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ANTENNA BEAM ELEVATION ANGLE FOR CONTROL

OF TROPOSPHERIC INTERFERENCE BETWEEN SPACE SYSTEM

EARTH TERMINALS AND TERRESTRIAL STATIONS

S. G. LUTZ AND W. J. HARTMAN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical ^ote /SO

Issued August 25, 1963

ANTENNA BEAM ELEVATION ANGLE FOR CONTROL OF

TROPOSPHERIC INTERFERENCE BETWEEN SPACE SYSTEM

EARTH TERMINALS AND TERRESTRIAL STATIONS

S. G. Lutz^ Hughes Research Laboratories

Malibu, California and

W. J. Hartman

Central Radio Propagation Laboratory

National Bureau of Standards

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington, D.C. 20402

price 15*

TABLE OF CONTENTS

1. Introduction 1

2. Terminal Beam Elevation and Tropospheric Interference

in Satellite Communication 1

3. Antenna Patterns 2

4. Methods for Calculating Transmission Loss with Elevated Beams 3

5. Variability of Tropospheric Scatter Loss 4

6. Transmission Loss Requirements for Interference Protection 5

7 . Calculated Re suits 6

8. Recent Results 11

9. Conclusions 12

10. References 12

ANTENNA BEAM ELEVATION ANGLE FOR CONTROL OF TROPOSPHERIC INTERFERENCE

BETWEEN SPACE SYSTEM EARTH TERMINALS AND TERRESTRIAL STATIONS

S. G. Lutz' and W. J. Hartman

High gain antennas, sensitive receivers and high power transmitters which are used

at the earth stations of space communications systems complicate the problem of sharing

frequencies with other surface services. However, because of the dependence of beyondthe-horizon tropospheric propagation on the scattering angle, it is possible to offset the

gain of the terminal antenna by elevating the beam,

Early studies of frequency-sharing assumed that the beam of the earth station was

elevated "sufficiently" to increase the transmission loss to that which would be obtained

if the actual antenna were replaced by an isotropic antenna. This paper studies the question of what constitutes a "sufficient" elevation angle, as described above, for the particular conditions of a smooth earth, 30-foot antenna heights and several combinations of antenna sizes. Some results are also given when there is an elevated horizon at the earth

terminal end of the path.

1. Introduction

It is necessary to predict the transmission

loss between two terminals in order to predict

whether frequency sharing between two communications systems is possible. When one of the

systems is a space communication system, the

prediction of the transmission loss is frequently

complicated because of possible different orientations of the earth station antenna. Early

studies [Lutz, 1961; Firestone, Lutz, and Smith,

1962; EIA-FCC, I960] of the possible tropospherically propagated interference between the earth station of space communications systems

and other surface radio services using the same

frequencies assumed that the antenna beam at the

earth station would be elevated "sufficiently" to suppress the power propagated via the main beam,

and thus justify the assumption of an isotropic

earth station antenna. It is clear that the required

computation time is greatly reduced if the trans- mission loss is calculated only once for an omnidirectional antenna rather than the many times

required for the different orientations of a high

gain antenna. This paper compares the expected

value of the tropospheric scatter transmission

loss via the elevated main beam of a 60-foot

* Hughes Research Laboratories, Malibu,

California

earth terminal antenna to that for an omnidirectional terminal antenna with arbitrary gain

relative to isotropic. For 10-foot, 6-foot, and

isotropic surface antennas, calculations are

given for beam elevation angles to 15 , path

lengths to 350 miles, and for frequencies between

1 Gc/s and 10 Gc/s. The results given here are

for the particular conditions of a smooth earth

and antenna heights of 30 feet at both ends of the

path, and have been obtained by consolidating and

extending the Hartman and Decker [ 1961 ] study.

Certain general features are exhibited,

such as the increase with distance of the elevation angle required to increase the transmission

loss via the main beam to that for an isotropic

antenna, and to values 10 db, and 20 db greater

than that expected for an isotropic antenna. Because elevated horizons are frequently advocated for reducing interference, some comparisons are made between paths with one elevated

horizon and smooth earth paths.

2. Terminal Beam Elevation and Tropospheric

Interference in Satellite Communication

Signals from early solar-powered satellites will be extremely weak,' consequently, their

earth stations will require high-gain antennas

and ultralow-noise receivers. DeGrasse, et al,

[ 1959 ] have shown that the use of a heliumcooled maser and a horn-reflector antenna
results in operating noise temperatures of less

than 30" K, which corresponds to 20 db less

noise than that from most conventional micro- wave receiving systems. However, such low

noise is achieved only when the antenna beam is elevated sufficiently (5° to 10° ) above the hori- zon to reduce the thermal noise from the earth,

which enters the antenna's main beam and near

side lobes. In addition, elevating the antenna

beam shortens the portion of the propagation path

through the earth's atmosphere, and thus

reduces the atmospheric absorption of the incoming signals and the atmospheric noise.

Because of this, early satellite communication

systems will probably avoid beam elevations

smaller than 5° to 10° . The maximum distance

between the earth stations of a satellite communication system will be limited correspondingly.

When satellite repeaters of sufficiently

high power can be used, low-noise reception will

be less essential,. Terminal antenna beams

could then be lowered to increase the maximum

distance between terminals if it were not for the probability of tropospherically propagated interference with microwave radio relay or other

surface radio stations in the vicinity of the terminal [Lutz, 1961; Firestone, Lutz, and Smith,

1962 ]. Such interference would be greatly increased by the gain of the terminal antenna if its beam were lowered toward such a station.

However, tropospheric transmission loss in- creases as the scattering angle increases, and

the effective scattering angle is increased by

the beam's elevation angle. When elevated

sufficiently, or when the beam is directed sufficiently away from the remote surface station in azimuth, the terminal antenna can be considered

to be equivalent to an isotropic antenna with

respect to tropospheric scatter propagation of interference; i.e. , the path antenna gain is reduced to a value equal to that expected for an

isotropic terminal antenna.

3. Antenna Patterns

Few three-dimensional patterns are available for large space communication antennae,

although a few azimuth directivity patterns are

available, such as that of the horn-reflector

antenna used for the ECHO experiments

[Crawford, Hogg, and Hunt, 1961 ]. Even with

the smaller microwave antennas, commonly used

in line-of-sight relay systems, usually only

azimuth directivity patterns are available, though

generally for both polarizations. Thus, it is not

yet well enough known how rapidly the gain toward

the horizon drops off as the beam of an antenna

is elevated above the horizon.

Some useful generalizations can be extracted from the available information for parabolic or horn-type reflectors at microwave

frequencies, although, as with all generalizations,

there will be some exceptions. The patterns for

these antennas may be considered to consist of essentially three parts: sector 1, the main lobe; sector 2, the near side lobes, which are 20 db or more below the main lobe, but still exhibiting a

definite lobe structure and gain above an isotropic antenna; and sector 3, the remainder of

the pattern, which may be described as having no well-defined lobe structure. For tropospheric scatter calculations,

the relatively strong sector 2 sidelobes generally

do not contribute significantly to the total power

received, since their gain falls off more rapidly

with their angle from the beam than transmission

loss via the main beam increases as a result of correspondingly increased scattering angle. For

example, a 60-foot parabola has its first sidelobes only a half degree from the beam center at

3. 8 Gc/s, and their gain would be at least 20 db

less than that of the beam. Measurements of tropospheric scatter loss versus beam elevation

do not show the sidelobe structure near the main

beam, as is observed on line-of-sight or just

beyond the horizon diffraction paths.
For sector 3, the scatter transmission

loss may be calculated as the loss using an isotropic antenna in place of the directional antenna,

minus the gain relative to isotropic of the perti- nent part of sector 3. Because of the foregoing, it is often

assumed for interference studies that the threedimensional pattern envelope is a solid of revolution, generated by an azimuth pattern envelope;

or by some usefully simple approximation to such

an envelope. One such idealization, used by

Electronic Industries Association Technical

Committees for Microwave Interference Studies

[EIA, I960], is the three-sector keyhole-shape

pattern, whose far-sidelobe sector is considered

isotropic. The three-dimensional version of this pattern resembles two cones projecting from a

unity-gain sphere. Figure 1 shows two such

HORIZONTAL

10-ft SURFACE COMMUNICATIONS ANTENNA

ELEVATED

60-ft SATELLITE SYSTEM ANTENNA

GROUND PLANE

SECTOR 3 SPHERES"

(ISOTROPIC OR LESS)

Figure 1: Three-dimensional keyhole -shape

antenna pattern envelope

patterns illustrating a surface microwave antenna

directed horizontally toward a satellite system's

earth antenna [EIA, I960 ]. The beam of the

latter is directed above the microwave antenna

at some minimum elevation angle which exposes

the isotropic sphere horizontally. The geometry

of this illustration suggests comparing the tropospheric transmission loss via the elevated beam

with that via an isotropic antenna, thereby determining the beam elevation angle for loss equality.

Such an analysis and comparison are made in

this paper.

4. Methods for Calculating Transmission Loss

with Elevated Beams

Both this study and the prior study

[ Hartman and Decker, 196 1 ] use a modification

of the Hartman and Wilkerson [ 1959 ] investigation of path antenna gain intended for tropospheric

communication paths, where both antennas are

directed at the horizon in the great-circle path

direction. Only the upper half of each of the beams clears the horizon, and the power from

the lower half is assumed to be reflected into the

upper half. For the present problem, the entire

elevated beam from an earth station antenna

clears the horizon, necessitating the modification. Results of the Hartman and Decker [ 1961 ] calculations were presented as a large collection

of curve families, most of which showed Time

Block II (winter afternoon) transmission loss versus beam elevation of one antenna for 50-mile

path length increments between 100 and 350

miles at one of four frequencies (1, 2, 6.5, or

10 Gc/s) and for the antenna combinations, 60-ft. and 10-ft. , 6-ft. , or isotropic, 30 feet above

smooth earth with a surface refractivity value

N =301. The calculations require the estimation of the smooth-earth basic median trans- mission loss which was obtained from recent NBS prediction methods [Air Force Technical

Order, 1961; Rice, Longley, and Norton, 1959].

In decibels,

L = Lb- Gp'

where L, is the transmission loss, L, the basic transmission loss between isotropic antennas (the path loss), and G is the path

antenna gain, given in terms of the free space

antenna gains of the transmitter and receiver

G = G^ + G - GT , p t r L,

and where G is the loss of path antenna gain as computed by the modified Hartman and Wilkerson

[ 1959 ] method. Note that the calculation of
Dms also involves the beam elevation angle l(j

in determining the effective scattering angle 0. For parts of the present paper, the

Hartman and Decker [ 1961 ] calculations have

been extended from \\i = 9° maximum to i|j = 12° and 15° , also path lengths down to 50 miles are

included. The validity of tropospheric scatter predictions for such short paths is questionable

because, at distances less than 100 miles,

diffraction becomes more important and eventually becomes dominant. No diffraction calculations are included here, however, because if diffraction is dominant when the beam is on the horizon, the beam elevations given are adequate

to offset the effects of the gain of the main beam

relative to the sidelobes.

5. Variability of Tropospheric Scatter Loss

It is well known that the hourly median

values of transmission loss over tropospheric

paths are highly variable. The NBS predictions are divided into two parts; predicting the median

of the hourly medians for winter afternoon hours

when the upper air is well mixed and predicting an empirically derived distribution of the hourly

median values about this median. These distributions are presented as curve families depending

on the scattering angle 9 and a percent of time

p during which the signal level will be exceeded

by p% of the hourly medians during each of

eight time blocks (see table I) and during all hours of the year. Based on extensive measurements, these curves include and allow for some

ducting and other propagation abnormalities.

For interference control, one is interested in low losses exceeded by, for example, 99.9% of

the hourly medians and corresponding to a signal

level exceeded for p = 0.1% of the hours from

these curves. Figure 2 shows a comparison of p = 0.1%

variability curves for several time blocks. All

such curves show the greatest variation near

9 = 10 mr, which generally corresponds to a

Table I Time Blocks

6 AM 1 PM 6 PM 12 PM

to to to to

1 PM 6 PM 12 PM 6 AM

November I II IU VIII

to April

May to IV V VI VII October

-BLOCK 7 (SUMMER, 12-6 AM)

BLOCK 4

-ALL HOURS, ALL YEAR (ALL BLOCKS)

Figure 2: Variability corrections as applied to the Time Block II median to obtain losses

exceeded by 99.9% of the hourly medians of the time blocks shown.

distance at which there is the most variable and

uncertain combination of diffraction and scatter propagation. In this range, the variability is

least for Time Block II and greatest for VII, reaching 56 db. Ducting is most probable in the early morning hours, and especially in summer,

when the upper air may be highly stratified and

undisturbed. Time Blocks VII and VIII tend to over-weigh the all-year curve, considering that

(non-military) communication is lightest during

these hours. Except for Blocks VII and VIII, Block IV is the most variable and reaches the same maximum as the all-year curve. Hence,

one can use the all-year curve with the assurance that it is typical of the worst of the heavily

used Time Blocks.

For further clarification, figure 3 shows some forward scatter path loss versus distance curves. Curve 1 is the basic (Time Block II)
140 50 100 150 200 250 300 350 400 450

DISTANCE, MILES Figure 3: Illustrations of the variability of tropospheric path loss for 30-ft-high isotropic

antennas, 6.5 Gc/s, smooth earth, with N = 301.

s

median loss which is predicted by NBS methods.

Curve 2 was corrected by use of the all-year

p = 99.9% curve and shows the maximum probable losses for which one might design tropospheric communication circuits. Curve 3 represents losses exceeded by 99.9% of the hourly medians on winter afternoons. Curve 4

shows corresponding losses during the worst

times, Block VII, when losses may fall toward

free -space values. An all-year 99.9% curve would lie between curves 3 and 4. There are several reservations concerning the applicability of these loss -variability

corrections to the surface interference coordination problems of satellite communication, most

of which reflect the prior research motivation

toward tropospheric communication.

1. The CRPL variability curves are supported by long-term measurements over more than two hundred paths, but at relatively

low frequencies, predominantly below 1 Gc/s.

More adequate experimental confirmation is needed of their applicability at probable satellite communication frequencies in the 3.7- to 8.4-Gc/s range, and particularly for elevated

beams.

2. Ducting the similar abnormal propagation which these curves allow for is improbable with the elevated beams of earth-station

antennas, except from low sidelobes.

3. The use of 6 as the independent

variable for loss variability is less applicable at the shorter distances and lower beam angles.

As an example of 3 above, consider two

ideal (tight beam, no sidelobe) antennas separated by a path for which 9 is only 2 mr. From

figure 2, the 99.9%, Time Block II, loss will be

5 db below the median. If one or both beams

are elevated, the scatter angle will increase,

and as a consequence the transmission loss will

increase. When 6 = 10 mr, the median trans- mission loss would increase about 20 db, but

figure 2 indicates a 32-db loss reduction 0.1%

of the time, hence 7 db less than with the beams

horizontal. This does not agree with available

data. Despite these observations, the CRPL

variability curves seem to be the best thus far

available. Consequently, the all-year variability curve will be used in this study.

6. Transmission Loss Requirements for

Interference Protection

Except in very specific cases, it is impossible to state the required value of trans- mission loss which will suppress interference to acceptably low and infrequent levels. Such

values depend not only on the transmitter and

receiver characteristics, modulation characteristics of both signals, etc. , but also on often

inadequately defined or agreed upon criteria of acceptability. At least two methods [Firestone,

Lutz, and Smith, 1962; Curtis, 1962 ] have been

used to determine transmission loss requirements and consequent smooth-earth separation

distances. In general, such losses should

exceed, for 99.9% of the hourly medians in a

year, values in the 150- to 180-db range, based
on presently probable system characteristics.

Lower values in this range may be used with

microwave relay stations, whereas the higher

values may apply between co-channel earth stations. The all-year median losses range from

20 to 40 db higher than the 99.9% values,

depending on the variability correction. Note,

however, that these transmission losses are the difference between the basic loss (isotropic

antennas) and the path antenna gain. With anten- nas horizontally directed toward each other, the path antenna gain could approach 100 db, thus

becoming more important than any probable

difference in required loss. For a given basic

transmission loss, any path antenna gain in- creases the required path length correspondingly.

7. Calculated Results

For reference purposes, figure 4 shows

280

260

HOURLY MEDIAN LOSS, WINTER AFTERNOONS

LOSS EXCEEDED 99.9 7o HOURS OF YEAR

50 100 150 200 250 300 350 400 450

PATH LENGTH, MILES Figure 4: Basic transmission losses; 30-ft-high

isotropic antennas, smooth earth, and N s =301.

Comparison of winter afternoon (Block II) medians with losses exceeded by 99.9% of the

hourly medians, all year.

basic median transmission losses and 99.9%

all-year losses between isotropic antennas, 30

feet above a smooth earth, with N = 301. One

should note that the median loss values below

100 miles assume only forward scatter propagation, neglecting diffraction propagation, which for

the stated assumptions may be neglected. However, at the shorter distances, only slight

changes in antenna heights or terrain profile are necessary to cause diffraction to dominate. Also,

if the primary consideration is to minimize

interference, earth stations should not be located on smooth earth and certainly not on hilltops.

Such stations should be shielded by surrounding

terrain while avoiding obstacle gain.

Figure 4 also shows that, beyond the horizon, tropospheric interference would be a minor

problem were it not for its variability or for

possible path antenna gain. In fact, Curtis [1962]

showed that line-of-sight separations are sometimes adequate in low-sidelobe directions.

Figure 4 also shows how increasing the frequency

increases the path loss. The importance of keeping the required loss low and the frequency

high is indicated. Seventy miles provides 160 db

at 4 Gc/s, whereas over 160 miles would be

needed for 180 db at 1 Gc/s.

Figure 5, which is a modification of the

240

180

DISH DIAMETER, ft 120 85 60

-I U 30

1 1 1 1 1 1 1

I 23456789 10 FREQUENCY, Gc Figure 5: Transmission loss for an elevated 5 mr beam compared with basic transmission loss

for winter afternoon medians and remote antenna isotropic.
Hartman and Decker [ 1961 ] curves, shows

median transmission loss versus frequency and

elevation angle + of an antenna beam of constant beamwidth (5 mr at 3 db), the second antenna

being isotropic. This beamwidth would require

a 120-ft antenna aperture at 2 Gc/s, but only

30-ft at 8 Gc/s. The dashed curves have been

added to permit comparison with propagation

between isotropic antennas. At 100 miles, elevating this beam about 6 cancels the effect of antenna gain. At 350 miles, however, an elevation near 12° would be required. This indicates

that the beam elevation required to suppress the gain increases with distance.

Calculations for 1 Gc/s and 10 Gc/s will

be used in what follows because these frequencies

bound the presently accepted "window" of usefulness for satellite communication. Within this window, the fixed microwave bands from 3.7 Gc/s

to 8.5 Gc/s are most apt to be used because of the greater difficulty of sharing frequencies with

other services. Relatively complete calculations

also have been made at 2.0, 4.0, and 6.5 Gc/s,

and the values are consistent with those at 1.0 and 10.0 Gc/s. In all of the remaining calculations, it is assumed that the earth antenna is a 60-foot parabolic dish.

Figure 6 shows the expected median

transmission loss as a function of the elevation

angle ip for 50-mile distance increments to 350 miles, at 1 Gc/s with the microwave antenna

isotropic. For comparison, the superimposed

curve farthest to the left shows the loss between

isotropic antennas at these distances. Hence,

the intersection values of \\> are the beam elevations at which the increased path loss via the beam reduces the path antenna gain to zero. At

this elevation angle, however, the antenna still will have a path antenna gain of some uncertain

value because of the additional propagation into

its sidelobes. Hence, the additional two superimposed curves intersect the beam-loss curves

240

230

1 i i i 1 1 1 !

( 350 MILES ~

., 300 X, 250 200 £ 220 - 150

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WINTER

AFTERO

ooo 7 //

'J" ^^IS0/IS0+l0db

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150

ISO

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140

1" D MILES V

30 ft 130 i 1 1 1 1 1 1

I 2 4 6 8 10 12 14 16 18 20

EARTH ANTENNA BEAM ELEVATION if/ , DEGREES Figure 6: Effect of antenna beam elevation angle

+ on median transmission loss; isotropic

remote antenna and 60 -ft steerable antenna at

1 Gc/s.

at the angles required for an additional loss of

10 or 20 db. Figure 7 shows similar curves, applicable

when the microwave antenna is directed horizontally at the earth station. The solid curves at 50-mile increments are for a 10-foot aperture,

whereas the two dashed curves show the comparison with a 6 -foot antenna, at 100 and at 350

miles. The loss difference is approximately the

4.3 db antenna gain difference between a 6 -foot

and 10-foot antenna. The superimposed curve

provides comparison with an isotropic earth

antenna, as before. These transmission loss

values are reduced from those shown in figure 6 by somewhat less than the gain of the micro- wave antenna. Figure 8 shows families of curves at

10 Gc/s , corresponding to figures 6 and 7. The

iso-iso locus shows elevation angles 4> , which

are approximately the same as in figure 6, but

corresponding to larger losses due to the higher
100

30ft J I I I I

I I 2 4 6 8 10 12 14 16 18 20 EARTH ANTENNA BEAM ELEVATION \f/, DEGREES

Figure 7: Effect of antenna beam elevation angle

i)j on median transmission loss; 10-ft-remote

antenna, and 60-ft steerable antenna at 1 Gc/s.

280

-QT3

260

240

220

200

180

160

140

120

i i i r ISO/ISO iso/iso+iodb

ISO/ISO+20db

T~

„-- 350 MILES, _- 300

- 250

200

ISO-60' CURVES

IO'-60' CURVES

2 4 6 8 10 12 14 16 18 20 EARTH ANTENNA BEAM ELEVATION,*//, DEGREE

Figure 8: Effect of antenna beam elevation angle

i|j on median transmission loss at 10 Gc/s.

frequencies. The 10-foot iso locus, however,

exhibits an increase in ij; with distance although

with a smaller increase in the loss when compared with the curve in figure 7. This frequencydependence is clarified by using the representations shown in figure 9 and figure 10. Both

270

260

COo 250

| 240

Li. 5

230

220

210

5 200

190 J_ 9 12 15

<//, DEGREES

18

Figure 9: Effect of antenna beam elevation angle on median transmission loss at 150 miles for

five frequencies; remote antenna isotropic.

CDa 230

CO8 220 < 210 Q

U

5 200

1 190

180

170

160 _i i i_ 12 15

\ft DEGREES

20 23

Figure 10: Effect of antenna beam elevation

angle on median transmission loss at 150 miles

for five frequencies; 10-ft-remote antenna.

figures show transmission loss versus elevation

angle for a distance of 150 miles and several

frequencies. Figure 9 applies when the micro- wave antenna is isotropic, and figure 10 when

the microwave antenna is a 10-foot dish. The

cross marks are plotted at losses equal to 0,
10, and 20 db relative to the transmission loss

when an isotropic earth antenna is used. In figure 9, the iji's are independent of frequency

within the limits of computational accuracy

(slide rule, curve reading, and plotting). Figure

10 shows that the corresponding equality-^'

s

become decidedly frequency-dependent when the 10-foot microwave antenna is directed at the earth station. The reason is that the gain of

this second antenna increases with frequency.

This gain increase is partly offset by the gain

loss term G . Curves such as these two were prepared

for distances in 50-mile increments, and the cross mark values of 41 were used to plot

figures 11 and 12. Figure 11 shows l)j versus

150 200

PATH LENGTH, MILES

300

Figure 11: Antenna beam elevation versus dis- tance for median loss equal to or 10 or 20 db

greater than that between isotropic antennas; remote antenna isotropic.

path length, at 1 Gc/s and 10 Gc/s, for beam

losses greater by 0, 10, and 20 db than losses

to an isotropic earth antenna, from an isotropic

microwave antenna. Figure 12 shows the marked

frequency-dependence when the microwave antenna is beamed at the earth station. The 1 Gc/s

curves are not much different from those in figure 11, but those for 10 Gc/s diverge to much

higher ijj's. However, both figures lead to an important conclusion. If the interference path

10 Gc/s

I Gc/s

_L 50 100 150 200 250 300

PATH LENGTH, MILES Figure 12: Antenna beam elevation for which

the median losses are equal to or 10 or 20 db

greater than if the terminal antenna were isotropic for a 10-ft remote antenna beamed

toward the terminal.

length is relatively short (50 to 100 miles) from

a low-power and relatively insensitive micro- wave station using relatively small antennas, a "reasonable" minimum elevation of the earthantenna beam will suppress its interference-gain

to that if an isotropic earth antenna were used.

Such a minimum elevation is reasonable because

it is about that (5 to 10° ) which will be used,

at first, to control atmospheric and earth-noise

degradation of maser receiver performance.

Interference at greater distances could be caused

by (co-frequency) high-power radar, tropospheric

communication stations, or other space communication earth stations having large antennas

beamed horizontally at the earth station concerned. The beam elevation necessary to suppress such interference to isotropic level

could become unreasonably large (i)j > 15° ) because such an increase in minimum elevation

angle could greatly reduce the satellite's earthcoverage or the maximum distance between

earth terminals.

Figures 13 through 16 are the same presentation as figures 6 through 12, except that,

instead of median losses, those exceeded by

99.9% of the hourly medians per year are used.
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11)11 -

_

flO 1 1 1 1 1 1 1 1 1 1 i 2 4 6 8 10 12 EARTH ANTENNA BEAM ELEVATION <J> , DEGREES

Figure 13: Effect of beam elevation angle on transmission loss exceeded by 99.9% of the

hourly medians; 1 Gc/s.

260

240

cr 220 O

X

UJI

1- 200

U-o

5

s* (»

180

(T) <J> >-mQ IRO UJo

UJ

UJo

X

UJ 140 -O•o

CO

CO 120 O

100

80

i r "i i i

1 r "i

1 r

Gc/S

ISO-60' CURVES

IO'-60' CURVES

2 4 6 8 10 12 EARTH ANTENNA BEAM ELEVATION )J> , DEGREES

Figure 14: Effect of beam elevation angle on transmission loss exceeded by 99.9% of the

hourly medians; 10 Gc/s.

!2l4

UJKa 12 o

10

i 4

L eeom=L iso + 20db

'-BeanrrLiso+IOcU'

. I Gc/S-^. 10 Gc/s

L Beom= Lrso

ISO.

50 100 150 200

PATH LENGTH, MILES 250 300

Figure 15: Elevation angle versus distance for the 99.9% level transmission loss equal to or arbitrarily greater than that using an isotropic

terminal antenna; remote antenna isotropic.

18

16-

cnW

UJ

Q

-£10-

yeUJl4|-

Cfl 2

0,

,=L,.„+IOct

100 150 200 PATH LENGTH, MILES 300

Figure 16: Elevation angle versus distance for the 99.9% level transmission loss equal to or

arbitrarily greater than that using an isotropic

terminal antenna; 10-ft remote antenna.

These losses are lower by 20 to 40 db, but the

shapes are changed only to the extent that the

variability correction changes with 9 (the scatter angle, or "angular distance") as was

shown in figure 5. Hence, the shape -change and

loss -reduction is most noticeable for short paths

and very low uVs. The preceding results have all been based

on the assumption of a smooth earth. If terrain

is used to shield the earth terminal from interference, the beam elevation required to increase

10
the loss via the main beam to that for an isotropic earth terminal antenna will increase.

Figure 17 shows two of the curves from figure 12

ISO 200 250 PATH LENGTH, MILES

350

Figure 17: Elevation angle versus distance for

the median level transmission loss equal to that using an isotropic terminal antenna, for smooth

earth conditions, and for a 200-ft obstacle

located 5 miles from the terminal end; remote

antenna isotropic, 60 -ft terminal antenna.

and the corresponding curves for a path with a smooth 200-foot high obstacle located 5 miles

from the earth terminal. It is assumed that this obstacle does not cause obstacle gain, but only

introduces path asymmetry and increases the scattering angle by 7.5 mr. These curves show

the loss equality angle increasing significantly

from the smooth earth to the elevated horizon

path with a substantially larger frequency dependence over the latter paths. However, the basic

loss is larger over the obstacle path than over

the corresponding smooth earth path ( ~ 13 db

at 50 miles and ~ 5 db at 300 miles for both frequencies for the example used here). Thus, the need for caution in assuming a smooth earth for arbitrary paths is illustrated.

8. Recent Results

Thus far, there has not been adequate

experimental verification of these predicted

effects at high beam angles. The results of one

early series of measurements between Boulder

and Haswell appeared as figure 3 of the Hartman

and Decker paper [ 1961 ]. The good agreement

with the predicted curve may result from the relatively low frequency, 409.9 Mc/s, for which

the loss in path antenna gain corrections were

relatively low. Similar experiments near 10 Gc/s

would be more critical but would be correspondingly more difficult to make because of the currently available transmitter powers and

antennas.

Recently, a program has been prepared

for a large digital computer to numerically integrate Hartman and Wilkerson's [ 1959 ] equation

3.2 with a Bessel function approximation for the

antenna pattern included in the integrand for

arbitrary orientations of the antennas. There

are several indications that the computer method

gives better results even though both methods

use the same formulas for evaluation. First,

in the Hartman and Wilkerson method, the final form for the calculations was derived empirically from many graphs to facilitate graphical

presentation in a report. Because of the intended

application of this report [ Hartman and

Wilkerson, 1959 ], the calculations were restricted to values of the parameters corresponding to conventional paths. The programmed

method is not subject to these restrictions. For

conventional paths (i. e. , low antenna heights

and antennas directed at the horizons) the two

methods agree within a few db. Unfortunately,

for situations where the two methods give

differences of 10 db or more, very few data are

available for comparison. These data agree

best with the programmed method, although the significance of this agreement is questionable.

At the higher frequencies/these new calculations show higher values of G for elevated

beams. For example, with 10 -foot and 60 -foot antennas at 10 Gc/s, the computed value of G

Xj was about 15 db higher when ty = 8 at 100 miles.

11
Such higher values of G. would reduce the frequency dependence, which was exhibited in figure 10. The new calculations also indicate a smaller difference in loss equality angles between

smooth earth and elevated horizon paths than was

shown in figure 17.

9. Conclusions

It should be remembered that the beam

elevation relations which have been presented were calculated by present path loss prediction

methods. These methods are partly empirical

and are based on extensive measurements which

typified tropo-scatter communication practices:

horizon-directed antennas and frequencies predominantly below 1 Gc/s. Additional extensive

measurements will be needed to confirm or modify these prediction methods for applicability

to the surface interference problems of space

communication. Nevertheless, the general propagation behavior dependence on beam elevation

angle should not differ greatly from that which

has been shown. It is believed that these studies

should give the space communication systems

engineer a better "feel" for the importance of controlling the minimum elevation angle as a tool

for controlling surface interference. More

generally, it should be remembered that physical

separation is not the only tool and that it is an

expensive one. The control of path antenna gain,

together with terrain shielding, should become

the engineer's best tools for the control of surface interference.

10. References

Crawford, A.B., D.C.Hogg, and L.E.Hunt

(July 7, 1961), A horn-reflector antenna

for space communications, Bell System

Tech. J. 40, 1095-1116.

Curtis, H. E. (May, 1962), Interference between

satellite communication systems and

common carrier surface systems, Bell

System Tech. J. 41, 921-943.

DeGrasse, R. W. , D.C.Hogg, E.A.Ohm, and

H.D.E.Scovil (January, 1959), Ultralow-noise measurements, using a hornreflector antenna and a traveling wave

maser, J. Appl. Phys. 30, 2103.

Electronic Industries Association (July 8, I960),

Technical appendix to submission on FCC Docket 11866.

Firestone, W. L. , S. G. Lutz, and J.Smith (May,

1962), Control of interference between

surface microwave and satellite communication systems, IRE Trans. RFI-4, 1-20.

[ This was based on the appendix to the EIA-FCC reference. ]

Hartman, W. J. , and M. T. Decker (December,

1961), Mutual interference between surface and satellite communication systems,

NBS Tech. Note 126. [This also appeared

as Chapter 5 of Frequency Allocations for

Space Communications, a report of the

Joint Technical Advisory Committee of IRE and EIA, available at the Office of the Secretariat, Joint Technical Advisory

Committee, IRE-EIA ($7.70). ]

Hartman, W. J. , and R. E. Wilkerson (Nov. -Dec.

1959), Path antenna gain in an exponential atmosphere, J. Research NBS 63D

(Radio Prop. ), No. 6, 273-286.

Lutz, S. G. (September, 1961), Frequency

sharing between satellite communication

and surface radio systems, Telecom. J. 28, 585-593.

Rice, P. L. , A. G. Longley, and K. A. Norton

(July, 1959), Prediction of the cumulative distribution with time of ground wave

and tropospheric wave transmission loss - Part I, NBS Tech. Note 15. (See

figures 2 - 12 )

U.S. Air Force Technical Order 31Z-10-1 (1961),

Path loss predictions made by this method

generally agree closely with those based

on NBS Tech. Note 15 (July, 1959), but

the computations are less tedious.

GPO 835-565 12
p. S. DEPARTMENT OF COMMERCE

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