Propagation Curves

[Propagation Curves]
[From the U.S. Government Publishing Office, www.gpo.gov]

National Defense Research Committee
\ Division 15

PROPAGATION CURVES

Issue 3 (Replacing Previous Issues)

October, 1.944/.

Report 966-ÓC

Bell Telephone Laboratories, Inc.

X-66618, ISSUE 1

PROPAGATION CURVES

TABLE OF CONTENTS

DISCUSSION ^^PaPe
» O • O) 0 o
1. Description of Charts.............................». . iv
2. How to Use the Charts.............................••°’ ₑ - vⁿ
3. Effects of Hills, Trees and Jungle..............................xv
4. Limitations.......................................**’..*.. xx
⁰ » w 0 0 Q ■ Ï ft 0 0 O 0
PROPAGATION CURVES ’
1. Vertical Polarization—Poor Soil..................." •« • *« 1-15
2. Vertical Polarization—Good Soil .............................16-30
3. Vertical Polarization—Sea Water..............................31-45
4. Horizontal Polarization—Poor Soil . . . .....................46-60
5. Horizontal Polarization—Good Soil . . UP • • • 61-75
6. Horizontal Polarization—Sea Water . tHulvil-Ç'" . . . . 76-90

PROPAGATION CURVES

The radio propagation charts given in this handbook can be used to estimate the received field intensity of ground wave signals when transmitting (1) between an airplane and ground station, (2) from one plane to another, and (3) from one ground station to another ground station, at frequencies from 200 kilocycles to 600 megacycles. This information covers plane elevations as high as 40,000 feet and distances up to 500 (statute) miles for both horizontal and vertical polarization and for three types of ground conditions, namely, sea water, good soil, and poor soil. Good soil means land of relatively high conductivity and high dielectric constant, land, such as clay, loam, and alkali soil. Poor soil means land of relatively low conductivity and low dielectric constant, such as land consisting largely of rock, gravel, or sand.
The propagation charts assume a smooth spherical earth with an effective radius of 4/3 of the true earth radius. Some information is also included for estimating the effects of intervening hills and of trees in the vicinity of the antennas.

1. DESCRIPTION OF CHARTS
a. Propagation Charts. The propagation charts are divided into two groups, one for vertical polarization, and the other for horizontal polarization. Each of these two groups is divided into three sections of 15 charts each, one section for each type of ground condition.
The first chart in each section indicates the field intensity at various distances when both antennas are at reference height. The reference height is ground level (0 feet) for vertical polarization and 10 feet above ground level for horizontal polarization. This chart presents a family of curves, one curve for each frequency.
The second chart in each section indicates the gain in decibels that results when the antenna at a ground station is raised from reference height to any other height up to 200 feet. For frequencies lower than those shown on this drawing the gain is negligible for antenna heights less than 200 feet.
The 13 remaining charts in each section cover the case when one antenna is elevated to considerable heights above the ground and the other antenna is at reference height. Each of these charts is for one specific frequency and presents a family of curves showing the field intensity when one antenna is elevated to various heights above the ground.

b. Power Correction. The propagation charts indicate the received field intensity in decibels above one microvolt per meter when the radiated power is one kilowatt. When the radiated power is less or greater than this value it is necessary to apply the correction shown in Figure 1 to the field intensity shown on the chart. In estimating the radiated power, the losses in the ground and in the transmission line or in other coupling units should be subtracted from the rated power output of the transmitter.

c. Antenna Correction. The charts for horizontal polarization and the charts for vertical polarization with one antenna elevated assume that the transmitting antenna is a dipole whose length is one-half wavelength or less*. The charts for vertical polarization with both antennas at ground level assume a vertical whip (one-quarter wavelength or less) working against a perfect counterpoise. In each case the field intensity shown on the charts is for a direction perpendicular to the radiating elements. In directions such that the angle is considerably less than 90 degrees, the field intensity is lower.
'’The computations are actually based on an ideal doublet but the difference between a halfwave dipole and the doublet is neglected since it amounts to only 0.4 db for the same radiated power. The change in input impedance which occurs when the height of a horizontal dipole is appreciably less than a quarter wavelength has also been neglected.

Figure 1. Power correction factor.

When a directional transmitting antenna is used, a correction should be made for its gain or loss, in decibels, relative to the reference antenna. The gain of a directional antenna is usually given in terms of its radiation compared to that from a half-wave dipole in the direction for which the field intensity for each antenna is a maximum.
For a given power, the field intensity from a whip antenna (connected to a perfectly conducting counterpoise) is 3 decibels less than that from a dipole elevated more than a quarter wavelength above the ground. This difference, which is in addition to the values shown on the height gain charts, may be taken into account by the corrections shown in Figure 2.

2. HOW TO USE THE CHARTS
a. Plane-to-Ground or Ground-to-Plane Transmission. In plane-to-ground or ground-to-plane transmission, the received field intensity can be determined by referring to the chart which corresponds to the given type of polarization, type of ground, and frequency. When one antenna is at reference height, the received field intensity is obtained directly from the curve corresponding to the height of the plane, provided the radiated power is 1 kilowatt and the transmitting antenna is a dipole. For other values of radiated power, the power correction indicated on

Ah ECTION FACTOR
WHI ITENNA CORR TING ANTENNA IS
GROUNDED IN TRANSMIT DIRECTIONAL
VERTICAL DIPOLE ANTENNA
WHEN USING CURVES FOR WHIP
Vertical polarization with both 0 +3 db 3 db plus directivity gain re-
antennas on ground ferred to dipole
Vertical polarization with one an- -3 db Odb directivity gain referred to
tenna elevated dipole
Horizontal polarization --- Odb directivity gain referred to
dipole

Figure 2. Antenna correction factor.

Figure 1 should be made. When the transmitting antenna is other than a dipole, the antenna correction is as indicated in Figure 2.
At frequencies above 20 to 30 megacycles, the antenna at the ground station is ordinarily mounted on a mast and therefore is considerably higher than reference height. This causes an increase in the field intensity by an amount corre-

spending to the height gain correction shown on the second chart in each section. However, when the sum of the field intensity shown on the chart plus the height gain factor is greater than the “free space” value shown on the chart, the free space value should be used*.
Example: A transmitter in a plane flying at 10,000 feet radiates 10 watts at a frequency of 150 megacycles, using a vertical half-wave dipole antenna. The receiving antenna is 100 miles from the plane and is mounted on a 50-foot mast. The intervening terrain is assumed to be good soil. The estimated received field for this example is:
42 + 19 — 20 4- 0 — 41 decibels above 1 microvolt per meter.
The first factor (42 decibels) is obtained from the chart on page 20 for vertical polarization, good soil and 150 megacycles for a distance of 100 miles and a height of 10,000 feet. The second factor (19 decibels) is the gain due to raising the antenna at the ground station from reference height (0 feet) to 50 feet above ground level, and is obtained from the height gain curve on page 17 for vertical
*The ray reflected from the ground may reenforce or weaken the direct ray, depending on the distance, antenna heights, frequency and other factors. Thus the field intensity may be as high as 6db above the free space field, and it may be 20 or 30 db below free space. The nulls are reasonably sharp, however, and in most conditions the field is equal to or greater than the free space value.

polarization over good soil. The third factor (—20 decibels) is the power correction factor obtained from Figure 1. The fourth factor (0 decibel) is the antenna correction factor indicated in Figure 2.
When the receiving antenna is 100 feet high this method would indicate a field of 42 + 25 — 20 + 0 — 47 decibels. However, this value may be in error since the sum of the first two terms (42 + 25 = 67) is greater than the free space field, which is about 62 decibels for 100 miles. For this case and for all greater heights at the ground station the estimated field intensity is 62 — 20 + 0 — 42 decibels above 1 microvolt per meter.
When transmitting to a plane from a ground station with a half-wave dipole antenna radiating 10 watts, the received field intensity at the plane is the same as that shown above for the same combination of heights and distance. However, when the transmitting antenna is a grounded whip, the estimated field at a plane which is at an elevation of 10,000 feet and at a distance of 100 miles is 42 + 0 — 20 — 3 — 19 decibels above 1 microvolt per meter. In this case the height gain factor is 0 decibel and the antenna correction factor is — 3 decibels as shown in Figure 2.

b. Plane-to-Plane Transmission. When transmitting between planes, both antennas are elevated to considerable heights above the ground, and it is usually

assumed that the received field intensity is equal to the free space field as long as the planes are well within line of sight. For greater distances, the field intensity decreases more rapidly and the assumption of free space transmission is no longer accurate. The maximum distance for which a line-of-sight path exists over a smooth spherical earth can be obtained from Figure 3*. In this chart the left-hand scale represents the height of one plane, the right-hand scale represents the height of the other plane, and the middle line represents the line-of-sight distance. The line-of-sight distance is obtained by laying a straight edge between the proper points on the left-hand and right-hand scales and by reading the distance at the intersection of the straight edge with the center line.
Example: Two planes are separated by a distance of 100 miles. The first plane is at an elevation of 10,000 feet and the second is at an elevation of 5,000 feet. The field intensity at one plane produced by a transmitter on the other plane radiating 20 watts from a dipole is obtained as follows:
A straight line on Figure 3 from 10,000 feet on the left-hand scale to 5,000 on the right-hand scale indicates a line-of-sight distance of 240 miles; so the planes are well within the line of sight at 100 miles. On any of the charts showing the
*The distances shown are slightly greater than the true optical line of sight, since an effective earth’s radius of 4/3 of the true earth’s radius is assumed to account for some refraction in the lower atmosphere.

free space field, the value for 100 miles is about 62 decibels above 1 microvolt per meter for 1 kilowatt radiated. The estimated field is 62 — 17 — 45 decibels above 1 microvolt per meter, where — 17 decibels is the power correction factor for 20 watts from Figure 1.

c. Ground-to-Ground Transmission. The field intensity for ground-to-ground transmission for antenna heights of less than 200 feet can be determined by the following procedure. The field intensity at reference height (0 feet for vertical polarization and 10 feet for horizontal polarization) is obtained from the first chart in each section. Add to this value the height gain factor for the transmitting antenna and the height gain factor for the receiving antenna as shown on the second sheet in each section. Finally, the power correction shown on Figure 1 and the antenna correction shown on Figure 2 should be added.
When one antenna is higher than 200 feet, the field intensity for ground-to-ground transmission can be determined in the same manner as discussed above for plane-to-ground transmission. When the lower antenna is at reference height, the field intensity is obtained directly from the chart for the desired polarization, type of ground, and frequency from the curve for the height of the higher antenna. When the lower antenna is above reference height this value should be increased by the height gain correction factor.

Figure 3. Maximum distance for line-of-sight path.

Example: A ground station operating on 30 megacycles radiates 50 watts from a 3-element vertical directive array located at a height of 100 feet. The gain of the 3-element antenna is assumed to be about 6 decibels referred to a dipole. The receiving antenna is mounted on a 50-foot mast and is 50 miles from the transmitter. The intervening terrain is assumed to be good soil. The estimated field intensity is
0 + 10 4- 5 — 13 -|- (6-]-3) =11 decibels above 1 microvolt per meter.
The first factor is the field intensity at 50 miles for both antennas at ground level. The next two factors are the height gain values for the 100-foot antenna and the 50-foot antenna, respectively. The fourth factor (—13 decibels) is the power correction for 50 watts from Figure 1. The last factor is the gain of the 3-element array over the reference antenna and consists of two terms: (1) a 6-decibel gain of the array over a half-wave dipole and (2) a 3-decibel gain of a half-wave dipole over the reference antenna (whip at ground level) as indicated in Figure 2.
In the alternate method the received field intensity is
13 4- 5 - 13 4- 6 — 11 decibels above one microvolt per meter.
The first factor (13 decibels) is obtained from the chart for vertical polarization, good soil, and 30 megacycles for a height of 100 feet at a distance of 50 miles. The second factor (5 decibels) s the height gain due to raising the lower antenna

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from reference height to 50 feet. The third factor (—13 decibels) is the power correction factor for 50 watts. The fourth factor is the gain of the 3-element array over the reference antenna (dipole).

3. EFFECTS OF HILLS, TREES, AND JUNGLE
a. Effect of Hills. The effect of hills can be divided into two parts, (1) the effect of placing antennas on hills, which is discussed in a later paragraph, and (2) the effect of hills and other obstacles in the transmission path.
Under certain conditions the field intensity behind a hill may be greater than would be obtained if the terrain between the antennas were level ground, but, in general, it may be assumed that intervening hills cause a loss in field intensity. This loss in decibels should be subtracted from the field intensity over a smooth spherical earth as obtained from the charts in this handbook. This additional loss, called shadow loss, is ordinarily small at frequencies below a few megacycles. but it may be 20 to 30 decibels or more at the higher frequencies.
An estimate of the probable magnitude of the shadow loss can be obtained from Figure 4 by the folio wing’procedure*:
1. Draw an approximate profile of the straight line path between the pro-
*This method is based on the theory of diffraction of plane waves over a knife edge

posed locations of the two radio terminals, using the elevations obtained from contour maps.
2. On this profile, draw a triangle similar to the one shown at the top of Figure 4. This triangle is formed by a line joining the base of the transmitting antenna with the base of the receiving antenna and a line from each antenna tangent to the hill that blocks the line of sight from that location.
3. From this triangle note: (1) the height H from the base of the triangle to the apex and (2) the distance Di along the base of the triangle from the nearer terminal to the foot of the perpendicular line H.
4. On Figure 4 draw a straight line from the point representing Di on scale
1 through the point representing H on scale 2 and extend this line until it crosses scale 3.
5. Draw a second straight line from the intersection of the first line with scale 3 through the point representing the frequency on scale 4. Extend this line until it crosses scale 5 and read the shadow loss at this intersection.
The example shown in Figure 4 indicates a shadow loss of nearly 10 decibels at 30 megacycles due to an “equivalent” 1000-foot hill located 10 miles from the nearer terminal. For a 1000-foot hill only one mile from the nearer terminal the expected shadow loss at 30 megacycles is about 19 decibels.
The above method considers only the straight line path between the trans-

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Figure 4. Shadow loss.

mitter and the receiver. In practice, reflections from hills near the straight line path may have an appreciable effect. In some cases a stronger signal is obtained by way of devious routes such as river valleys or mountain passes than can be expected by diffraction over the straight line path.

b. Effective Antenna Height. The effective height of a dipole or other type of balanced antenna located above ground which is level for the first half mile or so in the direction of the other antenna is the height of the center of the antenna above the ground level at the base; that is, it is usually about equal to the height of the mast. The effective height of an antenna on the edge of a precipice (falling off in the direction of the other terminal) can usually be taken as the difference in elevation between the center of the antenna and the bottom of the precipice. In the intermediate case where the antenna is placed on a hill sloping downward in the direction of the distant terminal, the effective height of the antenna depends on the steepness and uniformity of this slope. In general, the effective height of an antenna placed on a hill is greater than the mast height but is ordinarily less than the mast height plus the height of the hill.

c. Effect of Trees and Jungle Conditions. The attenuation due to trees is less for horizontal polarization than for vertical polarization, except at frequencies

xvm

above 300 to 500 megacycles where it tends to be independent of the type of polarization. For horizontal polarization the average loss due to moderately thick trees is negligible at 30 megacycles and may be 1 or 2 decibels at 100 megacycles. For vertical polarization the corresponding loss is 2 to 3 decibels on the average at 30 megacycles and 5 to 10 decibels at 100 megacycles. These losses are doubled when both antennas are located in woods.
With both antennas in clearings so that each is more than 200 or 300 feet from the edge of the woods, the attenuation due to trees is small for vertical polarization as well as for horizontal polarization. With vertical polarization, there may be large and rapid variations in field intensity within a small area, due to reflections from nearby trees and buildings. These fluctuations may occur even over line-of-sight paths when trees or buildings are within a few hundred feet of the direct transmission path.
In jungles or in swamp land with heavy undergrowth, considerable attenuation for ground wave transmission is to be expected with horizontal as well as with vertical polarization. The attenuation due to the jungle can be minimized by locating the antennas in clearings and by raising the antennas near or above the top of the jungle. An alternate method of jungle communication is to use sky-wave transmission. In this case, half-wave horizontal antennas are best for distances up to 100 or 200 miles, and a frequency in the range of about 2 to 8 mega

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cycles should be used, the optimum operating frequency depending upon the time of day and the season of the year.

4. LIMITATIONS
The field intensities estimated by the above-described methods are for the “ground wave” and do not include components of the signal that may be reflected from the ionosphere. In the frequency range of about 2 to 20 megacycles, which is frequently called the “short-wave range,” there are reflections from the ionosphere which provide sky-wave transmission over greater distances than are feasible with ground waves. The sky-wave field intensities vary considerably, depending on the frequency, time of day, latitude, and season of the year. Information on the maximum and minimum frequencies to use for sky-wave transmissions can be obtained from the monthly bulletins prepared by the Interservice Radio Propagation Laboratory of the National Bureau of Standards and distributed by the Communication Liaison Branch, Plans and Operations Division, Office of the Chief Signal Officer, War Department, Washington 25, D. C.
The ground-wave data given in these charts are limited by several assumed ideal conditions, since it is impossible to take into account all factors affecting radio propagation. The principal assumptions are a smooth spherical earth with

uniform ground constants, and a standard atmosphere in which the dielectric constant of the air varies uniformly as the height above the earth increases. The average bending of radio waves due to refraction in the standard atmosphere is included by assuming that the effective radius of the earth is increased to 4/3 of its actual value. Under other atmospheric conditions the field intensity at distances beyond the line of sight may be greater or less than the values shown on these charts. This dependence of radio propagation on the weather is small at frequencies of less than about 30 megacycles, but its importance increases as the frequency increases.
The charts may be in error at short distances when the distance between antennas is less than one or two wavelengths or when it is appreciably greater than the horizontal distance along the ground.
The ground constants used in these computations are shown in the table on the next page.
The information on the effects of hills, trees, and jungle agrees reasonably well with the available experimental evidence. However, the evidence is meager and further experience may indicate some modifications of these views.

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TABLE OF GROUND CONSTANTS

CONDUCTIVITY DIELECTRIC
GROUND CONDITION MHOS PER METER E.M.U. CONSTANT
Sea Water 4. 4x10-“ 80
Good Soil .02 2xl0'13 30
Poor Soil .001 10“ 4

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