end of the spring (fig. 78(2)), the tendency for the spring to continue past its equilibrium position is more pronounced. Also the period of oscillation, the time for one complete to-and-fro motion, is lengthened. The period is longer for a weaker spring, that is, for one with a weaker restoring force; and it is shorter for a stronger spring.
44. Electrical oscillations.—Electrical counterparts of the mass and spring are an inductor and a capacitor, furnishing inertia and restoring force, respectively, for electronic transfer. If by some
Figure 79.—Oscillatory circuit.
electrical force a separation of charge within the ideal (no resistance) closed circuit of figure 79 is made to occur such that some electrons are taken from the lower plate of the capacitor and transferred to the upper plate, a certain amount of energy is stored in the capacitor in the process. On removal of the electric force the energy stored in the capacitor is free to transfer the electrons back to the lower plate. As the electrons in question are released, their flow through the inductor sets up a magnetic field about it, and this magnetic field, once es
64
TM 11-455
RADIO FUNDAMENTALS	44
tablished, tends to prevent any decrease in the flow of electrons which might be expected after the original charge distribution is reestablished. As a matter of fact, the energy in the electrostatic field of the capacitor is transferred to the magnetic field of the inductor with the flow of charge; and at the instant of resumption of the original charge distribution the total energy of the circuit is associated with the magnetic field. The energy in the magnetic field is now available to transfer even more electrons from the upper plate to the lower, until the energy of the magnetic field is entirely diverted back to the electrostatic field. These energy relations are similar to those in the mechanical example. In the latter situation energy originally stored in the spring is released on removal of the compressing force with an ensuing transfer of energy from potential form in the spring to kinetic form in the mass until, at the instant the spring is expanded to its normal length, the energy—except for heat losses—is completely associated with the motion of the mass. This energy of motion carries the mass past its equilibrium position; and when the mass finally comes to momentary rest at the end of its swing, the energy of its motion has entirely disappeared, and energy is now present as potential energy in the extended spring, ready to send the poised mass in the opposite direction toward its equilibrium position again. At this point in the electrical case the capacitor is recharged exactly to its original magnitude but in opposite polarity, and the discharge proceeds in the opposite direction. The rate of charge and discharge which follows can be controlled by varying the capacitance or inductance, or both, just as the spring vibration frequency is controlled by varying the spring tension and/or the mass. The alternate charge and discharge of the capacitor does not continue indefinitely in an actual circuit, but damps out after a brief interval in the same manner as does the spring-mass combination, and for the same reason, that is, resistance. If the friction between the spring-mass and the table is reduced, perhaps by using a glass table top, the duration of the oscillatory motion is prolonged. If all the friction in the system could conceivably be removed, the oscillations should continue indefinitely. In the electrical circuit resistance develops from the collision of the electrons of the current stream with the constituent entities of the conductor traversed. The energy shared in this manner is ultimately all converted into heat, manifesting itself by a rise in temperature of the conductor and of the surroundings, and being lost for all practical purposes. If the inherent resistance of the oscillatory circuit could be reduced to a small magnitude, just the small amount of energy necessary to replenish
476847 0 - 42 -5
65
TM 11-455
44-46
SIGNAL CORPS
that lost in the form of heat on each cycle could probably be delivered periodically in escapement wheel fashion from an external source to sustain the oscillations in the LC circuit indefinitely. This is precisely what occurs in a vacuum tube oscillator. The tube and the associated circuit equipment serve as an escapement mechanism to trigger off energy from the power supply at appropriate intervals.
45. Simple oscillator circuit.—A simple scheme to achieve this end is illustrated in figure 80. The voltage across the capacitor C of the oscillatory circuit is applied to the grid of a vacuum tube so that variations in the vacuum tube output current correspond exactly with variations of the capacitor potential. This circuit is exactly the same as that employed previously to obtain regenerative amplification of an impressed signal whose frequency was that of the natural frequency of the tuned circuit (/= 7=)
X 2?ryLC/ In the regenerative case the feedback was definitely restricted so
§7 T7 ~l
FEED-BACK WINDING,
11.-2,660
Figure 80.—Simple vacuum tube oscillator circuit.
that the presence of the input signal was essential; that is, the signal served to control the frequency, and amplification followed accordingly. Here the feedback voltage is sufficiently large that the signal voltage is unnecessary (once the action is started) ; and sustained currents are obtained at a frequency w’hich is controlled only by the natural frequency of the tuned LC circuit.
46. Practical oscillator circuit.—a. Intermittent feedback impulses as supplied by a class C amplifier are quite adequate for sustaining the oscillations and are more economical of power than is the continuous feedback obtained with a class A amplifier, since with the former there is a smaller proportion of power loss within the tube itself. An arrangement to provide intermittent feedback impulses is shown in figure 81. Here the grid is biased by the voltage developed across the resistor R in accordance with the grid current, which in turn is determined by the magnitude (not frequency) of the potential of the capacitor. (See par. 37.) After oscillations are once established, a fixed battery bias to maintain the class C operation would serve. However, to permit self-starting of
66
TM 11-455
46
RADIO FUNDAMENTALS
the oscillations, the bias must be such that some plate current flows initially, since it is the first pulse of plate current that contributes the necessary pulse of potential across the LC circuit to set off the oscillatory action. Grid leak bias regulates itself to the requirements ideally. Figure 82 depicts the manner in which grid potential
R
m A—
Tc | i
FEED-BACK WINDING,
TL-2681
Figure 81.—Simple oscillator with grid leak bias.
grid current /?, and plate current 4 vary as oscillations are initially built up. The persistence of oscillations in thd LC resonant circuit with only intermittent pulses being released by the tube is commonly referred to as the circuit -flywheel effect.
b. During the build-up of the oscillations, as the amplitudes of the oscillatory current and of the grid and plate currents increase,
- i	i?
/-Ip	z------------------s
/ MiUlU
/ i z_________\
yX n ____________________________v III IIIII 111
- +
c------ TL-26S2
Figure 82.—Build-up of oscillations in grid leak bias oscillator.
the accompanying losses increase (and in proportion to current squared). If the direct current source is able to furnish only a limited amount of power, the magnitude of the oscillations is determined by the power available. Otherwise the amplitude of the
67
TM 11-455
46-47	SIGNAL CORPS
oscillations increases until the operation extends to the knee of the tube’s ip-eu characteristic, where the transconductance falls off and with it the output of the amplifier, too. This results in a decreased feedback and so a decreased grid swing, with a consequent increase in the transconductance which affects the peaks of the swing. In this way any tendency for the oscillations to assume a magnitude above or below a certain critical value is counteracted with an opposing effect by the instantaneous transconductance, which acts to keep the level of the oscillations uniform.
c. The frequency of oscillation is given to a good approximation by
7 27r-y LC
where Rp is the internal plate to cathode resistance of the tube; and R is the resistance of the LC circuit, including that resistance which is effectively introduced into the tank circuit when a load is coupled into the tank circuit to draw power from it. Variations in Rp occur with any slight changes in the vacuum tube electrode potentials. R/Rp is usually very much less than 1, so these changes produce only small, probably one part in 10,000, shifts in frequency. Nevertheless, demands on frequency stability are sufficiently exacting to warrant such design as will minimize the effects of plate resistance variation. The equation above suggests the use of the smallest possible value of R and the largest possible value of Rp which are consistent with other factors for good operation. A low R is the result of a low inherent resistance in the tank circuit together with a small load. The load might be the input to a sufficiently biased intermediate amplifier, but it should not be a radiating system, for example. For a particular frequency (which fixes the product LC) and for a given Q, the value of R can be reduced and stability encouraged by using a small L (low L to C ratio); the smaller the inductance, the smaller the dimensions of the coil, and the lower the inherent resistance. Further, a large C in itself is an effective aid to stability because small variations in capacitance due to mechanical vibration or to the presence of external bodies (hand capacitance) produce only a low over-all percentage change in the capacitance of the resonant circuit.
47. Oscillator circuits in general use.—a. A number of variations of oscillator circuit design are shown in figure 83. All of them are fundamentally alike, differing principally in the disposition and in the manner of coupling of elements. The feedback in the tuned-plate tuned-grid circuits is through the plate-to-grid capacitance within the tube.
68
TM 11-455
47
RADIO FUNDAMENTALS
b. An oscillator circuit is usually required merely to control the frequency and not to deliver any appreciable amounts of power. Power is developed by amplification in the succeeding circuits, where load changes have a much smaller effect on frequency. The electron coupled oscillator combines the functions of oscillator and power amplifier with one tube. The cathode, control grid, and screen grid of the tube serve as a triode oscillator. The coupling between the oscillator and
—own	mF
| Y m - L.i.K
£--------L|.|4--- I 
TUNED PLATE-UNTUNED GRID	HARTLEY
o Q—r-
i 7____L | I i
j o U________________________#hll
COLPITTS	TUNED PLATE-TUNED GRID
^OrTTI ............. (gW~
i------*	=- z |	7"
..	..— ■ -JWWV_|-	CRYSTAL
__||_ (TUNED PLATE-TUNED GRID)
ELECTRON COUPLED HARTLEY	tl-2683
Figure 83.—Basic oscillator circuits.
amplifier circuits is through the electron stream. Capacitive coupling is reduced to a minimum by operating the screen at ground potential as far as r. f. currents are concerned. This effectively isolates the output from the input circuit, so that the frequency of oscillation is relatively independent of load variations. Further, an increase of plate potential causes a frequency shift in one direction, whereas an increase in screen potential causes a frequency shift in the opposite direction.
69
TM 11-455
47-48
SIGNAL CORPS
By properly adjusting the screen tap on the voltage divider, the frequency may be made independent of any variations in the plate supply.
c. The crystal oscillator provides a remarkably steady frequency output. The oscillator proper is a crystal of quartz, which exhibits the property of developing an electrical potential across its faces when mechanically strained, and vice versa, expanding or contracting on the application of a potential. At the natural period of the mechanical vibrations of the crystal the two actions may be made mutually self-sustaining by feeding back a sufficient portion of the amplified potential to replenish the energy which is dissipated during each cycle as heat. The equivalent electrical circuit of the crystal, shown in figure 84, has a very high Q and a very high L to C ratio. in figure 84 represents the capacitance which exists between the mounting
O--------------------p-----------------
L jf
C	—C,
R I
o--------------------1-----------------
TL-Z68+
Figure 84.—Equivalent electrical circuit of oscillating crystal.
electrodes. Z, Z, and R represent the electrical equivalents associated with the vibrational characteristics of the crystal. At frequencies above that which corresponds to series resonance in LCR^ LCR behaves as an inductance. This inductance and C\ form a parallel tuned circuit, the antiresonant frequency of which is the frequency of the sustained oscillations. Since C is in general very much smaller than Cr, the series resonant frequency and the antiresonant frequency of the crystal lie very close to each other.
48. Oscillators for very high frequencies.—a. Resonant circuits for very high frequency oscillators sometimes take the form of short wires joining cathode and plate, with the necessary capacitance being furnished by that existing between electrodes within the tube itself. In other instances a pair of parallel wires (transmission line), short-circuited at the far end, is employed as a resonant circuit. A quarter-wave-length line (par. 85c) exhibits the properties of a high Q parallel resonant circuit.
1). At the ultrahigh frequencies, 50 megacycles and above, mechanical problems are encountered in the reduced sizes of the tube and cir-
70
TM 11-455
48-49
RADIO FUNDAMENTALS
cuit elements, while electrical difficulties arise in the form of frequency instability and decreased efficiency. Several forms of electron oscillator have been developed to replace the conventional vacuum tube at the ultrahigh frequencies.
Section VII
CONTINUOUS WAVE TRANSMITTERS
Paragraph
Oscillator-amplifier transmitter--------------------------------------------- 49
Neutralization_______________________________________________________________ 50
Parasitic oscillations_______________________________________________________ 51
Keying systems-----------_------------------------------------------------- 52
Frequency doublers----------------------------------------------------------- 53
49. Oscillator-amplifier transmitter.—a. A simple oscillatoramplifier combination for transmitting is shown in figure 85. The r. f. choke RFC\ and the capacitor C\ act as a filter to by pass radio frequency current around the oscillator plate supply. Because of the
osc.	amp.
—--a
£	Cl|li |RFC3 ______________Ci___Bli
RFC,	BIAS BATTERY	RFCZ
— —	Tl-2685
Figure 85.—Oscillator-amplifier transmitter.
low r. f. impedance of CA there is no appreciable r. f. voltage drop across this capacitor, and the lower end of the oscillating tank circuit is practically at ground potential as regards r. f. voltage. Capacitor C3 blocks the oscillator d. c. plate voltage from the grid of the amplifier, while at the same time offering a low impedance path foi radio frequency current. The r. f. choke, RFC3^ in series with the amplifier bias battery, is necessary to maintain a high impressed r. f. voltage on the amplifier grid. Capacitor C2 and r. f. choke coil RFC2 serve to direct r. f. currents around the amplifier plate supply.
b. If the amplifier is to be coupled into an antenna, the arrangement of figure 86® is preferable to that of figure 86®. In either case energy can be coupled through the inherent capacitance existing between the two coils. Such energy transfer must include some harmonic component because of the low impedance offered to high frequencies by this capacitance. However, in ® the electric field
71
TM 11-455
49-50
SIGNAL CORPS
across the inherent capacitance (indicated by dotted lines) is negligible, since the lower end of the amplifier tank coil is approximately at ground potential by virtue of C2. As a consequence the coupling in © is almost entirely magnetic, and harmonic transfer is held to a minimum; whereas in (2) the coupling has a greater capacitive component, hence a greater harmonic transfer.
50. Neutralization.—a. The radio frequency plate and grid circuits of the amplifier of figure 85 form a tuned plate-tuned grid oscillator; and unless some action is taken to prevent it, the amplifier will self-oscillate. One function of the amplifier is to isolate the oscillator from the ultimate load, the radiating system, in the interest of stability. An oscillating amplifier fails to serve this end. Only
AMR	AMR
° TO ANTENNA
Ct-- ’LL—>.	Cx--
_	jo TO ANTENNA
(T)	@	I	Tl-26o6
® Correct method.	® Incorrect method.
Figure 86.—Coupling amplifier tank coil to antenna.
when the amplifier operates as a nonoscillating amplifier is the frequency relatively independent of any variations of its plate load impedance.
b.	A favorite technique for suppressing oscillation within an amplifier consists of neutralization, that is, the introduction of a feedback voltage from the plate to the grid circuit which is equal in magnitude and opposite in phase to that which occurs as a result of the plate-to-grid capacitance within the tube. Figure 87 shows such a neutralizing arrangement adapted to the amplifier of figure 85. In figure 87, the amplifier tube interelectrode capacitance is indicated by dotted lines. Neutralization is accomplished by operating the oscillator normally and removing the plate potential to the amplifier stage. CN is adjusted until a minimum reading is obtained on an r. f. milliammeter coupled to the tank, coil, Z2, of the amplifier stage. Other r. f. indicators such as neon tubes held in the field of the tank coil will indicate neutralization at minimum glow.
72
TM 11-455
50
RADIO FUNDAMENTALS
Under these circumstances, CN and the tube capacitance are such that potential variations coupled through them from the grid circuit into the plate tank circuit are equal and opposite. Then with the d. c. plate voltage applied to the amplifier, feedback from the plate circuit into the grid circuit through the tube capacitance is exactly counterbalanced by that through ; and the amplifier acts in a simple nonoscillatory manner, reproducing in its output circuit only those effects impressed on its input circuit from the oscillator stage ahead.
c.	Cross neutralization of a push-pull amplifier is accomplished by joining the plate of the number 1 tube with the grid of the number 2 tube through a neutralizing capacitor, and the plate of the number 2 tube with the grid of the number 1 tube through another neutralizing capacitor (see fig. 88®). The r. f. voltage
AMP. osc	------
i T —lT I Tc" r In
~-----------• I" 11 KmJ-l--tz_L 	——2
rfc!	r , y______I
----ii^----1
------------------------—•N'Hvmz-1—
rfc2
TL-2687
Figure 87.—Transmitter with neutralized amplifier.
across each neutralizing capacitor counteracts the r. f. voltage across the interelectrode capacitance of the tube to whose grid it is connected. Another method of amplifier neutralization known as the Rice system is shown in figure 88®. This arrangement is similar to that of figure 87 except that the Rice system utilizes a split input circuit in place of a split output circuit.
d.	Tetrodes and pentodes eliminate the problem of neutralization of the interelectrode capacitance of the tube. However, other considerations frequently preclude their use as power amplifiers in transmitters. They are more expensive than triodes, and they require additional screen power to operate. The higher power sensitivity of tetrodes and pentodes means that less driving power is required, but at the same time increased difficulties are encountered with these tubes due to stray coupling effects between output and input circuits. These undesired input effects increase in importance as the normal input signal magnitudes are decreased.
73
TM 11-455
51-52
SIGNAL CORPS
51. Parasitic oscillations.—Circuit conditions in an oscillator or amplifier may be such that secondary oscillations occur at frequencies other than that desired. Such oscillations are appropriately termed parasitic oscillations. The energy required to maintain parasitic oscillations is wasted so far as useful output is concerned. A circuit afflicted with parasitics has low efficiency and frequently operates erratically. Figure 89 shows some of the incidental circuits which may give rise to parasitics in the transmitter of figure 87. The dotted ’•~T|
—Qr - <1 l(J
V----------------I------------ tl-2 6fl6
Cn @
® Push-pull stage neutralization.
® Rice system of neutralization.
Figure 88.—Neutralization circuits.
lines of figure 89© outline a high frequency circuit, and those of @ outline an ultrahigh frequency circuit. That part of the transmitter which constitutes a possible low frequency parasitic circuit is sketched in @. Parasitic oscillations may be suppressed by placing resistors or r. f. chokes at appropriate positions in the circuits, or by slightly modifying the existing values of circuit elements; and by using care in the physical arrangement and wiring of parts.
52. Keying systems.—a. A good keying system should prevent completely the radiation of energy from the antenna when the key is
74
RADIO FUNDAMENTALS
TM 11-455
52
------- Rfc3^	Cn
f	cz ~ i
gRFC,	gRFCz
c	(7)	>
]+	1 +
—n=——°--------------------------!
„ Cz	g rfc2
(2)	r
FT-s---------------——1
( A----------------GftTOpW
c4 J p	—I—,
L'l	I"03	r	k
i	1	ClT	II	1
/t—--------------------------U J ■
/ ■*'
POWER SUPPLY FILTER ____/
CAPACITANCES
(3)	tl-2689
® High frequency.
® Ultraliigh frequency.
@ Low frequency.
Figure 8!».—Parasitic oscillatory circuits in transmitter of figure 87.
75
TM 11-455
52
SIGNAL CORPS
open, and it should cause full power output when the key is closed. It should perform these operations without causing keying transients, or clicks, which cause interference with other stations.
b. For various reasons some energy may get through to the antenna during keying spaces. The effect is then as though the dots and dashes were simply louder portions of a continuous carrier. The backwave, or signal heard during the keying spaces, may appear almost as loud as the keyed signal; under these conditions the keying is hard to read. A pronounced backwave often results when the amplifier stage feeding the antenna is keyed. It may be present because of incomplete neutralization of the final stage, allowing some energy to get to the antenna through the grid-to-plate capacitance of the tube, or because of magnetic pickup between the antenna coupling coils and one of the low power stages. Such a condition can often
RFC	/	<0
----------'Tw-------------/ •--------------------------- --------------------------
100 VOLTS	-------------- IZZ 1.000 VOLTS
KF vH 100.000$ OHMS>
-----------------------1--------- H-2691
Figure 91.	—Blocked-grid keying of amplifier.
d. The grid circuit of an amplifier is generally chosen for keying because of the relatively small currents therein. Figure 91 illustrates blocked-grid keying. With the key up, two-thirds of 1,000 volts, or 667 volts, is across the 200,000 ohm resistor; that is, 667 volts is applied to the plate; and one-third of 1,000 volts, or 333 volts, is across the 100,000 ohm resistor, so that 333 + 100=433 volts negative bias is applied to the grid. No plate current can flow under these conditions. With the key down and short-circuiting the 100,000-ohm resistor, the full 1,000-volt plate supply potential appears across plate to cathode, while the grid bias is reduced to 100 volts, under which conditions the amplifier operates normally.
53.	Frequency doublers.—Self-excited oscillators for transmitters offer the advantage of flexibility of adjustment to various frequencies. Crystal controlled transmitters, on the other hand, operate only on fixed frequencies as determined by the crystals available.
77
TM 11-455
53
SIGNAL CORPS
Crystal controlled transmitters are widely used, however, because of their excellent stability. Oscillating crystals designed for very high frequencies are quite thin and fragile, and low radio-frequency crystals are generally employed in conjunction with frequency doublers. A frequency doubler is an amplifier which is so constructed and operated as to yield an output current which is of twice the frequency of the input voltage. An ordinary distorting amplifier with its plate circuit tuned to the second harmonic of the input frequency is often employed as a doubler. More efficient doubling, without employing tube distortion characteristics, is obtained from a double ended push-push amplifier. The two tubes in a push-push amplifier have their grids connected in push-pull and their plates connected in parallel as in figure 92.
MW?
--7	TL-2692
Figure 92.	—Push-push amplifier.
Thus the tubes work alternately, and the output circuit receives two impulses in the same direction for each r. f. cycle at the grid circuit, giving all second harmonic or double frequency output in the plate circuit. Figure 93® shows plate current pulses of a push-push amplifier with tubes operated class C. The missing half cycles are supplied by the tank circuit to produce a continuous second harmonic output as in figure 93®.
a- a	।	' T
GR.ID CIRCUIT CYCLE	!
©	Lu v v v V u
OUTPUT	TL-2693
CIRCUIT FREQUENCY
@
© Plate current pulses.
© Output current for push-push amplifier with tubes operated class C.
Figure 93.
78
TM 11-455
54
RADIO FUNDAMENTALS
Section VIII
MODULATED TRANSMITTERS
Paragraph
Amplitude modulation--------------------------------------------------- 54
Degree of modulation--------------------------------------------------- 55
Power relations in modulated transmitter------------------------------- 56
Modulation methods----------------------------------------------------- 57
Radiotelephone transmitter--------------------------------------------- 58
High and low level modulation—x---------------------------------------- 59
Reduction of normal carrier power for	phone operation------------------ 60
54.	Amplitude modulation.—Modulation is the variation of a radio wave at audio frequencies. Frequency modulation, variation of the frequency of the radiated wave, is discussed in section XII. Amplitude modulation, variation of the amplitude of the radiated signal, is accomplished by introducing both an audio frequency signal, say of 500 cycles, and a radio frequency signal, say of 1,000,000 cycles, into a nonlinear amplifier. The resultant sum and difference fre-
1 SECOND----------------------------*
TL-2694
____ 10 CYCLES	PER SECOND CORRESPONDING	TO	CARRIER
....q CYCLES	PER SECOND CORRESPONDING	TO	LOWER SIDE	FREQUENCY ....i2 CYCLES	PER SECOND CORRESPONDING	TO	UPPER SIDE	FREQUENCY ....10 CYCLES	PER SECOND CORRESPONDING	TO	MODULATED WAVE
Figure 94.—Modulated wave showing “carrier” and “side bands.”
quencies, 1,000,500 and 999,500, which convey the intelligence, are known as the side bands. The 1,000,000 cycle component is known as the carrier. Figure 94 illustrates a modulated wave as a sum of carrier and two side band frequencies. For simplicity of illustration the frequencies chosen in the figure are very low; however, the effects are similar to those in a modulated radio wave. An actual telephone transmitter side band consists of not one single frequency but contains as many frequencies as are present in the modulating signal, so that for reasonably good quality voice operation, side bands
79
TM 11-455
54-56
SIGNAL CORPS
occupy at least 3,000 cycles of the frequency spectrum either side of the carrier. Broadcast stations require side bands up to 10,000 cycles wide to convey musical programs properly. Those tuned circuits, both in transmitters and receivers, which accommodate modulated r. f. currents must be sufficiently broad to give good response to all the side band components.
55.	Degree of modulation.—The degree of modulation is expressed by the percentage of the maximum amplitude deviation from the normal value of the r. f. carrier. If the positive peak r. f. current reaches twice the normal value carrier current, the negative peaks being zero, the wave is said to be modulated 100 percent (fig. 95). The effect of a modulated wave as measured by receiver response is proportional to the degree of modulation. A 10-watt carrier modulated 100 percent is about as effective as a 40-watt carrier modulated 50 percent.
u ”	v v u	VU«
TL-2695
Figure 95.—100 percent modulation.
56.	Power relations in modulated transmitter.—The amount of power required to modulate a transmitter depends on the percentage and type of modulation.. To modulate a carrier 100 percent with a single sine wave of audio frequency requires an audio power equal to one-half of the r. f. carrier power. This is because with 100 percent modulation the amplitude of each side band is one-half the amplitude of the carrier. Power is proportional to current squared; thus each side band carrying one-half the current of the carrier requires one-fourth the power. However, the power required under modulation is one and one-half times the normal unmodulated power. With voice modulation the greater portion of the audio frequency components will not modulate the carrier 100 percent, so that the power increase for voice modulation is considerably less than for single tone modulation. Since the power is increased during modulation, the reading of an antenna ammeter rises when the transmitter is modulated. One of the operating tests of a modulated transmitter is to whistle into the microphone and watch for an increase in the
80
TM 11-455
56-58
RADIO FUNDAMENTALS
antenna current. For 100 percent modulation with a single sine wave the antenna current increases approximately 22 percent.
57.	Modulation methods.—Various methods of modulating a transmitter are in use. The audio frequency modulating voltage can be applied to the plate of one of the transmitter amplifiers to cause the output to vary in accordance with the audio frequency. This is known as plate modulation. Application of the audio frequency voltage to the control grid is referred to as grid, or as grid bias, modulation. A pentode power amplifier can be modulated by applying the audio frequency to the suppressor grid. This is known as suppressor modulation. The screen grid can also be modulated in
OSC________ REAMR________	__
j aKTOAHTEHHA
HH—M—11— ’	[_]—I ---*
[ft[ I
||OFt IO"4" lil
—‘H l f t ? y । A 7
MICROPHONE	| 1-1-1-[ J	ft
INPUT TRANS. ' MODULATORS g 4-
T TU-2696	*
Figure 96.	—Radiotelephone transmitter.
a tetrode. Cathode modulation, in which the audio voltage is applied in the cathode circuit, is a combination of plate and grid modulation.
58.	Radiotelephone transmitter.—Figure 96 shows the basic circuits of a complete radiotelephone transmitter. Sound waves impinging on the diaphragm of the microphone alternately compress and release the carbon granules of the microphone button, thereby varying the resistance of the microphone circuit and giving rise to a voice frequency pulsating current in the input transformer primary. The potentiometer R across the secondary of this transformer is a volume control to regulate the amplitude of the modulating signal. The grid-to-cathode resistance of the modulator tubes
476847 0-42-6
81
TM 11-455
58-59
SIGNAL CORPS
varies with the intensity of the signal, and resistors are connected across the inputs of these lubes to keep the net load impedance on the first speech amplifier tube nearly constant. These shunting resistors are much lower than the grid-to-cathode resistances, so that changes in the grid-to-cathode resistances have only a slight over-all effect on the load to the preceding circuit. The modulator output varies the plate voltage applied to the amplifier stage of the transmitter to produce corresponding variations in the radiated energy (fig. 97). The input speech amplifier stage can be converted into an audio frequency oscillator by providing a switch which, when closed, will
=p jt	IL-2697
Figure 97.	—Production of a modulated wave.
couple enough of the plate circuit energy through a capacitor back to the grid circuit to cause sustained oscillations to take place. In this way a combination continuous wave, tone, and voice transmitter results.
59.	High and low level modulation.—When the final r. f. stage of a transmitter is modulated, the modulation is described as high level, since the modulation takes place at the highest power level in the system. If the modulation takes place in an intermediate stage with a higher power amplifier or several such stages following, it is called low level. In low level modulation, amplifiers which are used to increase the power output from the modulated stage are operated as linear amplifiers, that is, in such a manner that their a. c. output potentials faithfully reproduce the applied grid potentials. These
82
TM 11-455
59-63
RADIO FUNDAMENTALS
62
63
64
65
amplifiers are operated class B, and include r. f. load resistors similar to those across the inputs of the modulators of figure 96 to prevent such distortion as might otherwise accompany the change of load impedance to the preceding stage with changes in the amplitude of the modulated signal.
60.	Reduction of normal carrier power for phone operation.—A modulated amplifier must handle peak currents which are twice the normal unmodulated magnitude. This means that during modulation an amplifier must be capable of handling up to four times the power it dissipates under steady intervals of unmodulated carrier output. For this reason in a transmitter which is designed for both continuous wave and phone service, the modulated amplifier stages are always reduced in carrier power output for phone operation.
Section IX
VACUUM TUBE DETECTORS
Paragraph Detection---------------------------------------------------
C. w. detection---------------------------------------------
Phone detection---------------------------------------------
Plate and grid detection------------------------------------
Comparison of detection methods-----------------------------
61.	Detection.—The conversion of r. f. energy in a receiver into audible sound frequencies to convey intelligence is accomplished by a process called detection or demodulation.
62.	C. w. detection.—For the reception of continuous wave (c. w.) signals, the output of a local oscillator, say 1,001,000 cycles, is combined with the incoming signal, say 1,000,000 cycles, and applied to a class B amplifier. The local oscillator is usually the amplifier itself, in which case the amplifier is called a heterodyne detector (fig. 98). The output of the detector contains current of the sum frequency, 2,001,000 cycles, and of the difference frequency, 1,000 cycles. The former, the r. f. component, serves no useful purpose and is bypassed to ground. The latter, the a. f. component, actuates the earphones or loudspeaker, usually through an audio amplifier.
63.	Phone detection.—For phone or tone reception a separate oscillator is not necessary, since the side bands differ from the carrier by the wanted voice frequencies. Here a simple diode rectifier, as in figure 99(T), is adequate. An analysis of the •equivalent circuit of figure 99© shows that the load resistance should be large (in comparison with the plate-to-cathode resistance) in order to develop a large a. f. output voltage across it. In practice the load resistance is
83
TM 11-455
63-64
SIGNAL CORPS
generally made from 20 to 100 times the internal resistance of the tube. Diode rectifier action is shown in figure 46.
wwvwwwww
LOCAL R.F.
to zp	AT OUTPUT
t
>	RESULTANT R. E
AVERAGE-k , ( 4 I I	, ,1,11.
PL'”‘
PLATE CURRENT
O --------------------
Figure 98.—Basic heterodyne detector circuit, and (right) how the local and incoming r. f. signals combine. The local, incoming, and resultant r. f. currents are actually alternating in nature, but the plate current of the detector tube remains direct current which is changed at the average rate, as the lowermost curve shows, to produce an audio frequency signal. For this reason, the “average plate current” curve is shown above the plate current zero line.
64.	Plate and grid detection.—A single tube combining both amplification and detection is sometimes employed to obtain increased output at the sacrifice of quality. In the circuit of figure 100 (T) the
PLATE TO CATHODE CAPACITANCE
f j	\J_	A RECTIFIER
______r		T 1 PLATE TO Z)to------------------—S	±	> CATHODE
KF.INPUT gg -L	f RESISTANCE
—3|	t rv t r;
----------1 | A.F OUTPUT	=T 1	A F-OUTPUT
0	0	Tl-2699
® Diode detector.	@ Equivalent circuit.
Figure 99.
tube operates at the heel of the ip-ep characteristic, as determined by the grid bias and by the plate potential, and amplification occurs before demodulation. The process is referred to as plate detection. In
84
TM 11—455
64—65
RADIO FUNDAMENTALS
the circuit of figure 100® the tube operates at the heel of the i^-eg characteristic, as determined by the grid leak bias and by the plate potential, and demodulation occurs principally first (in the grid circuit) with amplification following. This process is called grid leak, or simply grid detection.
fl J SIGNAL
„ /	.VOLTAGE
v T r—a	RFC
I	_L |
\l V Z-------------------^LATE CURRENT
R.MMPUT ||	+	'
________J C_________ T 5________________111 I , L_f	AVERAGE ' III	PLATE CURRENT
---II---
(T)	A.F BYPASS
signal voltage	i—II—। Z\ A
SAME AS TIG. (a)	IJLI (....
I---	I __________(PLATE CURRENT
._________ J	o) Q	*"	X A fl A
_L	5=	3 e A.F. OUTPUT r fl li Il A
RF.NPUT 3 g	T	I P	r	VtMlftf
______________J U-----------------11 I i 11 V lJ-----------------------------------------™°°	“ 'll1 v'
___11___ average' "_______PLATE CURRENT
® Plate detector.
© Grid detector.
Figure 100.
65. Comparison of detection methods.—Both diode and grid leak detectors draw current from the tuned circuit, thereby lowering both the selectivity and the gain of the tuned circuit. A properly designed diode detector gives less distortion than either of the other types; and usually the plate detector gives less distortion than the grid leak type. Grid leak detection was formerly used extensively for the detection of weak signals, but the development of improved radio frequency amplifiers has made the grid leak detector almost obsolete. The d. c. component produced in the diode detector is frequently used for effecting automatic volume control.
85
SIGNAL CORPS
TM 11-455 6G
Section X
RECEIVERS
Paragraph
Tuned r. f. receiver_________________________________________________ 66
Volume control_______________________________________________________ 67
Circuit of tuned r. f. receiver-------------------------------------- 68
Superheterodyne receiver_____________________________________________ 69
Pentagrid mixer_________________________________________■_________ 70
Converter____________________________________________________________ 71
Circuit of superheterodyne receiver---------------------------------- 72
Crystal filter------------------------------------------------------- 73
Multiband receivers__________________________________________________ 74
66. Tuned r. f. receiver.—a. Figure 101 shows the schematic circuit of a tuned radio frequency (t. r. f.) receiver. This receiver has one stage of tuned radio frequency amplification, a plate detector, and one stage of audio frequency amplification. The two resonant circuits are tuned by a ganged variable capacitor which has all rotor
_GANGED TUNING _
Xx	/r.EAMP	/ DET.	afamp
trimmer/ (*•***■) I	, \	HTTCTP—|	I LOUD
/ I	II 7 ©SX	II	b SPEAKER
t JiiT I®
-J—I—li_—11——Hi ||—
RF BY-PASS7	'BIAS RESISTOR	"
AND CAPACITOR.	t
’ I* I'।	A.F. BY-PASS7	T L-2701
Figure 101.—Tuned r. f. receiver.
plates connected to the same shaft so that both circuits can be adjusted with a single dial. Small trimmer capacitors connected in parallel with the main capacitors compensate for inequalitiepin circuit constants. These trimmers are. usually screw driver or socket wrench controlled from the rear or bottom of the receiver chassis.
b. The trimmer adjustment should be made at the high frequency end of the band, that is, with the main capacitor plates out of mesh. If this adjustment were attempted at the low frequency end of the band with the capacitor plates in mesh, it would take a large change in the trimmer capacitance to cause any noticeable change in the total capacitance. On the other hand, with the tuning capacitors out of mesh and presenting only a small capacitance, a minute change in the trimmer capacitance represents an appreciable change in the total capacitance, and it is possible in this way to get a critical adjustment.
86
TM 11-455
66—67
RADIO FUNDAMENTALS
c. To compensate for slight inequalities of the two tuning capacitor sections, the end plates are sometimes slit in such a way that just a portion of these plates can be bent slightly. This provides a means of alining the two circuits at various settings of the tuning capacitors so that the circuits track over the entire band.
67. Volume control.—a. Volume controls can be inserted in almost any circuit. Some receivers employ volume controls in more than one circuit. A popular method of volume control is the use of a variable resistor in a cathode return circuit to regulate the bias on a variable ju, tube. Automatic volume control, abbreviated a. v. c., may be had by taking the bias for one or more r. f. variable p tubes partly from the rectified input signal. In this way the amplification given to a particular tube can be decreased in accordance with the strength of the signal to provide a fairly uniform response at the expense of a certain amount of sensitivity. Automatic volume con-
XL-2702
rotor plates
R.F. INPUT g + 'X.NOT GROUNDED
I----------<^\AAA'dJ-----—
------w/L --------------II-------*-A.F. OUTPUT
TO GRIDS OF f d-JpiLTER
VARIABLE T FILTER
TUBES—1 A~
Figuke 102.—Automatic volume control circuit.
trol is particularly desirable for receivers in mobile craft wherein the signal strength changes as the vehicle is maneuvered. Figure 102 shows a detector diode with an a. v. c. circuit.
b. The variable p tube is designed to operate with a minimum bias of about 3 volts. This minimum bias is usually provided by a cathode resistor, and the a. v. c. bias is in series with it. A disadvantage of ordinary a. v. c. is that with it even the weakest signal reduces the amplification slightly. An adaptation which avoids this is shown in figure 103 and is referred to as delayed automatic volume control (d. a. v. c.). In this particular circuit the a. v. c. diode is separate from the detector diode, and both are housed in the same vacuum tube along with a pentode amplifier. The tube is called a duplex-diode pentode. Part of the energy which is fed to the plate of the detector diode is coupled to the a. v. c. diode section by the small capacitor C. By means of a cathode biasing resistor R the plate of the a. v. c. diode is maintained at a negative voltage which keeps it from rectifying and
87
TM 11-455
67
SIGNAL CORPS
producing the a. v. c. voltage until the peak voltage coupled to it through C counterbalances this diode’s negative voltage. For very weak signals, which do not produce enough voltage on the plate of the
R.F OR IF ->--? ■	--- \	----[ /	| .
INPUT TO	I
PENTODE SECTION J	----
H	—n—
+ * v .
A VC. _________ A A A A	_____________________.A.E OUTPUT
VOLTAGE	f J	----j +FROM DETECTOR
FILTER |r J_ |
a L_i±
I	Tl-2703
Figure 103.—Delayed automatic volume control.
a. v. c. diode to overcome the existing negative potential, no a. v. c. voltage is developed, and thus the sensitivity of the receiver remains the same as if a. v. c. were not being used. On the other hand, when normal strength signals are being received, which do not need the set’s
Triode with plate and
1st R.FSTAGE 2nd RESTAGE	3RD RF STAGE GRID CONNECTED TO FORM
-------------,-----------------------■’A DIODE DETECTOR.	TI.X7O,
V : ----------- / ---------- < ---------- ' —p-q	r—I—,
’ZtN ZvX	iZ-LN	Z-V' AFSTAGE
i,'	J I	J '	Jl	>	. ।_	LOUD
I	Sf? I 1 _>!? 1	St? I	<	11 I S SPEAKER
f} t if} I if} I if} r r n
| -----------------------------11--
----------- j •"	' pi-
AUTOMATIC VOLUME CONTROL	TO AU HEATERS '' |__ I'l'l
S.__.____	*--------1	"B" SUPPLY
manual volume control	1-------------
Figure 104.—T. r. f. receiver with automatic volume control.
maximum sensitivity, enough voltage will be coupled to the a. v. c. diode to overcome the small negative plate potential and produce an a. v. c. voltage drop across resistor Rx. This voltage has the a. f. and r. f.
88
TM 11-455
67-68
RADIO FUNDAMENTALS
components filtered from it and is applied to the grids of the variable H tubes just as in the case of the ordinary a. v. c.
68. Circuit of tuned r. f. receiver.—Figure 104 shows the complete circuit of a tuned radio frequency (t. r. f.) receiver employing
triode with plate and
1ST RF STAGE	2ND R E STAGE	3RDRFSTAGE grid CON N ECTEO To FORM
rn	-------------,-------------(--------,A diode DETECTOR	11-2.10$
''	: r--	/ I---	i--- r+~i----W	I---r~i
'z+x	>'/+x	'z+x	ZTX	/+t A.E3TAGE
J /	J I	J /	J /	LOUD
kfc> '	k|o '	k|o '	k[o	> II	k SPEAKER
0 tiIf w t £ t ■ n ta
*, hJjfL	if rrll—LrlrjI--------------Ll—L_1|7
m------------ii---------j—1---------U
I	—II—
-----------P®	)	j ---------------- p r-
AUTOMATIC VOLUME CONTROL	TO ALL HEATERS ।
X,	*-----1	"B" SUPPLY
manual volume control	'-------------
Figure 105.—T. r. f. receiver. Ground potential elements denoted by heavy lines.
three r. f. pentode stages with automatic volume control on the first two stages only. A switch is provided for short-circuiting the a. v. c. when it is desired to use the manual control (detector output po-
Triode with plate and
tSTRF STAGE 2ND RF STAGE	3RDRFSTAGE grid CON N ECTED TO FORM
V	,------------,----------------------,A DIODE DETECTOR.	TL-JTOb
I .	/ ,	! I ■	r/—r—r 1—I
'/AY	>'/X\	//■'Xx	l/TX	Z^tx^r A.F STAGE
{/	J '	1	J >	{ ._,, I r LOUD
'	Yf? '	1 -Se '	be '	<	'J I S SPEAKER
0 I Ip t ip I ip	r r	n
h Z	h Z	h m	M Z
4b Im I ■1-t.Lt i I------------LdZ I--------LU----I—
-d -----------H-----------1 ----------1
----------- 1	-	—p r
AUTOMATIC VOLUME CONTROL	10 *lL HEATERS ।
X	*-----1	"B" SUPPLY
MANUAL VOLUME CONTROL	--------------
Figure 106.—T. r. f. receiver. Elements at high r. f. potential denoted by heavy lines.
tentiometer R) exclusively. Figures 105 through 109 reproduce the same receiver wiring diagram with various circuits emphasized to facilitate study.
89
TM 11-455
69
SIGNAL CORPS
69. Superheterodyne receiver.—a. The superheterodyne has replaced almost all other types of general purpose receivers at the present time. Figure 110 shows the scheme of a superheterodyne receiver
Triode with plate AND
f ST R E STAGE	2ND R E STAGE	SRDRFSTAGE GRID CONNECTED TO FORM
VT7	....-.......-I.... ,-----------•••••’A DIODE DETECTOR.	Ti-zroj
V ' 1-------- ' I-------	' I-	[7—T—W	1 1-1
"’Z—N	' *F STAG£
-. UU-77 “l .—_lUlLT/—	।—_/LL'-'-’-yl	—_ /IL — )	JUT-V-~r
JI	J '	J I	JI	L____III	LOUD
MIMWF ti.P
•if- I  It It;; 1----------------n. I----UJ------1—L;i_
-T-----------TJ------------------------1 I
i:	Ji	—11—
-----------ii.	„____n r
AUTOMATlC VOLUME CONTROL	TO AU HCATEOJ I’।'' ’ |	~ I'l'l
_______X.__________	*-------1	"B" SUPPLY
MANUAL VOLUME CONTROL	1----------
Figure 107.—T. r. f. receiver. D. c. plate supply shown by heavy lines.
by means of a block diagram. The two novel features of a superheterodyne receiver are the mixer stage and the intermediate frequency (i. f.) amplifiers. The mixer stage is often referred to as the first detector. In reality it is a heterodyne detector which, in
.	triode with plate and
1ST R.E STAGE	2nd RESTAGE	SRDRFSTAGE GRID CONNECTED TO FORM
J	----------1----	--------,------/A DIODE DEFECTOR.	tiztob
rZ'i'x	l/T\	AF STAGE
n> nW nfn n>©r i#
/j,	!	(L !	7 I	Al	' L-I II	LOUD
< JSg •	/)<£ '	W”1 _L -S SPEAKER
lib f ib fib f Ib	f F In I a
IF b l.fj -----------------------slfl I--LJ------L_L||_
U I I I
■’	I	—II—
_____n r-
AUTOMATIC VOLUME CONTROL	TO AU UEATOU I'l'l' [ ' ~~,l'lll
-----S.		*	1-"B" SUPPLY MANUAL VOLUME CONTROL----------------------------I	_______
Figure 108.—T. r. f. receiver. Detector circuit shown in heavy lines.
conjunction with an appropriately tuned local oscillator, converts the r. f. input signal to a lower frequency, still r. f., which for identification is called the intermediate frequency. The second de
90
TM 11-455
69
RADIO FUNDAMENTALS
tector, or demodulator, converts this intermediate frequency energy to audio frequency.
b. The mixer may be operated as a plate detector with the exception that instead of filtering the r. f. from the output, here a band pass filter is used which eliminates all frequencies except the intermediate
triode with plate and
1st RF STAGE_ZjwWi STAW.-3--	___-Td\odTo^c°o^. F0RM
V	__ f  -------- / I------- rj—.—pq	r-—n
I I	l L	, -L	ZTxi A. F STAGE
./On On -*©
/	I ''	J /	J I	i _.._ I	loud
(o '	Id, •	Sfo '	S ' I S । SPEAKER
IMMKtHIKjHIp h nil®
II h IH	It Iji i—Ltlrj Li-------Lil-----1—Ln— ;
-I ---------1-----------1 ---------1
F I	L—II—
---------,L---------------------—	p p-
AUTOMATIC VOLUME CONTROL	T° AU “EAT^_I „ '
|	D QUrrLl
MANUAL VOLUME CONTROL
Figure 103.—T. r. f. receiver. A. v. c. circuit shown in heavy lines.
band which is desired. This band pass filter is actually a transformer with both primary and secondary tuned. The coupling is such as to produce a resonance curve which is flat topped to accommodate a narrow band of frequencies in the manner of the curve of figure 29®. The fixed intermediate frequency permits simplicity of
Y
“ AMPLIFIER “ MIXER — AMpUiF)£R “ DETECTOR ” AMPLIFIER ~<\SP£AKER
OSCILLATOR
TL-2710
Figure 110.—Scheme of superheterodyne receiver.
design to concentrate on optimum selectivity and amplification at this one particular frequency. At the relatively low intermediate frequencies employed, of the order of 500 or 1,000 kilocycles (kc.), stray capacitances are not especially troublesome, and standard pentode tubes give good voltage gain. Tuning the plate circuits allows
91
TM 11-455
69-70
SIGNAL CORPS
the i. f. amplifier tubes to work into high impedance plate loads, another factor contributing to high gain.
c. The local oscillator circuit capacitor is ganged to the tuning capacitors in such a manner as to generate oscillations of a frequency which differs from the signal frequency by an amount equal to the fixed intermediate frequency. Suppose, for example, that the desired incoming signal has a frequency of 1500 kc. and that the i. f. amplifier is tuned tq 465 kc. Then if the local oscillator is adjusted to 1965 kc., the mixer stage will yield (among other frequencies, which are rejected by the tuned circuits) 1965 — 1500=465 kc. Incidentally, a 465 kc. output of the mixer stage also results if a 2430 kc. incoming signal is present at the same time: 2430 — 1965=465 kc. In such a case 2430 kc. is referred to as an image frequency, since it is an image
::	j-24-30
465
'■	‘ 1965
465
1	■■ I5OO
tl-2711
Figure 111.—Illustrating image frequency.
in the frequency spectrum, so to speak, of the 1500 kc. frequency about the 1965 kc. oscillator frequency (fig. 111). The tuned circuits ahead of the mixer stage are tuned to the desired frequency, 1500 in this case, so that the preselector amplifies the desired signal much more than it does the image signal. The ratio of desired signal to image signal at the mixer input is known as the image ratio and in a good superheterodyne receiver may be around 1,000.
70.	Pentagrid mixer.—The plate detector mixer has the disadvantage of effectively coupling a load back through the interelectrode capacitance into the input circuit, so that tuning the input circuit affects the frequency of oscillations. A pentagrid (five-grid) mixer tube connected as in figure 112 isolates the oscillator and mixer circuits. The signal and mixer grids are screened by the two adjacent grids. The fifth grid is a suppressor.
TM 11-455
71-72
RADIO FUNDAMENTALS
71.	Converter.—A single tube performing the functions of both oscillator and mixer is known as a converter. Figure 113 shows the connections to a pentagrid converter. The grid nearest the cathode forms the control grid, and the next grid, the plate of a triode oscillator. The first grid is called the oscillator grid and the second, the anode grid. The signal is applied to the fourth grid, the third
t.r.f. J \ J _ i.f.
AMPLIFIER. S	AMPLIFIER
----------- +-
OSCILLATOR
----------- TL-A7I4
Figure 112.	-—Pentagrid mixer.
and fifth grids forming electrostatic screens. The oscillator “plate” current, which is flowing in pulses, causes electrons to shoot through the openings in the anode grid in spurts at the oscillator frequency. The effective plate current, is similar to that for the pentagrid mixer, in which two grids modulate a continuously emitted space charge. Pentagrid converters offer the advantage of simplicity of tubes and of wiring. However, at higher than broadcast frequencies, that is, for
TRT _______________/	-^1	\_,~L IE _
AMPLIFIER	(-------I	F AMPLIFIER-
oscillator. C--	+
	tl-2713
Figure 113.	—Pentagrid converter.
most communication frequencies, their usefulness is limited by interaction which occurs between the oscillator and signal sections. A detailed circuit of the pentagrid converter is shown in figure 114.
72.	Circuit of superheterodyne receiver.—Figure 115 shows the circuit of a six-tube superheterodyne receiver. This receiver has one stage of t. r. f. preselection, a local triode oscillator, a mixer, two stages of i. f. amplification, a diode detector with delayed a. v. c.,
93
TM 11-455
72-73
SIGNAL CORPS
and a power pentode output stage. A beat frequency oscillator is included for c. w. reception. Particular individual circuits of the receiver are emphasized in figures 116 to 119.
| Z: • W T I ... JU.
RE INPUT‘S !	xru S /	—L '   *F.
_ ;	~7>	~	TRANSFORMER
I —ii—3—	TJ [
OSCILLATOR-*;	~	/W-1	~
L.t_____________/ I	Tl-2714
~ +•
Figure 114.	—Pentagrid converter circuit.
73.	Crystal filter.—a. The characteristics of the quartz crystal make it particularly suitable for use in an i. f. stage of a superheterodyne receiver. The crystal serves two functions, namely, to increase the over-all selectivity of the i. f. amplifier and to permit the
1ST IF-BEAT \	J- ZK
FREQUENCY	T
t-tz	STAGE	hWZI J1	2. ND IF -AVC STAGE _
X TRF STAGE	-X.	AT 5TAGE,
•	--------t--\ MIXER	----h fc-y	r-----q	L jn
\	\ Zr^X	rr "	loud
\	/r._..A-_l \ \f I. k—.	_____J ,(J I  f7l rU.r *i	I speaki
ifiwipp WiW yQTt W>
B t Lt • x -hLfp1 u	\	,	, v	, .  .	___<'—I—> Control
OSCILtATORaSo T T- < I” ’
g T±r W"	"B" SUPPLY
, J-----
TO AU HCATtRS |.|_1	TL-WI*
Figure 115.—Superheterodyne receiver.
rejection of an interfering signal which is very close to the desired signal in frequency. These selectivity and rejectivity features are due to the fact that as the frequency of the exciting voltage is varied, the crystal behaves as a series resonant circuit at one frequency,
94
TM 11-455
73
RADIO FUNDAMENTALS
/r, and as a parallel antiresonant circuit at another frequency, /0; fa is very slightly (usually less than one part in a thousand) higher than fr. A typical crystal circuit in an i. f. stage is shown in figure 120(1).
1ST If- BEAT \	_L
FREQUENCY	ZT‘ gjjo'
SJ7	STAGE_aaAA-t-II_+1 2ND IE-AVC STAGE
___T5£ srAG£..	- T	I	*1 A F. STAGE
\-------------MIXER	f I I	‘ ""I I L AT"!
'	/r‘~‘ \	-\	(\ nV I I I ---Wt-I I (r~--'A LOUD
\ (I fr") | \ \( 1“—tn , * - , c IS r\ *- izUe4"U men ivr'”/ k SPEAKC“
ifiuputTT+ MfjM in®
I’M \ L M j I qLJM ■
I '	, v • z y	, x I / >___, x	L—< v.d»J Control
TMi mm hr—‘) local sjg I' ivry oscillators^r T f p°r
jo _ -dr	*B” SUPPLY
’—।i—l--w^^.-L....	, 1111—
TO All HEATERS |	n.„io
Figure 116.—Superheterodyne receiver. Second detector circuit shown in heavy lines.
&. At the series resonant frequency the crystal acts as a pure resistance, the magnitude of which is very small compared with the load impedance. As a consequence, at this frequency the voltage across
S.
*N0 If-AVC STAGE ,
A/ STAGE
I - ? in	I [--••A louo
—rwFCiJ/l PiCi lxF”“/ L speaker
f± ’■ r± > li t ’ ? j r L IF -
I	J k	» \	/ -	* j l—.	■  Control
—" v Mi	i 1
LOCAL gj|o 1^
OSCILLATORag f T f p-
I g —	—	"B" SUPPLY
— ------1 i—l----wy~-—.—L| < | • (p—j
TO ALL HEATERS ,
J	fU-W.7
Figure 117.	—Superheterodyne receiver. D. a. v. c. circuit shown in heavy lines.
95
TM 11-455
73
SIGNAL CORPS
the filter output (fig. 120®) is very nearly equal to that across A, that is, the output voltage is approximately one half the impressed voltage across the secondary coil ZjZ2. At all frequencies sufficiently
1ST IF - BEAT \	J_
FREQUENCY	gjjo
W	STAGE	r-AAAA-rll-^1	2 ND IF - A VC STAGE
TRF STAGE	I*	-----------—। A F STAGE
------------y mixer	~	1 lv .	 A
' /I \ ’• '/I \	(T "" nk	<	(F-—4 loud
' I I” “Tn d J1 ______________? I k Jrr I   (JI , i  l_k— J I speaker
W M? WhF ■'! ffl C -P
I - Hm’-h? Up1 ■	z s	. » z.  , s z s .	___r vJ—I Control
II	I
LOCAL gfe r"*L
OSCILLATORag £ -p> jrr
I jo “="	■=■	"B" SUPPLY
JLe—: I__q—1---VW	^^-|.|.|»—-^	TL—2718
TO All HCATtRS _J
Figure 118.	—Superheterodyne receiver. Local oscillator shown in heavy lines.
remote from the crystal’s resonant frequencies, the crystal presents a capacitative reactance due to the capacitor formed by the holder plates and the quartz dielectric. This reactance is relatively high as
Ji la?©
1ST I F - BEAT	X 3g
FREQUENCY
T. .................	otf] n
\ /flw	4, ।
t I	.( s	I	(k -jrr I 44-, -f J .-4,—L. 44--- I I speaker
imp Kn W Tr	E ® F
T i -ih P" _|i- ri r"	'-n/rln :z
L • Fl \ 1 ■ h ■ f L fl -I ______________________zx I  zv	--------[—
C----L------------- < JL
TL-27ZI
Figure 121.	—Power supply for receiver or for small transmitter.
output assists the regulation in that it presents a constant load, and any change in the current drawn by the equipment causes a reduced percentage variation in the total load. A potentiometer voltagedivider and bleeder resistor may be one and the same unit.
Section XII
FREQUENCY MODULATION
Paragraph
General------------------------------------------------------------------ 78
Simple modulator and demodulator----------------------------------------- 79
Wide swing frequency modulation------------------------------------------ 89
Practical frequency	modulator------------------------------------------- 81
Frequency modulation reception------------------------------------------- 82
General consideration in frequency modulation---------------------------- 83
Facsimile---------------------------------------------------------------- 84
78.	General.—Frequency modulation is the process of varying the frequency of a radio frequency wave at an audio frequency rate,
99
TM 11—455
78-79
SIGNAL CORPS
without varying its amplitude. The essential difference between frequency modulation (f. m.) and amplitude modulation (a. m.) is shown in figure 122. In this figure, © represents an unmodulated r. f. carrier; © shows the result of amplitude modulating the carrier; and (3) shows the result of frequency modulating the carrier. In @ during the modulation period the amplitude rises and falls in accordance with an impressed audio frequency signal. In @ during the modulation period the frequency increases and decreases in accordance with the audio signal, but the amplitude remains constant.
® Unmodulated r. f. wave.
® Amplitude modulated wave.
® Frequency modulated wave.
TL-2.72 2
Figure 122.—Amplitude and frequency modulation.
79.	Simple modulator and demodulator.—a. A simple form of frequency modulator is that of a condenser microphone shunting an oscillatory circuit, as in figure 123. The diaphragm in the condenser microphone (see par. 103) forms one plate of a capacitor. Sound waves striking the microphone compress and release the diaphragm to vary the capacitance at the voice frequency. Since the microphone capacitance is in parallel with the oscillatory circuit capacitance, the effect is that of changing the frequency of the r. f. oscillations at the voice frequency rate.
100
TM 11-455
79-80
RADIO FUNDAMENTALS
&. Any parallel resonant circuit, such as those in the r. f. stages of an ordinary a. m. receiver, will act as a frequency modulation detector if the operation is such that the unmodulated carrier is tuned in on one side, rather than at the center, of the response curve, as shown in figure 124. The effect of such “detuned” operation is that, as the frequency swings above and below the normal unmodulated carrier frequency, the resulting output varies in amplitude accord-
... rp-er-]
CONDENSER =F °	7-	___
MICROPHONE
............-_______-
tl-1723
Figure 123.—Simple frequency modulation scheme.
ingly. This amplitude modulated signal is rectified in the ordinary manner in the detector circuit.
80. Wide swing frequency modulation.—Frequency modulation presents an appreciable signal-to-noise ratio improvement over amplitude modulation provided the frequency swing with modulation is several times as much as the highest audio frequency transmitted. A 25-kilocycle swing to each side of the unmodulated carrier fre-
I
it FREQUENCY UNMODULATED CARRIER )
TUNED IN AT THIS POINT 7	TL-2724
Figure 124.—Parallel resonant circuit as a frequency detector.
quency is common in communication work; 75-kilocycle swings are usual in broadcast service. To obtain a wide frequency swing with modulation, the oscillator frequency is usually made much lower than the desired carrier frequency, and frequency doublers are employed to multiply the original modulated frequency. If the oscillator output has a normal frequency of 10 megacycles and.is modu
101
TM 11-455
80-81
SIGNAL CORPS
lated to produce a maximum swing of 10 kilocycles, then doubling twice results in a 40-megacycle carrier with a 40-kilocycle swing.
81. Practical frequency modulator.—Both the simple modulator and the simple demodulator described in paragraph 79 are relatively1 inefficient. A practical frequency modulator is shown in figure 125. Modulation is accomplished by variation of the effective reactance of the oscillatory circuit at an audio frequency rate as controlled by
***	J+
g RFC	OSCILLATOR
*—gi	y, I I
AUDIO INPUT	o
«-- I-----J }---------- ^OSCILLATORY CIRCUIT
Ci -j- > R( > Rx	q*c
Is < _____________________ tl-2725
Figure 125.—Frequency modulator circuit.
the microphone sound impulses. C2 is a d. c. blocking capacitor. C-l, Rrj and R2 form a grid bias arrangement for the control grid of the modulator. R2 is a very high resistance of the order of a megohm. The essential elements of the circuit are the small capacitance C across the modulator control grid and the high resistance R, which is in series with C across the oscillator tank circuit. The current through R-C is very nearly in phase with the voltage across R-C (fig. 126), which voltage is that across the oscillatory circuit, and the voltage
ErMR
EeiXc	L
TL-2726
Figure 126.—Voltage relations in K-U circuit of figure 125 (/( very much greater than Xc).
across C is practically 90° behind the voltage across the oscillatory circuit. The r. f. voltage across C (from the oscillatory circuit) is impressed on the control grid of the modulator. Audio voltages from the microphone circuit are impressed on the second grid of the modulator. The resulting audio frequency modulated r. f. plate current responses are in phase with the control grid potential, therefore 90° behind the voltage across the oscillatory circuit. The radio frequency choke in the plate circuit of the modulator offers a large re-
102
TM 11—455
81-82
RADIO FUNDAMENTALS
actance to the r. f. component of the plate current, so that this r. f. component flows through the tuned circuit of the oscillator. The total lagging current due to the voltage across the oscillatory circuit is greater than it would have been if it were not for the modulator circuit. The same effect could have been produced by connecting an inductor in parallel with the oscillatory circuit. Thus the modulator acts as an inductance. As sound waves strike the microphone, the potential of the second modulator grid is varied, and in turn, the modulator plate current. This varies the effective inductance of the oscillatory circuit, which then varies the frequency of the oscillations.
82. Frequency modulation reception.—a. A block diagram of a practical f. m. superheterodyne receiver is shown in figure 127. It will be seen that a receiver for f. m. differs from one for a. m. in two main aspects, the limiter and the frequency discriminator. A further essential difference is the width of the band to which the i. f. amplifier is tuned. For an a. m. receiver it is necessary only that
Y
“AMPLIFIER " MIXER "AMPLIFIER " LIMITER "OISCMMINATOR-AMPLIFIER
--------- H
OSCILLATOR
________ Tl-2727
Figure 127.—Block diagram of f. m. receiver.
the i. f. amplifier pass a range of frequencies 30 kc. wide at the most, corresponding to the separation between the two extreme side band frequencies. In an f. m. receiver the i. f. amplifier must accept a band from 50 to 150 kc. in over-all width depending upon the maximum frequency swing employed in the transmitter. The limiter serves to remove any amplitude modulation of the signal, and the frequency discriminator extracts the audio frequency signal to feed it into an ordinary audio amplifier. The limiter stage of an f. m. receiver is shown in figure 128. The actual circuit is that of an ordinary i. f. stage, but the operating conditions are quite different. The resistor R1 broadens the band pass characteristics of the i. f. transformer Tl. The d. c. voltages applied to the limiter tube are very small, about 15 volts on the plate and 10 volts on the screen. Consequently, even a rather weak signal arriving on the grid saturates the tube, and the maximum possible plate current is reached when the instantaneous voltage on the grid is considerably below the maxi
108
TM 11-455
82
SIGNAL CORPS
mum value of the signal voltage. The result is that as long as the signal is above a certain minimum strength, all traces of amplitude modulation are removed by the limiter action, as shown in figures 129 and 130. Since the original transmission contained no intentional amplitude modulation, any amplitude modulation present in
- t ■ > y c—t---------------©j—	L g*
IE 	o? K	J <—
---2--------7 k----K VW--------< —  1|----
------II------------vXv---"■
__ /Ze—n:
—	/	—	TL-272S
VOLTAGE DIVIDER, FOR SCREEN
Figure 128.—Limiter circuit of an f. m. receiver.
the signal as received is the result of extraneous effects, including static and man-made noise; these unwanted components are removed by the limiter.
b. Figure 131 shows the actual combination of the limiter stage and the frequency discriminator stage of a typical f. m. receiver. The discriminator is the equivalent of the second detector of an ordinary
INPUT VOLTAGE,
I II III Ml fl IIIIIHIII
Tl-2.729
Figure 129.—Input signal to limiter tube, containing both a. in. noise and the desired f. m. signal.
superheterodyne, and performs the similar function of demodulating the carrier. (See sec. XI.) The i. f. transformer 7'2 has a centertapped secondary, £1-£2, feeding the plates of a double diode rectifier. The cathodes of the latter terminate at the ends of a center-tapped resistor 7?2, one end of which is grounded. The direct current return path of the rectifier circuit, between the center tap of xSl—/Sr2 and the
104
TM 11-455
82
RADIO FUNDAMENTALS
center tap of 7?2, is completed through, the inductor L. This coil has another function, to be explained later.
(1)	The operation of the discriminator circuit in recovering the audio modulation from the f. m. signal depends on three factors. First, the center tap of Nl-S2 causes a division of the signal voltage across the tuned circuit xS1-aS'2-Z2, this voltage being induced by the primary P in the usual electromagnetic fashion. The second factor is that the
WHK
OUTPUT CURRENT
TL- 2730
Figure 130.—Output of the limiter tube. The a. m. impulses have been chopped off.
signal voltage across P is also impressed across inductor L because of the direct connection afforded by the coupling capacitor Cl. Note that the coil L is common to both halves of the secondary of T2 with respect to the signal voltages applied to the two sections of the double diode. This means that two voltages are impressed on each diode: the magnetically induced voltage in the secondary circuit and the voltage which appears across inductor L. The third and controlling fac-
LIMITER	frequency discriminator.
I	I I ■ I
Cl	I
_________DOUBLE DIODE_ X
/iy 5/|i	*4- AT.
Tl	(-~.T~| P 3 fgSl M IV	i .[.AMPLIFIER
U I 01 r I *——''i—
I -V e_lpww------II— ------------------------ ■+
I----1|-WW----
B+
Figure 131.—Limiter-discriminator section of f. m. receiver.
tor in the operation of the discriminator is the complex phase relationships between these voltages. These are determined by the relation between the incoming signal frequencies and the resonant frequency of the secondary circuit xSl-xS'2-<72. (See sec. II.)
(2)	Suppose the incoming f. m. signal is not modulated; that is, the microphone at the transmitter is idle and the transmitted signal is a steady one of the carrier frequency. Although the mixer action
105
TM 11-455
82-83	SIGNAL CORPS
of a superheterodyne changes the carrier frequency to a lower one, this retains the characteristics of the original carrier, and we can therefore say that the discriminator is being fed an unmodulated signal of one frequency. When the secondary circuit xSl-xS'2-(72 is tuned exactly to this frequency, it shows only a resistive effect to the incoming signal, and the phase relations between the two voltages on each diode are such that no audio frequency variations appear across the output resistor R2. In other words, no sound is heard from the receiver.
(3)	Suppose now that the microphone at the transmitter is actuated by voice or music. The frequency of the carrier swings accordingly. In the receiver, the secondary tuned circuit xSl-$2-Z2 is not resonant to the higher and lower frequencies represented by the modulation. At frequencies above resonance, the circuit presents an inductive effect; below resonance, a capacitive effect. The phase relations between the voltages impressed on the diodes shift in such a manner as to unbalance the diode outputs. This unbalance occurs at the audio frequency rate at which the frequency of the incoming signal is changing wTith modulation. The alternating voltages that develop between point X and ground have the same wave shape as the original audio modulating voltage at the transmitter, so the audio amplifier and its associated loud speaker or earphones reproduce an audible sound corresponding to that impressed on the transmitting microphone.
83. General consideration in frequency modulation.—a. Frequency modulation transmitting apparatus, in general, is relatively simple; very little power, almost none, is required to accomplish modulation. Receiving equipment, on the ether hand, is somewhat more complicated than in the amplitude-modulated system. Receivers for frequency modulation are essentially superheterodynes of advance design. Special consideration is given the limiter and discriminator portions of the circuits.
b. Frequency modulation is necessarily restricted to very high frequency channels, above 40 megacycles, in order to accommodate any reasonable number of operating stations each requiring an overall spread of from 50 to 150 kilocycles. At these high radio frequencies radio waves behave somewhat like light waves, so that the service area of a transmitter is approximately confined to the “line of sight” range of its antenna. Incidentally, static is generally lower at these high frequencies than it is at the lower communication frequencies.
c. A disadvantage of frequency modulation from a tactical standpoint is the fact that of two stations operating on closely adjacent frequencies, a receiving station normally hears only the stronger one, signals from the weaker station being entirely inaudible in the back
106
TM 11-455
RADIO FUNDAMENTALS	83-84
ground of the stronger one. This particular characteristic is probably a very satisfactory one for broadcast (entertainment) service. However, in the event that the inaudible signal is actually the desired one, a receiving operator has no indication of the presence or absence of the weaker station. For military communication this may be a handicap in some situations. With amplitude modulation the receiving operator experiences at least a heterodyne whistle or an unintelligible background jumble, sufficient response to give positive indication of the presence of a weak operating station, so that the receiving operator can proceed accordingly.
84. Facsimile.—a. Facsimile involves the transmission of any intelligence which can be recorded on paper, as, for example, letters, photographs, sketches, and maps. Facsimile differs from television in that facsimile transmits only still subjects such as pictures and printed pages, whereas television deals with living scenes. The problems of the former are much simpler than those of the latter. In fact, perhaps the principal problem of any facsimile scheme is that of obtaining a transmitting medium capable of high fidelity reproduction of audio frequency currents. Just such a medium is provided by a frequency modulation radio system.
Z>. The facsimile transmitter employs a light and lens arrangement which is such .as to illuminate a small spot, about y1Oo inch in diameter, of the copy being transmitted. Reflected light from the surface of the paper carrying the copy is focused on a photoelectric cell, which responds with a current which is in proportion to the light. The magnitude of this current controls the amplitude of an audio oscillator, which in turn modulates a radio transmitter. A mechanical arrangement shifts the light spot across the paper from side to side, the intensity of the reflected light varying with the degree of the blackness of the copy and modulating the transmitter accordingly. At the end of each line of the paper scanned, the spot is shifted down by one diameter, and a new line is scanned until the complete copy has been exposed.
c. The facsimile receiving system contains a rectifier which operates from the output of an ordinary receiver. The output of the rectifier presents a varying d. c. potential, one side of which is applied to a Moo inch diameter steel stylus. The other side of this potential is applied to a metal drum on which is wrapped a specially treated recording paper. The stylus makes contact with the paper, and the passage of current through the paper causes a chemical coating to be removed, thereby exposing a black spot, the density of which is related to the magnitude of the current flowing. By the use of a small
TM 11-455
84-85	SIGNAL CORPS
motor rotating at a predetermined speed, the speed being fixed in accordance with the transmitter scanning rate, the recording stylus is moved across the paper exactly in step with the scanning light of the transmitter.
d. In the transmitter each time the scanning device shifts the light spot to the next line an extremely short low-tone impulse is transmitted. In the receiver as the end of a line is reached, the stylus is shifted to the next line and held there by a stop, and the output of the rectifier is transferred from the stylus to an electromagnet. The next impulse actuates the electromagnet, which releases the stop and permits the recording to continue on the new line in synchronization with the transmitted subject.
Section XIII
ANTENNAS
Paragraph
Radiation_____________________________________________________________ 85
Antenna systems_______________________________________________________ 86
Feeder systems________________________________________________________ 87
Loading----------------------------------------------------------___	88
Propagation of radio waves________'_________________________________ 89
Fading-------------------------------------------------------------- 90
85. Radiation.—a. Two types of electromagnetic field are present about any conductor carrying an alternating current. One is the familiar induction (or magnetic) field, which gives rise to transformer action and to choke coil effects. The induction field falls off rapidly a short distance from the conductor, so that its effects are purely local. The second type of field accompanying an alternating current is a radiation field, which moves off into space. This radiation field may be properly modulated at its source and then intercepted and demodulated by a receiver to convey intelligence from one point in space to another. Some radiant energy is released from conductors carrying currents at the usual 25- and 60-cycle power frequencies; however, the amount is exceedingly small. Highest efficiency radiation is achieved at the higher frequencies, that is, at frequencies normally classified as radio frequencies, 50,000 cycles and above. This accounts for the use of these high frequencies in radio communication.
b. The commonly employed resonant radiating system is essentially a two-wire open-ended transmission line (fig. 132). Energy which flows along the line from the generator is not delivered instantaneously to the far end on the closing of the switch but proceeds
108
RADIO FUNDAMENTALS
TM 11-455
85
along the line at a finite rate, which rate depends on the frequency of the generated voltage and on the inherent capacitance and inductance per unit length of the line. For a sine wave generated voltage, sine wave currents and voltages occur at each point along the line with a phase displacement from the original which is dependent on the distance from the generator. Electrical reflections of the voltage and current waves occur at the far end of the line, so that if the length of the line and the frequency of the generator are in the proper relation, the reflected waves reinforce the advancing waves always exactly in phase to produce large amplitude standing waves of current and voltage along the line. This is the condition of resonance under which the radiated energy is a maximum.
@ Parallel wire transmission line.
@ Partly opened-out transmission line. ® Dipole.
Figure 132.—Resonant radiating systems.
c. If the velocity of propagation of waves along the parallel wire transmission system is v feet per second, and if the generator frequency is f cycles per second, then the voltage variations at a point which is at a distance A=yfeet from the generator are just one cycle I)
behind those of the generator. Similarly, at a distance 2A=-y-feet from the generator the voltage variations are exactly two cycles behind those of the generator. The distance A (Greek letter, “lambda”) is the wave length of the radiated wave, that is, it is the distance between two points in the wave which differ in phase by one complete cycle. The condition of resonance is obtained when the over-all length of the radiating system is an integral number of half
109


@	TL-2732
TM 11-455
85-86	SIGNAL CORPS
or quarter wave lengths depending on the particular type of radiating system.
d. (1) It is now the universal practice to designate radio waves in terms of frequency, which is expressed as so many cycles, kilocycles or megacycles. Formerly, waves were designated in terms of wave length, the unit being the meter. Wave length figures are convenient in discussions of antenna systems, as the wave length gives some idea of the actual physical dimensions of the wires. For instance, a “half-wave” antenna for 50-meter transmission is 25 meters (a little more than 25 yards) long.
(2) It is well to remember that an inverse relationship exists between frequency and wave length; high frequencies correspond to short waves, and low frequencies to long waves. The following simple formulas show how one is converted to the other.
x	300.000,000
Frequency (in cycles) = ™r
J Wave length (m meters) or
xv i /• a \	300,000,000
Wavelength (in meters) =	——-—A-----------r—v
Frequency (m cycles)
• / \ ! । / »
VOLTAGE-* । /	\ ,'	\ V
y \	I	j«- CURRENT
CURRENT-*	\/	/\	/	\
@ ,',*'V0LTAGE
!	\ /	i	/\	T«--2-733
•	\/	■	• \	/	'
v	I \|/	;
® Fundamental frequency.	@ Second harmonic.
Figure 133.—Standing waves of current in Hertz antenna. Voltage distribution in dotted lines.
86. Antenna systems.—a. The parallel wire antenna confines the moving electric field to one direction and hinders the propagation, of waves from progressing generally in all directions. More nearly uniform radiation in all directions is secured as the transmission line
110
TM 11-455
86
RADIO FUNDAMENTALS
is altered in stages from © to © of figure 132. In © the radiation field is small in extent and is confined mainly to the end, while in © the complete electric field forms a part of the radiation. The disposition of © is referred to as an electric dipole or as a Hertz antenna. The standing wave resonance distribution of the current in such an antenna at the fundamental frequency (lowest frequency at which
A
4
4
^CONDUCTING PLANE	Tl-2734
Figure 134.—Lower half of Hertz antenna replaced by extensive conducting plane.
the antenna can resonate) is shown in figure 133©, maximum at the center and minimum at the ends. The wave length of the corresponding radiated wave is twice the length of the antenna. Current distribution in the same antenna excited at twice the fundamental frequency (second harmonic) is shown in figure 133@. The wave length of the corresponding radiation is equal to the length of the antenna. The voltage distribution in the dipole is shown by dotted lines in figure 133.
VOLTAGE-*', /	\ ;	(	/♦•CURRENT
/	\	KK
/ \	: \	!	:♦'VOLTAGE
CURRENT—/ \	/ \	\/X	N**
—	—	TL-273S
® ®
® Fundamental frequency.	® Third harmonic.
Figure 135.—Current distribution in Marconi antenna.
b. If the lower half of the antenna is replaced by an extensive conducting plane (fig. 134), no disturbance is caused in the propagated waves from the upper half. A practical form of such a radiating system is the so-called Marconi antenna in which the lower terminal of the generator is connected to ground and the earth’s surface serves
111
TM 11-455
86-87
SIGNAL CORPS
as the extended conducting plane. Current and voltage distributions in such an antenna at the fundamental and third harmonic frequencies are as shown in figure 135. The wave length of the radiation at the fundamental frequency is four times the length of the antenna, and at the third harmonic it is four-thirds the length of the antenna. In the grounded Marconi antenna the voltage is necessarily a minimum at the ground, so that the antenna resonates only when excited at odd harmonic frequencies.
87. Feeder systems.—a. Standing waves could not arise in a parallel wire transmission line which is infinite in length, because the advancing waves should never reach the far end to produce the necessary reflected waves. As a consequence, since a resonant condition would not be attainable in an infinite line, the radiated energy in such a line would be negligible. Electrically, the condition of an infinite line can be obtained by terminating a finite line in the impedance which a corresponding infinite line can be calculated to present, namely,
7 __ /L
°~yc
where L and C are the inductance and capacitance per unit length of the line, respectively. Zo is called the characteristic impedance of the line. A transmission line terminated in its characteristic impedance is useful as a feeder system for transferring radio frequency energy without radiative losses from a transmitter to an antenna in case the two are necessarily separated by some distance.
b. Because the effective voltage and current vary along an antenna (figs. 133 and 135), the impedance of an antenna varies according to the positions of the connections used to couple it to the power source. A number of methods are possible for properly coupling a nonresonant line to an antenna. Two of them are shown in figure 136 © and ©. In the arrangement”of figure 136© the appropriate impedance match is secured by varying the spacing, A, between the feeder connections. The single-wire line of figure 136© is a modification of the two-wire line in which the ground supplies a return circuit through the antenna to ground capacitance. The proper adjustment of a nonresonant transmission line can be determined by checking for the absence of current maxima or voltage maxima along the line with an inductively coupled flashlight bulb or with a neon bulb in contact with the wTire, respectively.
c. If the transmission line is not terminated m its characteristic impedance, resonance effects result. A resonant transmission line, as
112
TM 11-455
87—88
RADIO FUNDAMENTALS'
illustrated in figure 136® and @, may be regarded as a portion of the antenna which is folded back on itself so that the radiation from one half cancels the out-of-phase radiation from the other half. The line of figure 136® is fed from the transmitter at a point of low current and high voltage. Such a feed system is commonly connected to the transmitter across the capacitor of a parallel tuned circuit. The line of figure 136© is fed from the transmitter at a point of high current and low voltage, and is normally connected in series with a series resonant circuit. Resonant transmission lines are less efficient than
* z	_ —— _
s''	h— a —>|
1	CURRENT--»-l
I
I I I I
I	TL-2736
© @ /
h----------L -----------H
v o lt7 t_____________________________i /j [\ ।
"-■v	r	z	/	\	।
j® /1	/	\ 2S
A \ I ' Ae-CURRENT	CURRENT-*/ f t \ 4
4 \	/	I	' I
|\J k	I ' I
@ and ® Nonresonant transmission lines. @ and ® Resonant transmission lines.
Figure 136.—Feeder systems.
nonresonant lines; however, they are easier to adjust, and they are suitable for use over a wider range of frequencies than are nonresonant lines. Resonant lines are satisfactory if the transmitter is located only a short distance from the antenna.
88. Loading.—a. Often one antenna system is used to transmit signals of various frequencies. In most instances it is impractical to vary the physical length of the antenna to resonate accordingly as the excitation frequency is changed. However, the electrical length may be changed by loading, that is, by lumped-impedance tuning.
476847 0 - 42 -8
113
TM 11-455
88-89
SIGNAL CORPS
If the antenna is too short for the wave length being used, it is resonant at a higher frequency than that at which it is being excited. Therefore, it offers a capacitive reactance at the excitation frequency. This capacitive reactance can be counterbalanced by introducing a lumped-inductive reactance as shown in figure 137®. Similarly, if the antenna is too long, it offers an inductive reactance, which can be corrected by introducing a lumped capacitive reactance as in figure 137®.
---1	—r
LESS THANA	MORE THAn£
—	Tt-2737
© ®
® To compensate for too short an antenna. ® To compensate for too long an antenna.
Figure 137.—Loading.
b. Figure 138 shows a typical antenna tuning unit. In ® the transmitter feeds the antenna system at a point of high voltage; in ® the transmitter feeds the antenna system at a point of high current. The arrangements of @ and ® provide antenna loading for use with a short antenna, for example, with a short mast antenna of the buggy whip variety as is conveniently mounted on moving vehicles.
89. Propagation of radio waves.—a. The radiation from an antenna is conveniently regarded in two parts, that which travels along the surface of the earth, called the ground wave, and that which is propagated at an angle above the horizontal, called the sky wave. The ground wave suffers energy losses because of earth currents which it induces and because of dielectric effects. The attenuation associated with dielectric losses increases with the frequency, so that the ground wave of high frequency transmitters is effective over only “relatively short distances. If the ground wave of a 1-mega-cycle radiation is effective over about 50 miles, the ground wave of a 10-megacycle radiation from a transmitter of comparable power may be essentially confined to within a 10-mile radius.
b.	The sky wave passes into the ionosphere, an ionized layer of the earth’s atmosphere, at a height of about 70 miles above the surface
114
TM 11-455
89
RADIO FUNDAMENTALS
of the earth. The depth of the layer, its degree of ionization, and its effective height above the earth’s surface vary with the seasons, solar radiation, and sunspot activity. The radiant energy of a sky wave is partly transmitted through the ionosphere, partly absorbed in it, and partly reflected back to earth. The angle of reflection depends on the frequency and on the angle of approach of the incident radiation. The reflected sky wave is in part reflected back on striking the surface of the earth and continues on its path from earth to ionosphere and back until it is completely absorbed. A. picture of the attenuated ground wave and of the refracted and reflected sky waves
Y _______	y
Wim,-----------Wu.
e-------------------------(a)-----t
AMMETER. VV	r	1
©A	@	A
mV	T
---------------1 ------------------1
e------0-------e----------0--------t
@ A	® A
TL-X73S
Figure 138.—Antenna tuning unit of radio transmitter BC—191-A.
from a transmitter is shown in figure 139. Those components of the sky wave which proceed upward at angles with the vertical which are less than the critical angle (fig. 139) are partly absorbed by the ionosphere and partly transmitted through it, but they are in no part reflected back to the earth. The critical angle is related to the frequency of the radiation, increasing as the frequency increases, so that sky waves of ultrahigh frequency are not returned to the earth at all, and communication on these frequencies is almost entirely by means of the ground wave alone.
c.	There is a portion of the earth’s surface, as shown in figure 139, which is reached by neither ground wave nor sky wave. The distance from the transmitter to the point where the first sky wave
115
TM 11-455
89-91
SIGNAL CORPS
returns to the earth is called the skip distance. Skip distances of several hundred miles are common on the higher frequencies.
A REFRACTED WAVE
'CRITICAL / /	\	\
'' AN&LE’ // s' \	\	\[	\ '
\ / //.s' Reflected \	\ /	\	\
' / ZZ' WAVES \ V \	\
TRANSMITTER.	\	/\ \
h—-f\\
NO signal FROM TRANSMITTER.	\\
RECEIVED IN THIS ZONE
TU-2739
Figure 139.—Ground and sky waves from transmitter.
90.	Fading.—Fading, random rising and falling of the intensity of a received signal, is attributed to the interaction of different components of the same radiation, which by virtue of having traveled different paths from the transmitter, arrive at the receiver in different phase relations. The condition of the ionosphere is continually changing, so that at one instant the several components of the received wave may be in complete reinforcement to present a very strong signal, and at a later instant the phase relations may be such that the combined effect is very weak.
Section XIV
MAJOR COMPONENT PARTS OF RADIO CIRCUITS
Paragraph
General___________________________________________________________________ 91
Transformers____’______________________________________________________	92
Power transformers________________________________________________________ 93
Audio frequency transformers______________________________________________ 94
Radio frequency transformers______________________________________________ 95
Autotransformers__________________________________________________________ 96
Inductors_________________________________________________________________ 97
Fixed resistors___________________________________________________________ 98
Variable resistors________________________________________________________ 99
Fixed capacitors_________________________________________________________ 100
Variable capacitors_____________________________________________________  101
Piezoelectric crystals___________________________________________________ 102
Microphones______________________________________________________________ 103
Headsets and loudspeakers________________________________________________ 104
91.	General.—a. In general the theory applicable to electrical communication circuits is the same as that applied to power circuits;
116
TM 11-455
91-94
RADIO FUNDAMENTALS
however, the components used in communication circuits differ widely in design from those of power circuits.
1). Certain convenient “symbols” are universally used in radio diagrams to represent these component parts. In effect they are shorthand pictures of the instruments themselves. In the illustrations accompanying this section, the symbols of the various parts are included so that the student can familiarize himself with them.
92.	Transformers.—If two coils are placed near to each other, one coil having an alternating current generator connected to it, the varying lines of magnetic force from one coil cut through the second coil, causing a voltage to be induced in the second coil even though there is no actual metallic connection between the windings. This is transformer action and the two coils in inductive relation to each other are called a transformer. The coil producing the original lines of force is the primary and the coil in which the voltage is induced is the secondary. Transformers used in radio fall into three general groupings as to application. They are power transformers, audio frequency transformers, and radio frequency transformers. The power and audio frequency transformers have cores of magnetic materials (usually some form of iron). The radio frequency transformers are generally of air core design; however, very small magnetic cores (usually powdered iron) are used in some intermediate frequency transformers.
93.	Power transformers.—Power transformers in general transfer power from one circuit, at a certain frequency, voltage, and impedance to another circuit at the same frequency, but at a different voltage and impedance. Power transformers used in radio receivers and transmitters transform the source voltage, usually 116-120 volts, 60 cycles, to either higher or lower voltages. When the voltage is raised, for example in plate circuit application, the transformer employed is called a step-up transformer; when the voltage is reduced, for example, in filament circuit application, the transformer employed is called a step-down transformer. Power transformers having both step-up and step-down windings on the same core are widely used in radio receiver and transmitter construction. Figure 140® shows a typical power transformer.
94.	Audio frequency transformers.—Audio frequency transformers are used to transfer voltages of wide audio frequency range rather than voltages of a single frequency as in the case of a power transformer. A transformer suitable for transforming voltages in the audio frequency range from one circuit to another must have certain design features not found in a power transformer. A perfect
117
TM 11-455
94
SIGNAL CORPS
transformer should transform without loss or phase change as a result of modifying the magnitude of the load impedance. To accomplish this the transformer would need to have unity coupling,
infinite inductance and no resistance in its primary and secondary windings, yet have a finite ratio of winding inductance. A good audio frequency transformer can only approximate these conditions by having the resistance of its windings small and the inductance of its
118
. I Primary	———
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Low voltage center tappea	a K	9 g IF Transformer
secondary nigh voltage secondary	—oj g—	<3 K	w-,m primary
Typical Power Transformer	JB
0)	Typical A F AnyRF
Transformer Transformer
NOTE: Either winding may be primary or secondary depending on direction in which diagram is drawn.
1	T \ These tines indicate^^lh,------
iron core	»>	____
Secondary	Primary ■ ---J----	91	K
; Secondary	_ C______
—1----- _ -E—fl?--------------------- Power or Aup/o P F Inductor
PF Auro Transformer Power Auto transformer	Inductor
(step-up ratio)	(step down ratio) (Filter Reactor or Choke)
(T) Multiwinding power transformer. Flexible leads from the various windings are brought out through holes in the bottom of the case.
® Audio amplifying transformer. This particular one is of the push-pull output* type, with tapped primary and secondary.
® Radio frequency transformer. This fits inside the round aluminum shield can (®i). ® Intermediate frequency, with attached midget variable air capacitors for tuning the primary and the secondary. This assembly fits inside the square aluminum shield can (®i).
Figure 140.—Typical transformers used in radio circuits.
TM 11—455
RADIO FUNDAMENTALS	94-95
windings large compared to the circuit with which they are connected, and by having high permeability cores with windings so placed that the flux linkage is a maximum. The frequency response of a transformer is limited at the low frequencies by the inductance of its windings and at the high frequencies by the distributed capacitance of its windings. The core dimensions are determined by the flux required. An audio transformer must be able to carry a limited amount of direct current in its primary winding without causing magnetic saturation of the core. By a suitable compromise of inductance and distributed capacitance, a transformer winding can be designed that will have practically a uniform response to audio frequencies over a wide range. As in a power transformer, the voltage ratio of an audio transformer is equal to the turns ratio of the windings. Figure 140® illustrates a typical audio frequency transformer.
95.	Radio frequency transformers.—Radio frequency transformers in receivers and transmitters are used to transfer radio frequency voltages of a comparatively narrow band; therefore, they act as band pass filters. At radio frequencies the transformer again requires additional design precautions. The distributed capacitance of the windings and the associated equipment in the circuit presents a low reactance at high frequencies that has a short-circuiting effect on the high impedance load. Thus, if an effort is made to approach the requirements of a perfect transformer by winding a large coil to obtain a large inductance, the distributed capacitance of the winding will cause parallel resonance. The reactance of the winding might even become capacitive, in which instance the coil would actually act as a capacitor. At radio frequencies, only a comparatively narrow band of frequencies needs uniform amplification, hence the capacitive reactance present can be used in combination with the self-inductance of the windings to obtain resonance and impedances comparable with the Rp of the associated tube. Resonance provides a band pass filter action which passes the wanted frequencies and rejects unwanted frequencies. In practice either one or both of the windings are tuned by variable capacitors of the proper capacitance range to allow tuning across a certain frequency range, for example, 550 to 1500 kc. in the case of a broadcast receiver. The coupling between windings in a radio frequency transformer is rather critical. Loose coupling will cause insufficient voltage transfer and a loss of wanted frequencies; tight or overcoupling will broaden the response or resonance curve and allow unwanted frequencies to pass. The induced voltage in the secondary at optimum coupling is seldom much
119
TM 11-455
95-97
SIGNAL CORPS
higher than the primary voltage; however, there is a resonant rise in voltage in the secondary due to the Q of the resonant circuit. The presence of iron cores at radio frequencies gives rise to eddy current and hysteresis losses that practically preclude their use except in the case of intermediate frequency transformers. Figure 140® shows a typical radio frequency transformer and ® shows a typical intermediate frequency transformer.
96.	Autotransformers.—It is possible to obtain transformer action with only a single coil if a connection is made somewhere along the winding between the extreme ends. If a step-up voltage
® Single-winding “tank-’ inductor, used in high power transmitters.
® Plug-in type r. f. transformer used in medium power transmitters.
® and ® Small r. f. transformers used in ultrahigh frequency receivers and transmitters.
® Small r. f. inductor or “choke coil” used in receivers and low-power transmitters.
Figure 141.—Typical r, f. inductors and transformers.
effect is desired, the winding between the tap and one end is considered the primary and the entire winding acts as the secondary. If a step-down effect is desired, the entire winding is considered the primary and the section between the tap and one end acts a,s the secondary. Such transformers are known as “autotransformers” and are used in both power and radio frequency applications.
97.	Inductors.—An inductor is any single-winding coil. If it has many turns of wire on an iron core, its inductance and therefore its impedance are high, and it is used mainly as part of low frequency filter systems, especially in a. c. power supplies for receivers and
120
TM 11-455
97-98
RADIO FUNDAMENTALS
transmitters. Iron core inductors, sometimes called “chokes,” resemble power and amplifying transformers in appearance (fig. 140® and ®).
a. Inductors consisting of small air-core spools of wire have a high impedance to radio frequency currents, and are therefore used as chokes in r. f. circuits (fig. 141®).
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® Fixed Resistor ZZ
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Those in group ® use resistance wire wound on ceramic tubes, with the wire itself covered by a protective coating of heavy enamel. The single resistor @ is also of the wire-wound type, but with a center tap and without covering for the wire. Those in group ® use a thin layer of metallized carbon on an insulating form, or consist of a solid carbon stick of small cross section.
Figure 142.—Typical fixed resistors employed in radio sets.
b. Inductors consisting of a few large turns of heavy wire are used as part of the ZZ “tank” circuits of transmitters. Since they are subjected to high voltages and must carry considerable r. f. current, their insulation must be very good (fig. 141®).
98. Fixed resistors.—Fixed resistors used in radio circuits are of many types and sizes. They range in resistance from a fraction of an ohm to several megohms. Resistors are rated in ohmic value and also according to the power which they can safely dissipate in the form of heat. Figure 142 illustrates typical fixed resistors of various values of resistance and wattage.
121
TM 11-455
99-100
SIGNAL CORPS
99. Variable resistors. — In many instances variable resistors are desirable for obtaining control of current flow in a circuit. There are many types of variable resistors used in radio circuits. Further classified as to application, they fall under the general headings of rheostats, potentiometers, voltage dividers, etc. Figure 143 illustrates these types of variable resistors.
100. Fixed capacitors.—a. Both fixed and variable capacitors are used extensively in communication circuits. Fixed capacitors are
Figure 143.—Typical variable resistors used in radio circuits.
of various types and sizes and, in general, are classified in terms of the dielectric used, as mica capacitors, paper capacitors, oil filled capacitors, etc. The electrolytic capacitor uses as a dielectric a thin film of oxide and gas which is formed chemically when voltage is applied to the unit. One of the conductors is usually aluminum foil and the other the electrolyte. Like batteries, electrolytic capacitors are manufactured both in the wet and dry types. Due to the extremely thin film of dielectric, very large values of capacitance without excessive physical size can be obtained. These capacitors have high leakage and low internal resistance as compared to other types and are useful only in pulsating d. c. circuits. Attention must be given to the proper polarity of these capacitors when they are connected in a circuit. Due to these operational limitations electrolytic units are used almost exclusively in power filter circuits.
b. Mica capacitors have low leakage and high voltage ratings but are limited in capacitance by cost factors to about 0.05 microfarad.
Arrow indicate moving corrtaci
—V\Aaaaaa/vv\a/\/—
Variable Resistor
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i®
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TM 11-455
100-102
RADIO FUNDAMENTALS
Paper capacitors consist of tinfoil and paper rolled together and impregnated with wax to exclude moisture. Capacitors of this type vary approximately in capacitance values from y10 to 2 microfarads. Where large capacitance and high voltage rating are required, oil filled capacitors are used. The actual dielectric material is oil-treated paper, and the whole container is also filled with oil to keep the assembly protected. Figure 144 shows a group of fixed capacitors of various types.
® Mica dielectric type.
@ Paper dielectric, wax impregnated.
® Paper dielectric, oil impregnated.
® Electrolytic.
Figure 144.—Typical fixed capacitors.
Fixed Capacitor. (all types)
101.	Variable capacitors.—Most variable capacitors used in communication circuits are of the air dielectric type. In general, they consist of two sets of metal plates insulated from each other and so arranged that one set of plates can be moved in relation to the other set. The stationary plates are called the stator; the movable plates, the rotor. The capacitance range of variable air capacitors is from a few micromicrofarads to several hundred. Figure 145 shows a typical group.
102.	Piezoelectric crystals.—Certain crystals, of which quartz is the principal one encountered in radio, exhibit a phenomena called the piezoelectric effect. Prepared sections cut from such a crystal will, when subjected to an electric field, be placed under stress and
123
TM 11-455
102—103
SIGNAL CORPS
slightly deformed. If the crystal section is placed under such mechanical stress, it will develop a difference of potential between its faces. Such a crystal section has a natural frequency of mechanical vibration determined by its position in the original crystal. A crystal section for use in radio is ground to a thickness which will produce a desired frequency, and is mounted in a holder with its faces in Contact with metal electrodes. An example of its use is control of frequency in a vacuum tube oscillator circuit. Some of the tuned circuit voltage is fed back to the crystal, causing it to vibrate and
Variable Capacitor.
(all types)
® Four-gang receiving type.
® Single, unit, with wide plate spacing, used in high-power transmitters.
® Midget •‘trimmer” or “padder” type.
® Midget type, with wide plate spacing, used in high frequency transmitters.
Figure 145.—Typical variable capacitors.
produce a varying potential between its electrodes. This is in turn amplified and impressed on the tuned circuit. Optimum output is obtained by tuning the circuit to the crystal’s natural frequency, which is highly stable. The use of crystals in filter circuits has been explained in paragraph 73. Figure 146 illustrates some typical crystals and holders.
103.	Microphones.—A microphone is a device for converting acoustical energy into electrical energy. The various types of microphones are named in accordance with the methods used to produce this conversion. Thus, there are carbon, condenser, dynamic, velocity, and crystal microphones. Carbon microphones use the variation of resistance between carbon granules, due to acoustical or
124
RADIO FUNDAMENTALS
TH 11-455
103
n- sound pressure on a diaphragm, to vary an electrical current at sound as. frequencies. Condenser microphones operate on the principle of a- acoustical energy causing variation in the spacing between two al J plates; the resulting variation of electrostatic capacitance causes a ;a )ii-of ,ed nd
turn it is mcy,
■ting Sof d to unit; the al or
Crystal Symbol
Figure 146.—Typical crystals and holders.

Figure 147.—Carbon microphone, T-17.
125
Microphone

TM 11-455
103
SIGNAL CORPS
variation at sound frequencies in a high d. c. potential applied between the plates. A dynamic microphone uses a low-impedance coil mechanically coupled to a diaphragm. The sound waves move the diaphragm and coil, the movement of the coil in a magnetic field
Figure 148.—Headset.
Figure 149.—Permanent magnet speaker and cabinet.
inducing currents in the coil at the frequencies of the sound waves. The velocity type or ribbon microphone also operates on the electromagnetic principle but uses a ribbon of dural (a metal alloy) suspended between the poles of a powerful magnet. When the ribbon is vibrated by acoustical energy, it cuts the lines of force and a current,
v r: f; Ci bi A so h V
is In of on vil wa tyi cip use sou fro
126
77.-2W&
Headset (oarphone)
Loud Speaker
TL-2799
TH 11-455
103-104
RADIO FUNDAMENTALS
which varies in accordance with the sound waves, is induced in the ribbon. One type of crystal microphone has a Rochelle salt crystal fastened to a diaphragm. Sound waves move the diaphragm and cause the crystal to vibrate, thus producing an alternating voltage between the crystal electrodes at the frequencies of the sound waves. All of the types mentioned except the crystal microphone require some source of current, magnetic field, or polarizing voltage. Figure 147 shows an Army microphone (type T-17) which is the carbon type.
104.	Headsets and loudspeakers.—A headset or a loudspeaker is a device for converting electrical energy into acoustical energy. In general, the headset or loudspeaker performs the opposite function of a microphone. When varying currents flow through the windings on the permanent magnet of a. headset, the diaphragm is caused to vibrate in accordance with these currents and produces audible sound waves proportional to the variations of current. Figure 148 shows a typical headset. One type of loudspeaker works on the same principle as the headset. Instead of a metal diaphragm, the speaker uses a paper cone, actuated by an armature, for setting up audible sound waves. Figure 149 shows a loudspeaker of this type removed from its cabinet.
127
TM 11-455
104
SIGNAL CORPS
Battery		Vacuum Tubes	ipiate D iode (two e le me nts)	/	)
		Filament type	I A/
Aerial Ground Fuse	I	glass or metal/\cJZ envelope S fjiamenr /CETplatfl Diode (cathode type) (^-x) cathodeHji , heater Triode (Three elements)F^yp,ate
A.C.Generator	—^7)—		Filament type	\Az nd filament
D.CGenerator		। plate Triode (cathode type)-4-® grid
Meters Voltmeter —(	y)— or —(g)-	cathode pfn Tetrode (four elements) z£xplate No.2grid —p— \ No.1 grid v7\y
Ammeter —(	—or —O	caThodeTPr Pentode (five elements)	.r No.3grid
Wattmeter		No.2 grid EE zzz) No J grid YSz cathode Pt------------	57	81
Surge impedance (characteristic impedance)----------------------- 87	112
Tetrodes_____________________________________________________ 26, 50	39, 72
Time constant due to R and C------------------------------------- 81	102
Transconductance, Gm_____________________________________________ 24	38
Transformer coupling______________________________________ 36, 95, 96	48,
119, 120
Transmission lines_______________________________________________ 87	112
Transmitter coils-----------------------------------------------  97	120
Triodes__________________________________________________________ 19	33
Tuned-grid tuned-plate oscillator-------------------------------- 47	68
Tuned r. f. circuits_____________________________________________ 68	89
Tuned r. f. transformers_________________________________________ 95	119
Tuning unit antenna, typical------------------------------------- 88	113
Ultrahigh frequency oscillators__________________________________ 48	70
Vacuum tubes:
Anode_______________________________________________________ 16	30
Beam________________________________________________________ 27	40
Cathode_____________________________________________________ 16	30
Characteristic curves______________________________________  18	31
Converters, pentagrid_______________________________________ 71	93
Duplex diode________________________________________________ 29	41
Electron emission___________________________________________ 16	30
Mixers__________________________________________________ 69,	70	90,	92
Pentagrid___________________________________________________ 71	93
Pentodes________________________________________________ 25,	50	39,	72
Plate_______________________________________________________ 16	30
Rectifiers______________________________________________ 17,	76	31,	98
Space charge____________________________________________ 27	40
Tetrodes_______•________________________________________ 26,50	39,72
Triodes_________________________________________________ 19	33
Variable n tube_________________________________________ 28, 67	40, 87
Variable capacitors_____________________________________________ 101	123
Variable resistors_______________________________________________ 99	122
Vectors:
Addition_____________________________________________________ 6	5
Capacitive reactance_________________________________________ 9	11
Capacitance and inductance in series_________________________ 7	9
Capacitance and resistance in series_________________________ 6	5
Capacitance, inductance, and resistance in parallel_____	10	15
Capacitance, inductance, and resistance in series____________ 9	11
Filter section______________________________________________ 15	27
139
TM 11-455
INDEX
Vectors—Continued.	Paragraph Page
Impedance and inductive reactance_______________________ 9	11
Parallel resonance_____________________________________ 10	15
Series resonance________________________________________ 9	11
Sine function,.^________________________________________ 6	5
Voltage and current in phase____________________________ 6	5
Velocity of radio waves____________________________________ 85	108
Voltage:
Amplification__________________________________________ 35	47
Distribution antenna___________________________________ 86	110
Feed_________________________________________________   87	112
Gain------------------------------------------------ 12,35	20,47
Volume control_____________________________________________ 67	87
Wave:
Length------------------------------------------------- 85	108
Modulated___________________________________________ 55, 58	80, 81
Propagation____________________________________________ 89	114
[A. Q. 062.11 (2-21-41).]
By order of the Secretary of War :
G. C. MARSHALL, Chief of Staff.
Official :
E. S. ADAMS,
Major General, The Adjutant General.
Distribution :
D 1, 2, 7,17 (3); B (2); R 1-7, 17 (5); Bn 3, 4,17 (5), 1,11 (10); IBn2,5-7 (5);IC1 (3),2-7 (10), 11,17 (15).
(For explanation of symbols, see FM 21-6.)
140
U.S.GOVERNMENT PRINTING OFFICE : 1942
For sale by the Superintendent of Documents, Washington, D. C.
Price 25 cents
UNT LIBRARIES DENTON TX 76205
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1002642931