HAGN, KEENAN AND NER: SIEBIDING FOR GROU-TEMINAL ANTENNAS

PIT SHEELDING FOR CO]MMUNICATIONSATELUITE GROUND-TERMINAL ANTENNAS

George H. Hagn,* Michael G. Keenen,** and Harry A. Turner,*Members, IEEE

Abstract

Several authors have suggested that hills, earthwalls, excavated pits, or same other such obstaclemight be used to shield ground-terminal antennasof satellite-to-ground communication links fromradiated interference propagating at low angles(troposcatter, groundwave, etc.). As the trade-off is between access or look angle and shieldingefficiency, such pits would be most useful forsynchronous satellite systems, but they might beapplicable for other systems. This paper presentsmeasured isolation provided by three representa-tive pits at two frequencies (2.3 and 9.0 Go)in the band presently proposed for comunicationsatellite systems (1-10 Go). One pit had a verti-cal wall, one a sloping wall, and the third arounded sloping wall. Isolation of 4tO dB appearsfeasible, and possibly more could be achieved withspecially designed pit walls.

Introduction

Several authors have suggestedl9*" that naturalterrain, earth walls, excavated pits, or sme otherearthen obstacle might be used to help shieldcomunication satellite ground-terminal antennas.This type of shielding could be used to reduceinterference whether the ground terminal is a re-ceiver or transmitter. It would be most useful fora stationary satellite system but could also beused with other systems. Such shielding would beeffective in protecting against energy propagatedat a low angle such as ground wave or troposphericscatter. It might not provide protection againstprecipitation scatter which can arrive at highangles.10 Predictions based upon diffractiontheory indicate that 20 to 30 dB of shieldingmight be obtained by using pits. One reportl de-scribing measurements at X-band (3 cm) concludedthat a trench with sloping sides could add atleast 32 dB of isolation. The results of thesemeasurements are reproduced here as TabJe I.The total-isolation column includes 128 dB offree-space loss.

Workers at the MIT Lincoln Laboratory8 measured102 dB isolation at I-band (8,000 Mc) for rela-tively shallow pits spaced 1 mile, The ntree-tops"horizons for transmitter and receiver were 1.60

*Comunication Laboratory.**Radio Systems Laboratory, Stanford Research In-

stitute, Menlo Park, Calif.***References are given at the end of the paper.

and 3.0°, corresponding to a 00 horizon equivalentdistance of 275 miles. For an 11 mile separation.the horizons had increased to 2.40 and 5.00 re-spectively (550 mile equivalent distance) and thepath loss in excess of free-space loss had in-creased to 116 dB. Their measurements includedpropagation effects between pits and agreedroughly with calculations based upon the tropo-spheric scatter mode. Information on both pit andpropagation loss is required in establishing co-ordination distances,9 but let us now considerthe shielding pit itself.

The following questions are pertinent to theproblem:

1) What shielding is realizable in practice?2) What is the most advantageous pit geometry?

Three representative pit geometries (wall shapes)are shown in Fig. 1. To answer the precedingquestions, we measure the shielding provided bythree one-sided "pits" approximating the abovegeometries. The measurement frequncies were 2.3Go (S-band), and 9 Go (X-band). These frequenciesare near the limits of the band currently beingproposed for communication satellite use (1-10 Gc).The equipment used, theory, and results are de-scribed.

Description of Equipment and Operating Procedure

General

A transmitter was set up on the upper level andback from the lip of each pit. The transmittingantenna was so oriented, with the aid of a transit,that the axis of the beam was tangent to the lip.The receiving equipment was mounted in a truckso it could be moved into and out of the shadowzone in the pit. Transmitter output was main-tained at a constant level, and the power recei-ved at several positions in the pit was measuredby dLrect comparison with a calibrated signalgenerator. At each position, the receiving an-tenna was rotated in both azimuth and elevationuntil it found the directions that yielded thestrongest signal. The receiving antenna invari-ably wound up pointing at the lip of the pit online with the source,, indicating that reflecttonsdid not appreciably affect the measurement.Readings, in dbm, from the signal generator dialprovided power measurements and, when comparedto a direct signal from the source, a measure ofthe effectiveness of the pit as a shield.

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IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY

Measurements were repeated to ensure that the datawould yield consistent results. The results re-peated within t 3 dB.

S-Band System

Transmitting System. Figure 2(a) shows thetra .ct,ngstebock diagram. The oscillatorwas a microwave triode with tunable cavities andwas capable of an output power level up to 4 wattsCW. A small portion of this power was extractedat the directional coupler and attenuated to alevel suitable for use in the thermistor powermeasuring circuit. The frequency meter in the linewas a direct-reading absorption-type cavity whichproduced a dip in the power reading when tuned tothe oscillator frequency. This meter determinedthe transmitted frequency to within 2 Mc. Theantenna was a pyramiidal horn with a 10-by-14.5-inch rectangular aperture. Calculated gain at2.3 Go (X-' 0. 425 ft) was 17 dB. For the sameparaveters, the calculated half-power beamwidthwas 26 degrees and the calculated first-nullbeamwidth was 59 degrees.

Receiving System. A block diagram of the recei-ving systemi-s given in Fig. 2(b). The basicelement was an AN-APR-L receiver with an S-bandtuning head (TN-5h/APR-h). The receiver inputterTinals could be alternately connected to theantenna or to the signal generator by means ofthe coaxial RF switch. This provided a convenientmea-ns of determining the received power at theantenna terminals by comparing it with the outputof the signal generator. The variable attenuatorand anplifier provided flexibility in terms ofthe range of input signal powers that could behandled. The frequency range of the equipnent was2.3 to 4 Ge. The antenna was a dipole-illuminatedparabolic reflector with a 2-foot diamaeter. Itscalculated gain was 20 dB. Calculated half-powerbeamwidth was 11 degrees and ca'culated first-nullbeamwidth was 30 degrees at 2.3 Gc.

Calitbration was performed in the following manner:The signal picked up by the antenna was fed throughthe attenuator and amplifier to the receiver, andthe attenuator was adjusted to give a convenientdeflection on the receiver diode current meter(about two-thirdst full scale). The receiver wasthen switcned frcm the antenna to the signalgenerator, and the output of the signal generatorwas adjusted to produce the sane meter deflection.The power level was read from the calibrated out-put dial of the generator.

X-Band System

Transitting System. Figure 3(a) is a blockdiagr noThe-n transmitting sytem. Theoscilator was a reflex klystron with an outputpower of up to 200 mw. Regulated beam and reflec-tor voltages were supplied by a klystron power su-ply. The klystron output was amplitude-modulatedby a 1000-cycle square wave by means of modulationapplied to the reflector voltage in the power sup-ply. This was done to provide a video output at

the receiver for convenience in calibration. Aportion of the output was extracted at the direc-tional coupler for monitoring frequency and power.The transmitting antenna for this system was apyramidal horn with a 3-by-4-inch rectangular aper-ture. For f a 9.0 Gc (Xm 0.109 ft), the calcu-lated gain was 18 dB; the calculated half-powerbeamwidth was 22 degrees; and the calculated first-null beamwidth was 50 degrees.

Receiving System. A block diagram of this sys-temir given ingFig. 3(b). The basic element wasa Type 46ADA receiver from a surplus SO-12M/Nmilitary radar. As in the S-band system, thepower level of the incoming antenna signal wasdetermined by camparison with a known signal gen-erator output. However, in the I-band case,, aside-wall coupler was used rather than a switch.While the antenna signal was being neasured, thesignal generator output was heavily attenuated andthus effectively isolated from the receiver output;when the gemnrator was being measured, the antennawas pointed away from signal sources. The signalgenerator was modulated by a 1000-cycle squarewave to match that of the X-band transitter.The video output of the receiver was displayed ona calibrated oscilloscope. Calculated antenna gainwas 26 dB. Calculated half-power beamwidth was5.5 degrees, and calculated first-null beamwidthwas 15 degrees.

Shielding Measuremrents

Twsical Descriptionl of the Sites

The three measurement sites were chosen to ap-proximate the geometries depicted in Fig. 1.Figure l(a) was simulated by a steep-wall gravelpit. Figure l(b) was simulated by the sloping ofa freshly prepared building site wall and Fig. l(c)by a smooth, round hill. Three such sites wereavailable locally. One of these sites is picturedin Fig. 4. The steep-wall pit had grass growingalong the lip and same trees in the great-circlepath between transmitter and receiver. The treesdid not obscure the line-of-sight path betweentransmitter and receiver. The sloping-wall pitwas a building site with no vegetation. Shortgrass was the only vegetation on the round-hillpropagation path.

The vegetation appeared to be unimportant exceptat the gravel pit, where the trees and grass onthe lip (longer than at the other sites) appearedto affect the results. On one measuring day it wasquite windy, and deep fading occurred at bothfrequencies, making it difficult to obtain cali-brated measurements. When the wind died down, thefading ceased, and the X-band signal was approxi-mately 8-dl3 stronger than the peak signals duringthe fade. The S-band signals were about the samemagnitude but nore stable. These changes in peakamplitude might be explained by the wind blowingacross the X-band receiver klystron causing thereceiver tuning to vary.

Figures 5(a), (b), and (c) give the path profiles.

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HAGN, KEENAN AND TURNER: SHIELDING FOR GROUND-TERMINAL ANTENNAS

Theoretical Considerations

Data Reduction Formula. The power received vialine-of-sight propagation from a source radiatingpower PT is TR8

pTGTuRd2(h-r dj2

where GT - the gain of the transmittingantenna in the direction ofthe propagation path

GR a the gain of the receiving an-tenna in the direction of thepropagation path

d - the distance separating the antennasA - wavelength

The isolation between the antennas is the powerratio

A .(4 -fd)2PR 2TGh

The isotropic isilation (OT a GR Il) is

S . (4-T-d)2

For data reduction, the measured isolation is cam-pared to isolation calculated for a free-spacepath. The difference is attributed to the pres-ence of the pit and is called the shadow loss, SL*

Thus, in decibels, SL, A (measured) - A (freespace).

Knife-Edge Diffraction. Huygen's method, ap-pliTedto iFi:pelifaction past a knife-edge(see Fig. 6), 9,10 leads to a Fresnel dlffractionequation which describes a field, E:

E LL(v)ejr- C(v) + j S(v)EO

3.2f (cos Ir v2 +8 sin f v2) d v

where Eo is the field at R with the straight edgeremoved, and

v h 2(1 + 1

A d2 dlJ

This: method is used to predict the shadow loss inthe steep-wall and Sloping-Wall Pits. Values forthe Fresnel integrals are given in Jahnke andEmde'l Table of Functions -3 and the convenientlyplotted byrGrenier.12

In the region v > 1.2E to a goodN Eao v

approximation. Notice also that for dl>>d2,2 -h d for v > 1.5,Ev h2 ad for v>1.5,-

0.159v/'dh

This is the expression that would apply for pro-tection against troposcatter where d would repre-sent the distance frmo the camon voiine to thelip of the pit and d2 would be very nearly the dis-tance from the lip of the pit to the antenna with-in the pit.

Diffraction over a Curved Surface. The problemof diffraction over a curved obstacle ip discussedin detail by Neugebauer and Bacjynski 3l and theproblem of diffraction by a parabolic cylinder istreated mathematically by Rice.l5 A simplifiedformula given by D b and Prycel6 has been experi-mentally verified. This formula was used hereto plot theoretical curves for comparison withdata taken in the round-wall pit.

Frcm Domb and Pryce, the rate of attenuation be-yond the optical horizon, expressed in decibels,is assmed to be linear with increasing distanceinto the shadow. The rate is 8.8 dB per unitdistance Doi where Do - (RA,/zr) 'A, R is the radiusof curvature, and a value of 322 feet was used forthe path being measured. For exaMple, at S-band,Do a 2.1l feet, and at Station 2, 81.1 feet intothe shadow, attenuation is 29.6 dB. [The calcu -lated results are plotted in Fig. 8(c) with themeasured re sults3Direct Measurements of Isolation Provided by Pits

In the measurements, the isolation provided bythe pits, their shadow loss, is a function ofarrival angle (6 to 9.5 degrees in each case) ofthe incident energy and the position of the re-ceiving antenna on the bottom of the pit. The re-ceiving antenna was always oriented toward thelip of the pit. The shadow loss is presented asa function of the angle, Qc, through which thepath is deviated by the diffracting obstacle.Figure 7 is an illustration of a typical setup.

T m location of transmitterR a location of receiver

*a arrival angle of incident energye - elevation (or look) angle of pit-shielded

antennaoc - angle through which the energy is diffracted

where 0, - 0

Figure 8 shows the measured shielding (shadowloss vs diffraction angle) at the two frequenciesfor each plt. Figure 9 compares the shieldingprovided by the pits at each frequency. Allmeasurements shown are for horizontal polarizatimn.Vertical polarization measurements were made, butno significant differences were observed.

It is believed that system paramters were notknown to an accuracy greater than t 3 dB. This,in conjunction with the variations between runs oft 3 dB, results in an over-all absolute accuracyof about t 6 dB.

In the steep-wall pit, the shadow loss variesfrom about 33 dB at small diffraction angles toabout 56 dB at the largest diffraction angle sam-

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PR

IEEE TRANSACTIONS ON ELECTROMAGNETIC CMPATIBIITTY

pled (19 degrees). The slight frequency depen-dence that one would expect from diffraction theorywas within the resolution of the measurements.In Fig. 8(a) and (b), curves of shadow loss thatresult from appJying Fresnel knife-edge diffrac-tion theory to the pit are shown by dashed lines.

In the sloping-wall pit, the shadow loss variesfrom about 18 dB at small diffraction angles toabout 53 dB at the largest diffraction angle(21 degrees). Again, wavelength dependence wasnot observable. Theoretical curves based onknife-edge diffraction theory for the pit areshown dashed.

In the round-wall pit, wavelength dependence wasdefinitely observable. The sudden change in slopeof the curves at cc- 8.5 degrees occurs where thereceiver could no longer see the lip of the pit.At these stations, the receiving antenna wasoriented on the strongest signal, which was alwaysalong the path to the lip of the pit. The lossbegins to increase rapidly as the receiver moves"under" the wall. At S-band, the shadow losvaries from 8 dB to 46 dB. At X-band, the shaowloss varies from 28 to 71 dB. In Fig. 8(c),theoretical curves azn shown as dashed lines.

In Fig. 9(a), the three pits are compared at S-bandand it can be seen that a pit with a steep wallgives tne most loss. Data on the round-wall pitindicate that it may provide the greatest am-ountof shielding when the diffractim angle is greaterthan about 10 degrees, that is, when the wallshields the receiver from the illuminated lip.Plotting the round-wall shielding versus diffrac-tion angle is potentially misleading, since thereis on:y an "effective Q7" for this case, andmeasurements were mnade quite close to the wall.

In Fig. 9(b), the three pits are compared at I-band.As at S-band, the steep-wall pit provides moreshielding than the sloping-wall pit. The snieldingeffect of the round wall is pronounced, especiallywhen the bulge of the wall becomes an obstacle be-tween the receiver and the illuminated lip.

The value of 32 dB of pit shielding by Lutz andWilliamn1 is difficult to compare with our measure-ments, because of differences in pit shape. How-ever, it appears that the measurements are of simi-lar magnitude when comparison is made with thesloping-wall pit. It is also difficult to gampareour results with those of MIT Lincoln Lab., ex-cept in a gross manner, in that the propagationpath loss and shadow loss are lumped. Using themedian tropospheric loss figures (corrected to8000 Mc) presented in the letter gives a negligibleloss due to the shallow pits. The data sample wasquite small, however, and the pits might have con-tribrt,Vd mnre loss than estimated by this technique.

Conclusions and Discussion of Results

These conclusions can be reached from the meas-urements:

1) For a given pit, shielding will

increase slightly with frequencyin the 1-10 Gc band.

2) Scalar knife-edge diffractiontheory applied to any of the pitswould predict too little isolation.The four-ray model is not requiredbecause foreground reflections werenot important, as evidenced by thesmooth decay of the field with in-creasing diffraction angle (N.

3) The pit with a steep wall providedmore shielding than the pit with asloping wall. These two pits hadsmall radius of curvature relativeto a wavelength. From the round-wall mreas-urements and the frequencytrend, it is expected that increasingthe radius of curvature of the liprelative to the operating wavelengthwill increase the isolation.

4) At least 4o dB of pit shielding isfeasible; 70 dB of shielding maybe possible with carefully designedpit walls.

Pit shielding appears to be a useful technique forreduction of interference caused or suffered bysatellite ground terminals. Due to the possibilityof reflections in enclosed pits, and because it isdifficult to make an accurate prediction of howmuch shielding any pit-natural or man-made--willprovide, it is essential that interference levelsat potential earth sites be measured. For a giveninterference situation, the possibility exists foran optimum size and shape for a shielding pit.

Figure 10(a) presents the authors' concept of anear-optimm pit shape for shielding the groundterminal of a satellite-to-ground communicationlink against an interfering signal propagated viathe tropo-scatter mode. The higher parts of thecomon volume contribute less interfering signalwhen the common volume receives uniform illumi-nation. A diffraction angle,6c, of 10 degrees hasbeen chosen as the maximum angle of arrival ofinterference against which protection is needed.The receiving antenna in the pit [Fig. 10(a)] is a60-foot parabolic dish. Ary surface causing dele-terious reflection could be covered with microwaveabsorber.* AlternativelyJ, the pit could be design-ed with a re-ceurved wall in the region of the un-wanted reflections as shown in Fig. 10(b); or acombination of such techniques could be used.

*Perhaps there is a better solution to the problemof eliminating "glints." It does not seem wise toline the pit with absorber as this would tend toincrease the receiving system temperature. Thiswould be particularly true in the case of a shallowparabolic dish antenna, where the feed can "see"the pit walls. Indeed, for this case one mightwant to line the pit walls with a conducting screen.

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HAGN, KEENAN AND TURNER: SHIEIDING FOR GROUND-TERMINAL ANTENNAS

Losses for the pit with the re-curved wall couldpossibly be estimated theoretically with a Fres-nal surface integrall8.

The use of actual terminal hardware to conduct aninterference survey is expensive, but usable re-sults should be obtainable with simple test appar-atus and a 10-foot parabolic dish mounted on atruck. In a large pit, such an antenna would ap-proximate a point sample of the diffraction field.Any actual antenna in such a shielding pit wouldhave a non-zero aperture; it would not be a pointsampler of the diffraction field, [i.e., the aper-t,ure would not be uniformly illuminated by the dif-fraction field--see Fig. 9(b)1. In fact, the il-

lumination of any large antenna would be a functionof antenna orientation as well as pit geornetry.Different parts of the aperture would subtend dif-ferent diffraction angles. Which one should beused in shielding calculations? A conservativeestimate would be obtained by using the smallestQC, whereas a more realistic estimate might be ob-tained by using the angle to the phase center ofthe antenna in the pit.

The use of a pit to add to the transmission lossexperienced by propagating microwave radiation is,of course, not limited to satellite system recei-ving ground terminals. These installations com-bine high-gain antennas and sensitive receivers,and therefore might require protection. However,recent considerations of allowable field strengthat a microwave relay station raise the possibilitythat pit shielding may be used to assist in theprotection of other systems from communicationsatellite ground-terminal interference as well.

Acknowledgment

Original research on this project was funded bythe National Aeronautics and Space Administration.

References

1. S.G. Lutz and D.D. Williams, "Pit protectionof Satellite Communication Terminal Antennas fromSurface Radio Interference," Report SRS-420,Hughes Aircraft Comparn, Culver City, California(1 February 1961).

2. "Frequency Allocations for Space Communi-cations," a report of the Joint Technical AdvisoryCommittee of the Institute of Radio Engineers andthe Electronic Industries Association (March 1961).

3. S.G. Lutz, "Twelve Advantages of StationarySatellite Systems for Point-to-Point Coomunication."1961 WESCON Convention Record, Paper 21/1.

4. W.R. Vincent and A.M. Peterson, "Interferencein Satellite Comunication Systems," paper pre-sented at URSI Symposimn on Space CnmmunicationResearch, Paris, 18-22 September 1961,

5. J.J. Downing, et. al., "Interference Consider-ations for Communication Satellites," 1961 WESCONConvention Record, Paper 29/2.

6. R.G. Gould, "A study of the Influence of Coim-mercial Comnunication Requirements on the Designof Caomunication Satellites," Final Report, SRIProject 3398, Contract NAS-586, Stanford ResearchInstitute, Menlo Park, California (January 1962).7. W.L. Firestone, S.G. Lutz, J. Smith, "Controlof Interference Between Surface Y.icrouave and Sat-ellite Comriunication Systeems," IRJ3 Trans. PRFG,No. 2, pp. 1-20 (May 1962).

8. W.E. Morrow, Jr., D. Karp, R.. Locke, Jr.,and W.C. Provencher, "The Influence of TerrainShielding on Radio Wave Propagation at 8000 Mc,"Proc. IEEE, Vol. 51, No. 6, pp. 955-956 (June 1963).

9. International Radio Consultative Cammittee(CCIR) of the Int,ernational Telecomm'iriicationUnion, Study Group IV, Documnent IV/63-E (UnitedKingdcm.) dated 11 December 1964. Draft Question:"Site Shielding-Factor to be Used in Calculationof Coordination Distance." This topic was discus-sed briefly in CCIR Document IV/157-E (Rev. 1)dated 1 March 1965, "Frequency Sharing BetweenCcomaunication-Satellite Systevs and Terrestial Ser-vices," Annex I.

10. A.S. Dennis, "Precipitation Scatter as anInterference Source in Commurnicatiorn SatelliteSystemp;s" 1962 IRE International Convention Record,pp. l45-151 (Mar,Y192,Siilar nontoappeared as: Research Memorandum 2, SRI Project3773, Contract NASr-49-(02), Starnford ResearchInstitute, enlo Park, California (Novemrber 1961).

II. J.C. Schelleng, C.R. Burrows, and E.B. Fer-rell, "Ultra Short Wave Propagation.," Proc. IRE,Vol. 18, (March 1933).

12. G.H. Grenier, "Obstacle Gain and Shado; Loss,"Microwave J., Vol. 5, No. 7, pp. 60-69 (1962).

13. E. Jahnke and F. Emde, Tables of Functionis(Dover Publications, New YorVk, 195).

1X. H.E.J. Neugebarer arnd M.P. Bachynsli, "Dif-fraction by Smooth Cylindrical Yountains," Proc.

, Vol. 46, No. 9 pp. 2619-1627 (1958).

15. J.O. Rice, "Diffraction of Plane Radio Wavesby a Parebolic Cylinder,"' Bell S -. Tech J.(Marell 1954). See also Bell ystem Monograph 2278.

16. C. Damb and M.H.L. Pryce, "The Calculation ofDiffraction of Radio Waves Round the Curved Sur-face of the Earth," as cited in the following re-fere3nce as about to be published.

IL7. J.S. McPetrie and L.P. Ford, "An ExperimentalInvestigation of the Propagation of Radio Wavesover Bare Ridges in the Wavelength Range 10 cm to10 rn," J. IEE (London), Vol. 93, Part IITA, No. 3,pp. 527-53C01946).

18. G. Mi]lington, R. Hewitt, and F.S. Immirzi,"Fresnel Surface Integral," Proc. IEE (British),Vol. ]09, Part C (Monograph 50dE) 130-437(September 1'962).

1L965 97

IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBIIITY

Table 1

AN ISOLATION MEASUREMENT AT X-BAND

Total Net Probability That IsolationIsolation Isolation Is Less Than Value Stated

(db) (db)

167 39 0.12164 36 0.025160 32 0.00 (minimum iso-

lation observed)

tSource: S. G. Lutz and D. D. Williams, HughesAircraft Company (Ref. 1).

(a) Vertical Wall (b) Sloping Wall (c) Round Wall

Fig. 1 Representative Pit Geometries

(b)

Fig. 2 (a) S-Band Transmitter(b) S-Band Receiver

98 June

E6IN, 1lUNA AND UPMis SILDEIK FMC CMOUND-DIILUL ANTENAS

(a) (b)

Fig. 3 (a) X-Band Transmitter(b) X-Band Receiver

Fig. 4 Steep-Wall Pit

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IEEE TRANSACTIONS ON ELECTROMANETIC COMPATIBILITY

INSERT DIMENSIONSS-BAND X-BAND

h3.75L 2.17

X 74' 74'(A _8 665

leso 6.5°1

12'

STATION: 00 0 3 10 16

(a) PATH PROF

LOCATIONS OF RECEIVING ANTENNA

-ILE OF STEEP-WALL PIT

DIMENSIONS

_ S-BAND X-8ANDh 3.4L 2.17s

_ 8S' 8B,8.5° 7.50

,8 6.3°__ 6.3°

STATION 4 3 2

2 3 4 5 6 7 8

LOCATIONS OF RECEIVING ANTENNA

(b) PATH PROFILE OF SLOPING-WALL PIT

CHOSEN AS THELIP OF THE PIT

STATION

DIMENSIONS

S-8AND X-BANDh 3.75I 2.17'

.4 0° 0°

RISE

LOCATION OFTRANSMITTINGANTENNA

(c) PATH PROFILE OF ROUND-WALL PIT

Fig. 5 Path Profiles

(a) Path Profile of Steep-Wall Pit(b) Path Profile of Sloping-Wall Pit(c) Path Profile of Round-Wall Pit

100 JuneS

HAGN, KEENAN AND TURNER: SHIELDING FOR GJROUND-TERNINAL ANTENNAS

hT

a a-DIFFRACTION ANGLE

OBSTACLE

g - d, in of - d2g -D

Fig. 6 Illustration of Knife-Edge Diffraction Parameters

Dif fraction Path

Fig. 7 Typical Measurement Setup

T

I

1965 101

(c)ROUND-WALL

PIT

70i

X-BA

ND(T

HEOR

ETIC

AL/

6C

BANDMEASURED

/S-B

AND(T

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ETIC

AL)

X-kB

AND(MEASURD

/50 40 3C

-/yS

BBAND

(MWE

ASUR

ED)

20 I0

05

1015

20a

degr

ees

Fig.

8Sh

adow

Loss

vs.

Diff

ract

ionAngle

atS-

andX-Band

(a)Steep-Wall

Pit

(b)Sloping-Wall

Pit

.0 -J U1)

adegrees

Fig.

9Co

mpar

ison

ofTh

ree

Pits

'Sh

adow

Loss

(a)-S

-Ban

d(blX-

Band

-j

C/O

Hi 0 H X C) 0X CO)a W 0 C]Q 1s3 H C-.

9HAGN, KEENAN AND TURNER: SBIELDING FOR GROUND-TBRIINAL ANTENlNAS

248'

93' -

(a)

(b)Fig. 10 Near-Optimum Pits

(a) Near-Optimum Pit for Protecting 60-Foot Dish Against InterferenceArriving 0 < at ' 100 (SL ' 40 db)

(b) Artists' Conception of Pit with Re-curved Wall for SuppressingUndesired Reflections and Imposing Double-Diffraction Loss

1965 103