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o WADC TECHNICAL REPORT 52-135 HH

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AIR-TO-AIR TR0P0SPHE2IC PROPAGATION OVER WATER

GARNER B. FANNING FRED P. MILLER, CAPT, VSAF

AIRCRAFT RADIATION LABORATORY

JUNE 1952

WRIGHT AIR DEVELOPMENT CENTER

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WADC TECHNICAL REPORT 52-135

AIR-TO-AIR TROPOSPHERIC PROPAGATION OVER WATER

Game? ;B* :EJsnning Fred P. Miller, Capt, USAF

Aircraft Radiation Laboratory

tune 1952

RDO No. 112-73-1

Wright Air Development Center Air Research and Development Gömmand

United States Air Force Wright-Patterson Air Force Base, Ohio

McGregor & Werner, Inc., Wakefield, Mass. Dec. 15, 1952 150

ABSTRAGT

I

Air-to-air trorpps;pheric one-way radio wave propa- gation tests over smooth fresh water are described and the resulting data analyzed. Signal strength for fre- quencies of 250, 1.0Q0, and 3300'mc was recorded simul- taneously in a receiving aircraft äs a function of separation distance from a transmitting aircraft at 1000 feet altitude«, Analyses include primarily those pheno- mena -due to earth reflections^ and, tö >a less extent, the effects of lower atmosphere refraction. The recorded interference lobe structure envelope compares' favorably with the theoretical except for a maximum range fore- shortening probably due to a substandard refractive- con- dition. The indicated lobe rates arid reflection coef-^ ficieiits closely compare, with the theoretical with spme:

scatter primarily due to rapid unavoidable excursions of aircraft altitude. Theoretical frequency dependent diffraction slopes calculated far signal strength beyond the horizon are closely duplicated in the data. No severe fades were observed that, co.uld not be at- tributed to earth reflection phenomena.

PUBKeAT^ON REVIEW

This report has been reviewed and is approved«

FOR TEE COMMANDING GENERAL:

tTDäKENGE H. LEWIS ColeÄ, USAF Chief,. Aircraft Radiation Laboratory Directorate of Laboratories'

WADC TR 52-135 iii

I !

TAELEOF CONTENTS

I* Introduction ................... = ..... 1.

II. (Conditions of the Experiment 1

III.. Data and Results . i ....... i . . 9

IV. Conclusions 14

LIST OF IIXUST.MfI0NS

Figure gage

1. Geometry of the Experiment ......... » . * 2

2. Map of Flight Path, .....*...... » 3

3. Center Point Shift from Center of Lake ...... 4

4. Fresnel Zone Geometry ... ........ * . , 5

5-. Terrain Profile of Flight Path .**,.........,.... 16

6. Recorded Data, 250 mc ......... , 17

7. Recorded Data, 1000 mc ...... --. .... , 18

8. Recorded Data, 3300 mc 19

9. Signal Level vs Distance, 250 mc . . 20

10. Signal Level vs Distance, 1000 mc. ..... . 21

11. Signal Level vs Distance, 3300 mc. . . 22

12. Structure of Last Interference Lobe 23

13* Lobing Rate vs Range, 250 mc c. ..... -. ... ., , 24

14. 'Lobing-'Rate '-vs :Range^ lG©0:;ines . .. ., ? * * -.- . . * -• • * * s • 25-

15. lobing ;tete vs Range, 3300 mc. *, 2b

16. Del^a h vs= Range . .. ; *.-.., ....*. 27

17. Reflection Coefficient, vs Grazing. Angle,. 250 mc. (Run l) . . . 28

MDC TR 52-135- iv

Figure,

18, Reflection

19, Reflection

20.. Reflection

21 k Reflection

22'. Reflection

23 ,; Reflection

24. Reflection

Page

Cbeffic/ient vs Grazing Angle-, 250 mc (Run 2) * . . 29

Coefficient TS Grazing Angle, 250 mc (Run 3) -,, . . 30

Coefficient vs^ Grazing Angle, 1000 mc i(Run 1). . . 31

Coefficient TS Grazing Angle, 1000 mc (Rum 2). . . 32

Coefficient TS Grazing Angle, 1000 mc (Run 3). . . 33

Coefficient TS Grazing Angle,, 3300 mc (Run 1). * , 3^+

Coefficient TS Grazing Angle, 3300 mc (Run 2). i # 35

WKDC TR 52-135=

I,- INTRODUCTION

It is well established knov&edge that all -propagation of radio waves I •within the troposphere is affected markedly by the presence of the earth and its atmosphere. At frequencies of ab.püt 40 mc and higher, signal level as a, i function of range is deviated chieflyv from theoretical propagation in free j spao^ By atmospheric refraction or bending, and by earth reflection phenomena« T-o determine, .propagation characteristics under these conditions, many factors are involved, such äsj, the geometry of the situation, the refractive index of the medium \(&ir) supporting the propagation, the frequency and polarization ; of the electromagnetic radiation involved, and the electrical characteristics j of the reflecting, earth, __ The geometry incliid.es the location of the, receiver ;

~~ and transmitter with respect to the earth sand the physics-1 contours of the earth's reflecting area. Electrical char act eristics of the ear-th that enter in are the dielectric constant and conductivity«

Several experiments of this nature have been conducted in the past» ^1, 2.) Such experiments, -with at least one notable exception (2), have usually been conducted for short ranges or have maintained either the receiver or transmitter, or both, fixed on the earth,- Considerable additional knowledge can be accrued by utilizing an air-tö-alr propagation link and recording in a receiving aircraft the signal level as a function of' range from a transmitting aircraft,. In the experiments reported -here^ the two aircraft opened range on reciprocal headings at 1000 ft abovs a body of smooth fresh water, while three separate and distinct propagation frequencies were operated simultaneous- ly. The frequencies employed w§re 250 mc, 1000 mc, and 3300 mc«

Heferring to Fig, 1, earth reflected signals affect the total signal j j level at the receiving antenna by vectprially combining with the direct I ; path signal to «produce an, alternate partial cancellation and r&inforcemeht [ of the signal I-evel at the receiver • The shape, period, and -magnitude of this regular fluctuation of signal, termed lobe structure, are determined theorem tically by the operating wavelength, altitude of transmitter and receiver, range between terminalsi and the nature of the reflecting surface* Such an experiment as depicted in Fig, 1 can yield much information for ready compari- son with theoretical,, such as, lobe rate for each operating wavelength, lobe envelope, maximum range, and earth reflection 'coefficients»,

II, CONDITIONS OF THE EXPERIMENT-

' In these experiments, it was intended to minimize the effects of atraos- 1 pheric refraction and emphasize the phenomena associated with specular earth. S reflections- ükdea-lized specular 'reflections -ca=n: ;fee> •jzee&fcz&ü better experi- i ' mentaily for the air-to-air case, while- flying over a large aitea of smooth ! homogeneous surface such as ah inland lake,/ Such a body of water- wag. selected. ! . for the present flight tests. In the. interest, of flying safety and antici- j ' pated optimum r-esults, the altitude of both aircraft during, the tests was I chosen as 1000 feet, .above the lake surface, such altitude being determined by

WADC TR 52-135,

radio altimeter» This lake,, called Grand Lake, is located in nort.hwestern Ohio and is shown in Pig. 2 on sections of Coast and Geodetic topographic maps. The early portion of the flight paths is also shown. The aircraft separated sharply over the center of the lake at 1000 feet altitude fee the start of each of three separate runs, the transmitting aircraft (type C-46) flying eastward, the receiving-recording aircraft (type B-17) going westward. Flying path accuracy arid location was determined precisely by the pilots logging exact times over predetermined check points. Such times were used also in the data reduction to- determine the distance between, aircraft äs a function of time, which, in turn, was employed to indicate range on the field strength recordings.

Although an effort was made to maintain the center point between aircraft fixed &t. the center o--f the lake, some constant, drift did occur. This drift was carefully computed and>is shown in Pig., 3 for each o=f the "three separate test runs performed* Since the=,most important data for purposes of reflection calculations were obtained for aircraft separations of 40 miles or less^ the limits of drift indicated can be considered unimportant.

Grand Lake is not ideally large. On the^ other hand-, its limited extent contributed to the mirror-like smoothness öf surface enjoyed on the day of these experiments»

TRANSMITTERS RECEIVERS

Figure 1. Geomefcrj- of the Experiment

WADC TR 52-135

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WAB3> TR 52-135

Where a large area of the earth is Illuminated' by the- radio-frequency «energy, earth reflections occur over quite an extent.. Obviously, for a smooth reflecting surface* not all the räys striking, the earth can reach the* vicinity of the receiving antenna. In fact, on the basis of pure optical theory -which may be applied, only one ray can so reflect with angle of inci- dence equal wangle of reflection. For hpth aircraft at the same altitude this;

reflecting point is the., center point between aircraft. For other reflecting rays that differ by one-half wavelength in. texact path travel between aircraft, the points of reflection define an ellipse on the reflecting, surface. The area of Ulis elongated ellipse is termed the First Fresnel. Zone. It is im- portant because most of the reflected energy arriving at the receiver vicinity is included in this bundle of rays. The region between this ellipse and a second ellipse defined by another hal f wavelength path difference is termed the Second Fresnel Zone, and so on for successive Fresnel Zones. The second and succeeding zones are deereasihgly important in determining the effect at the receiver of the reflections. Fig« 4 illustrates these zones.

- ! •U

Figure 4. Fresnel Zone Geometry

An excellent discussion of these Fresnel Zones and their importance can be found in the literature (3). With regard to the experiments reportad here, it is important to know the extent of these elliptical Fresnel Zones for the conditions impos.edi Useful approximations- have been derived by Kerr (.'3,) to

WADC TR. 52.-135' 5

i

yield the length of the major and minor axes of the successive. Fresnel Zones, For both aircraft at the same altitude, the approximations can <be further re- , •duced to:

**/y- 'f—H* 4h* 1/« andy

'inXr where x • length of major axis in miles,,

y = length of minor axis in miles, n = number of Fresnel Zone considered* r ** distance between aircraft in miles,, h - altitude of both aircraft in miles. X^ operating wavelength in miles.

Upon close examination of these expressions it is apparent that the ellipses are markedly elongated along the line connecting, the propagation terminals — such -elongation increasing very rapidly äs the range increases to the. horizon. For the geometry and. operating wavelengths employed, Table I give3 the dimensions of the first two Fresnel Zones at various- ranges.

TABLE I

r (miles} freq. (mc) xj,(miles) .x^miles) Ji(miles) y2(miies)

20 250 6,14 8.31 0.122 0,172. 20 1000 3.17 4,29- 0.062 0.08& 20 3300 1.76 2.49 0.034 0.048

30 250 10.95 14.65 0.150 0.211 30 1000 5.53 8.04 0.075 0,106 3.0 3300 .3.22 4.55 0.041 0.058

40 250 16,5: 21.6 0.172 0.243 40 1000 8.9. 12.2 0.087 0.122 40

i i

3300 4.96 6.99 0.047 0.067

From thg'se data it can be concluded that for aircraft separation, ranges up to approximately 40 miles the most important First Fresnel Zone is con- fined to> the surface, of the lake. Moreover, calculations related to earth reflections can be valid assuming the major contributing reflections to be

j from, the mirror^smooth fresh water surface. J

I For some calculations involving ranges greater than 40 miles the actual earth*s surface is important. At these ranges the dominant reflecting areas

| e^eäräv beyond the' lake böünäärTies . However, the surröühmhg terrain is J relabively smooth. This is indicated in a terrain profile, Fig. 5,. which was, I derived for that part of the flight path shown on the map in F ig • 2« The i small town of St, Marys on the east edge of the lake was estimated to have

isolated buildings or struptures of approximately 50 feet in height. This estimated region of the; profile is shown in cross hatching.

WADG TR 52-135

Again, the profile, can be important in determining the theoretical maxi- mum range for which the -direct and reflected, rays can reach the receiving air- * I craft. All rays or signal received beyond this total horizon range is due to | di-ffraction« This very rapidly decreasing signal with increasing range is i indicated by the steep slope near the; horizon range. Such slops theoretically is a function of operating wavelength, with steepness increasing with frequency, i The possible shortening of the total horizon range by the- terrain irregulari- i ties indicated is almost negligible. In fact, a 100-foot change in altitude ö'f the center point (the worst case and equivalent, to both aircraft changing altitude by 100 feet from 1000-feet) results in a change in total horizon range of approximately 4 miles. For comparison purposes the'total horizon, -range is. indicatad in. Table II# assuming various values for the adjusted earth radius» ! The 3/3 .earth radius applies for geometrical range; 4/3-and 5/4 earth radius ; apply for accepted standard a-fihospheres. with linearly decreasing index of re- ' fraction with altitude».

TABLE II

Altitude of each total Horizon Range (Miles) Actual Earth Radius Airciaft (Feet) to be Multiplied -by a Factor of:

3/3 5/4 4/s

500 54.9 61.2 63.3 600 60.2 67*0 69. Z 700 65.0 72.3 74.8 800, 69.4 77.4 80.0 900« 73.7 82.1 84.9

1-Ö0Ö 77.7 86.5 • 89*5

1100 81.4 90.8 93.9 1200 85.1 94.8 98.1 1300 88.6 98.7 102.1 1400 92.0 102.4 106.0 1500 95.2 106.0 1Q9.6

i

Another "possible result of irregular reflecting surface is that of diffuse scattering of' the radio waves- rather than uniform and specular reflect- ions. A measure of surface roughness is indicated by K. Ai Norton (4) and is dependent on grazing angle, height of the irregularity* and operating frequency. According to Norton a realistic criteria for deciding when the smooth-earth theory may be used with confidence is when.

h,sih-vr<X/32

where,, h is the height of the irregularity \lf is the grazing angle i\\ is the operating wavelength»

WADC TR 52-135

During the test nans of these experiments- the roughness of the earth's surface- can- Be described generally by the above, criterion for three separate regions*

(1) Essentially smooth out to ranges short of where the First Fresnel Zone leaves the lake sitrface.

(2) Essentially smooth at far ranges (and small grazing, angles) where h sin Nif becomes vanishingiy small.

(5) Intermedia-t.e region between (1) and (2) for which essentially rough conditions are possible, especially for the s=hörter wavelengths'. Under rough conditions the signal level can have either an irregular* lobe structure or none at all, but never a regulär lobe structure»,

•Manifestly, the propagation of radio waves is affected to same escbent by the refractive index of the medium traversed. In the present experiments; this medium is, of course, the bottom 1000= feet of the atmosphere, next to the. earth over the flight path. Refractive index, or more important, refractive index gradients,, are difficiilt to obtain oveir such an extent'; such measure- ments usually involve- the accurate measurement of. pressure, dry temperature, and partial vapor pressure. More recent devieeä have been, successfully de- veloped independently by G. Birnbaum at the National Bureau of Standards and C. M. Crain, University of Texas, EERL, musing microwave cavity techniques. Flying safety prohibited these airborne measurements to be taken at very low altitudes. As the m:or.e important phenomena related to earth reflections., particularly at ranges under 40 miles, are not affected seriously by refractive effects, the present experiments involved no extensive meteorological measure^ ments. Although some unexplained deviations in the data from theoretical can be attributed to atmospheric refraction, no serious discussion of this will be attempted«

Some mention of the general weather situation is in order. These tests were performed oh 20 March 1951 from about 1130 to 1530 EST. The entire area was blanketed with approximately two inches of snow which had ^een deposited during the passage, of a low pressure cell some 36- hours before. Although the surface air temperature was approximately 0°C, there was no indication of ice on the mirror smooth lake. A weak high pressure cell extended over the test area with visibility excellent and clear cky. Although of ho particular im- portance, the surrounding radiosonde stations within 200 miles indicated a moderate subsidence at 5000 feet =MSL.

The propagation measuring equipment used in these tests was used extensive- ly in collecting propagation data on previous high altitude airrto-air flights. Although, details of its< ehara.dt_etis.tica have, b.een .given in a. previous, .report, ..on these earlier tests (5), general, characteristics are offered here for complete- ness. Three separate propagation links were operated, simultaneously; namely, 250 mc and 100Ö mc. interrupted continuous wave, crystal controlled at 1818.18 cps; and 3300 mc, 2,25 microsecond pulse, 320 pps<. Ä:ll polarizations were vertical. The power output of all three transmitters was monitored continuously

Ü7ADC TR >52-135 g

M3__t__________iE__M£«i

and found to vary less- than- one^halP-decibel. The- main1. inverter[ vöi-täge- in. both aircraft was als:p recorded continuously and found -to- -have negligible •variation» The receiver-recorder installation was calibrated with appropriate signal generators several times before and a=fte-r -each of the three test runs-. All recordings-were per formed us ing the ModeOi AW. Esterllhe-Angus, zero to one milliampere,. ink recording milliammeter, with <a chart speed of three inches per minute. The 0,5-sec time constant of this instrument was actually the limiting factor for the frequency- response of the propagation recording systemj such response weis a maximum of about two cycles per second.

In order to ensure that no observed signal variations were ;due to antenna patterns, while the aircraft yawed or pitched, all antennas were located' and measured such that gain variations over a 20° cone directly to the rear of each aircraft were negligible.

As a result of exhaustive tests both on the bench and in the airs. it. can be concluded that all fluctuations in signal strength observed and recorded .represented actual changes in field strength due to propagation phenomena,

III. DATA AND RESTJLTS-

The original recorded data are shown in Fig. 6, 7, and 8. No data are shown for the third run for 3300 mcj the receiver was inadvertently detuned just prior to the run and the> rapid excursions of signal level prevented imr mediate.retuning. Increasing mileage between aircraft is indicated from right to left at the bp ttoih of each chart.. Signal level decreases from a maximum at the top to receiver noise level near the: bottoms As there is some nonlinear! ty over the dynamic range of the receivers, a calibration for each run is indicated on each chart.

In order to examine further some features of the recorded data it was found convenient to replot for each mile on linear-linear graph paper the signal level in db (decibels), below Q.l volt (50 ohms) vs separation of aircraft. Such replots are shown in Figs, 9, 10, and II, where the signal level envelope is, plotted over the region of rapid Ibbe rate. Superimposed on each of these replots are curves representing the theoretical envelope of the interference:

lobe structure and the theoretical curve for propagation in free space. These theoretical curves were derived from common formulas available in tlie literature (6), The lake surface was assumed to have, a dielectric constant of 80 and a conductivity of 1 x 10"^ emu. It. should be noted that for the frequencies in- volved,, the shape of these curves is essentially independent of the operating wavelength.

S"evertcl remarks- -concerning these-, replots &r-Q gus:t-i-fied*. I-n all the. data; the free.; cpace trend is apparent,, with good correlation out to ranges well within the interference r.egion. Moreover, the signal level is fairly well con- fined, to the theoretical envelope.

! f

WAtiC TR 52-135

It appears that the total horizon range obtained in thes.e experiments is somewhat le3s than theoretical. In this respect, it is interesting to examine the location .and character of the last interference lohe (first, one back from the total horizon range). This lobe is well defined in all the: data and can serve as ä yardstick for comparison with theoretical. Theoretically,, the exact location of the maxima of this lobe is a function of .altitude of each aircraft, operating frequency, and the particular linear refractive index lapse rate .assumed for the atmosphere. Although the: details of these cal- culations are- "tedious^ they have been greatly simplified by methods and charts recently available^ (7). By making use of these expedients and assuming various index profiles, the exact location of thes:e lobe maxima .have been de- rived for each of the three pertinent operating wavelengths and an altitude for both aircraft of 1000 feet. The assumed profiles were derived from assuming an effective earth's radius of 3/3, 5/4, and 4/3 of actual. The use of 3/3. earth radius is tantamount to- assuming a constant index vs altitude, whereas, the, conventional use of 4/3 results from assuming a linettr lapse rate of twelve (n - 1) 106 units per thousand feet. The use of the 5/4 earth radius representation has Deuii. alloyed extensively by Booler (ö),. Fig. 12 repre- sents the actual plot of the experimental data for all the iast lobes with theoretical locations of maxima* assuming various earth radii. From these plots it a,pp_ear3 that the actualT conditions encountered for these tests were such that the integrated refractive effect over the propagation path ap- proached that to be expected from 3,/3 earth- radius conditions. More simply, the actual location of the lobe maxima compares favorably with a theoretical that assumes a constant index of refraction profile (zero gradient),» This situation, referred to aij a substandard condition, is rather uncommon. A more likely explanation would involve perhaps a marked substandard condition for the lower 1Q0 feet or so of air next to the earth with an ordinary lapsing index on up- to 1000 feet. Propagation through such a medium can result in an overall effect similar to= that indicated in the data*

At this time it is interesting to compare the slope of the decaying signal near the end of the-runs for -each frequency. As the signal beyond the total horizon range is due predominately to diffraction, this diffraction field can be computed using first mode theory and the well established rela- tionship due to Booker (9)': /y

Rate of signal level drop -a 25.8 db »per j^ (ka) miles

Where?. X is operating wavelength in miles a is actual earth's radius in miles k is assumed factor modifying the earth's radius

T-.ab.le. I-I.I .glides various diffpacti^n slopes, .derived, from this expre.jSsions.. i

\

WADC TR '52-135 10

TABLE III DIFFRACTION SLOPES IN DB. PER. -MILE

Operating Frequency ;(mc) 3/3 5/4 4/3

250 1000 3300-

1.14 0.98 0.938 1.81 1.56 1.495 2.68 2,28 2.22

Differences between theoretical and actual slopes are difficult to reconcile and will not be- discussed further except to note that, pure diffraction con- ditions were not duplicated in these experiments, and even the theoretical, slope shown (that due to first mode) is valid =only for ranges well beyond the horizon. It does appear that the slope resulting from assuming a 3/3 earth's radius approximates more closely the experimental data. These theoretical slopes are indicated in the right portion of Figs. 9, 10 and 11.

Turning again to the original data recordings of Fig's. 6, 7, and 8, it is fruitful to examine the- actual lobe rate for the various frequencies' in- volved. This lobe frequency is dependent in a complicated' way oh altitude, range and grazing angle, and operating, frequency. In more simple terms, the signal level will change from any maximum to an adjacent minimum when the path length difference between direct and reflected rays is half the operating wavelength. The theoretical values of this lobe rate for the three frequencies employed have been calculated' and are presented in Figs«. 13, 14, and 15, as a continuous solid line. The actual values derived from measurements on, the original recordings are shown as single points,. These curves indicate quanti- tatively a characteristic apparent in the recording; namely* a considerable irregularity in lobe rate. For some portions of the recording there appears to be a slight frequency modulation superimposed; in other mor§ severe cases, one or more lobe maxima or minima appear to be washed out altogether. In many portions of the data, the maximum- or minimum has been appreciably dis- placed in range.

The above phenomena of irregular lobe rate has a ready explanation, sug- gested by Dr. Donald E, Kerr during extensive private consultation on these experiments. As -indicated before, a change of path difference between direct and reflected rays by a half wavelength will slide the signal from a maximum to a minimum. This change in path length difference ..is a.cc.ut.e.ly sensitive to aircraft altitude for the geometry involved. This is- especially true for the shorter wavelengths. The exact relationship for both aircraft at 1000 feet is depicted graphically in Fig. 16. Consider, for example, the case of -2-0 miles, set3ar.at-i.0n-:. If -either -a-i-rcra-ft. :ch==•"=•-«-^ • ingsd ai=titude:

3J500-JT1C s-igh'fel l-eve-l would- -chang-s -franr maximum -t'o- -minimum« T"hö- :5:airre" ef-f e.et; would be. experienced for'the 10QP-mc and 250-mc signals* respectively, by either aircraft changing altitude by 2'7 feet, and- 103 feet. If excursions in altitude of both aircraft occur, this -effect can be doubled. If one aircraft, "bounced" through a minimum it, might do 30 with such speed that the Ssterlino-

WADC TR 52-135 11

lobe structure, but remain near the free space level», in this case, the indi- cated £ would be zero,. This reasoning can be used in an attempt to .ex- plain the "very w5.de .scatter in & evidenced in Figs.» <?3 and 24. In fact-, toward the end of each run in the original data, the definite lobe structure seems to be. absent except for 250 mc. The corresponding reflection coeffi- cients for this region would approach zero,

IV. CONCLUSIONS

Results of these experiments and; subsequent calculations suggest .several important conclusions. The conditions of the experiment were satisfactory for observing phenomena peculiar to air-to-air tropospheric propagation over smooth fresh water. Such phenomena include the effect of earth reflections and, to a lesser degree, the refractive effect of the earth's atmos?phere,

Ercm-an examination of the data presented herein, the following specific cöriclus iöhs can be drawn:

i(l) From the best fit free space curves indicated in the replotted data, Figs.. 9, 10, II, the general trend of signal strength vs range follows an inverse distance curve,

(2) The actual lobe structure envelopes compare favorably with the theoretical, especially at close ränge. Notable exceptions are evident in Figss 9, io, and. 11, where the lower envelope boundary sags below theoretical in all cases. At long ranges the envelope correlation is poor. As the theoretical exceeds in range the actual in all cases, a substandard refractive condition is sug- gested,

(3) The experimental diffraction slopes approach closely those fre- quency dependent slopes theoretically calculated assuming first mode diffraction theory. Close examination indicates better correlation of actual with theoretical slopes when a- 3/3 earth's radius (substandard atmosphere) is assumed,

(4) The position of the last lobe maxima for all three test runs occurred near that anticipated for a 3/3. earth's radius,

(5) No severe or extended fades in signal level were occasioned that could not be explained on the basis of earth reflections,

(6) The actual signal lobe rates follow in general the theoretical -value for each operating -frequency 5 -The oceas-iona-i scatter of actual rates is legitimately attributable to unavoidable vertical motion of the aircraft during the test runs,

(7) Reflection coefficients calculated from recorded signal maxima and

FADC "TR 52-135 Ik

minima compare favorably with theoretical values for smooth fresh water. Small departures c-l-ose/ in can stem from sudden vertical motion-of the aircraft by changing the relative phase of direct ' •• and reflected rays. The major deviations, howevers for low grazing i angles (far ranges) result from an apparent reduction or loss in ground reflec^M- ray* Possible causes of this aret j

(a) Partial diffuse scattering of the earth reflectje.d .rays, and

(b) Actual physical obstruction of these rays coupled with anomalous refraction and shift ing grazing angle» ;

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FIGURE 17. Reflection Coefficient vs Grazing Angle, 250 mc.(Run r)

WADC TR 52-155 2g

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E-IGURE IS, Refiectiqn Coefficient vs Grazing Angle, 25Q mc. (Bun 2)

WADC TR 52-135 29

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WADC TR 52-135 30

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SHiGURE 21.e Eeflegtion, Coefficient vs Grazing Mgle, 1000 oc. (Eun 2)

MDC. TR 52KL35 32

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lÄDC TR 52^55 35

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FIGUP 23. Reflection Coefficient vs. Grazing Angle, 3500 mc. (Hun l)

WADC TR 52-ir35 3k

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FIGURE 2l+. Reflectioia Coefficient vs Grazing Angle,, 3300 mc. (Hun 2)

WADC TR 52-135 35

c

REFERENCES.

1, Kerr, I). E. Propagation o f Short, Radio Waves. Volume 13. Radiation Laboratory Series. MoGraw-Hirl~Böok Company, Inc., New York, 1951. Sections 5.6 and 5^9.

2. Interim Report on tjTfcra High^Frequency Propagation Tests Over, Land and Water. Electronics ^Tiest"!DiVision.'""Naval Air Test, Center, Patuxent, Maryland* August- 1948.

3« Reference No« 1, Section 5.4«

4* Reference No, 1,. Section 5.4 footnote p. 416»

5. Miller, F. P»$ and Alerlahd, G-. J*., Air-to-Air Electromagnetic Wave Pr,opagatjon. USAF Technical Report 6254;, USAF, Air Materiel Command» Gctoher 1950«

6. TeHnan, F. E. Radio Engj&eers Handbook« McGraw Hill Book Company* Inc., New York, 194?. pp 699,. 703.

7», Reference No. 1, Section 2.13

8« Booker, H. G., Walkinshaw, W. Meteorological Factors in Radio Wave Propagation. Joint Report by The Royal Meteorological and The

^äical Society, London, 1946.

9. Reference No. a, pp* 80 - 127.

10» Reference No. 1, Section 2.

11. Reference No. 1, Section 5.1.

12. Reference No, 6, pi 709.

FABÖ TR ,52^135 U>

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