National Bureau of Standards

Library, N.W. Bldg

APR 2 8 1965

^ecltnlcal ^iote ^o. 307

PHASE AND AMPLITUDE DIVERSITY IN OVER-WATER

TRANSMISSIONS AT TWO MICROWAVE FREQUENCIES

H. B. Janes, A. W. Kirkpatrick,

D. M. Waters and D. Smith

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

<€ecknic*l ^©te 307

Issued April 12, 1965

PHASE AND AMPLITUDE DIVERSITY IN OVER-WATER

TRANSMISSIONS AT TWO MICROWAVE FREQUENCIES

H. B. Janes, A. W. Kirkpatrick D. M. Waters and D. Smith

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: 40 cents

TABLE OF CONTENTS

Abstract 1

1. Introduction 1

2. General Description of the Experiment t 2

2.1. Path and Equipment Location 2

2.2. Data Output 3

3. Phase Variation Data 4

3. 1. General 4

3.2. Power Spectra of Phase Data 7

3.3. Comparison of Power Spectra with Theory .... 10

3.4. Cumulative Distributions of Phase Data 11

4. Signal Amplitude Data 12

5. Sea Reflection Screen 14

6. Conclusions 14

6. 1. Phase Variations 15

6.2. Signal Amplitude Variations 15

7. References 16

8. Appendix 17

8. 1. Phase and Amplitude Measurement

Equipment 17

Figures 19-63

in

PHASE AND AMPLITUDE DIVERSITY IN OVER-WATER

TRANSMISSIONS AT TWO MICROWAVE FREQUENCIES*

H. B. Janes, A. W. Kirkpatrick, D. M. Waters, and D. Smith

Propagation measurements at 9.4 and 9.2 Gc/s were

made over a 47 km, line-of-sight, over-water path at

Eleuthera Island in the Bahamas. The purpose was to

study the signal amplitude and phase variations at the two

frequencies, and in particular, the variations in the

phase difference and amplitude ratio ("differential amplitude") of the two frequencies. The results include power

spectra of phase and phase difference variations (from

about 10 to 4 c/s) and cumulative distributions of

amplitude, differential amplitude, and phase difference.

1. Introduction

A microwave signal transmitted over a fixed line-of-sight path

through the troposphere will exhibit time variations in both phase-ofarrival and amplitude. Such variations are caused by fluctuations in

the three-dimensional structure of the atmospheric refractive index

along the radio path. Phase-of-arrival variations have been investigated in an extensive experimental program carried on by the

National Bureau of Standards using predominantly over-land paths

[ Herbstreit and Thompson, 1956; Norton, et al. , 1956; Thompson and

Janes, 1959; Thompson, Janes, and Kirkpatrick, I960] • Although

these experiments were not designed to study amplitude variations,

the signals were observed to undergo occasional deep and prolonged

fading on paths of the order of 15 km in length. Two questions which

these previous experiments have left unanswered are: (1) how will

the signal phase and amplitude behave on an over -water path, and (2)

to what extent will the phase and amplitude variations be correlated in

two signals separated in frequency.

*This work was supported by the U.S; Air Force Missile Test Center

(MTDRM) under Air Force Order No. (08-606) 63-2147, Sept. 11, 1962
An essential part of the Mistram precision missile trajectorymeasurement system being installed on the Atlantic Missile Range

consists of microwave links which transmit phase information from

the outlying terminals of two baselines to a central station where it is converted to target position and velocity information. Each link

involves the simultaneous transmission of two signals in the neighborhood of 8 Gc/s separated by 256 Mc/s. The successful operation

of the system requires that the phase and amplitude variations of one

signal relative to the other remain below certain limits.

At the request of the Air Force Missile Test Center, the

National Bureau of Standards in the fall of 19&2 made a series of

measurements at the Mistram installation on Eleuthera Island,

British West Indies, to determine the magnitude of these relative

phase and amplitude variations. The purpose of this report is to

present the results of this work.

2. General Description of the Experiment

2.1. Path and Equipment Location

The baseline microwave link chosen for the measurements at

Eleuthera extends from the Mistram central site near the Auxiliary

Air Force Base in a southerly direction to the Powell Point site at

the southwestern tip of the island, a distance of 47. 32 km (figure 1).

Except for the immediate foreground at each end, the entire path is

over water. The antenna at the central site terminal is a 3-meter

parabolic dish mounted on a building roof 4. 1 meters above local

ground and 32 meters above mean sea level. At the Powell Point

terminal, a 2.4-meter parabolic antenna is mounted on a guyed steel

tower at a height of 59 meters above local ground and 71 meters above

mean sea level.

The klystrons and mixers were placed as close as possible to

each antenna, and coaxial cable was used to carry the IF signals

from that point on. (At Powell Point, the mixing took place directly

behind the antenna; at the Central site, the mixer was at the end of

about 20 m of waveguide, almost all of which was inside an air-conditioned building). Consequently, the phase and amplitude variations

discussed here are only those contributed by the propagation medium

and by any mechanical motion of the two Mistram antennas involved. A

description of the phase and amplitude measurement equipment is given

in the appendix.

-2-
Meteorological measurement equipment consisted of two microwave refractometer sensors mounted on the antenna tower at Powell

Point at heights of 6 and 43 meters above ground (18 and 55 meters

above mean sea level). Another refractometer sensor was mounted

on a mast 5. 5 meters above ground (33. 5 meters above m. s. 1. ) at the

Central site. In addition, wet and dry bulb thermometers and aneroid

barometers were placed in shelters 1.5 meters above ground at each

terminal. Anemometers to record horizontal wind velocity were located

at each terminal, 8 meters above ground at the Central site and 13

meters above ground at Powell Point.

2.2. Data Output

Experimental data were recorded during three periods, each

approximately two days in length. In general, they consisted of time

variations in the following parameters:

1. The phase-of-arrival of a 9.2 and a 9.4 Gc/s

signal transmitted over the 47 km path, each phase

record, in effect, being relative to an arbitrary

phase reference at 9.2 and 9.4 Gc/s, respectively.

Hereafter, these will be referred to as the "single

frequency phase" or <j> data. These variables were

each split into two recordings, a low-pass recording with a pass band from to 10 c/s, and a bandpass recording with a pass band from about 0.25

to 10 c/s. The low-pass recording circuits were

equipped with an automatic scale -shift feature

which shifts the variable in precise increments of

180 degrees whenever it threatens to exceed the

scale limits of the recorder. The resulting records

combine relatively high resolution with long-term

continuity.

2. The difference between the phase-of-arrival of

the 9.2 and 9.4 Gc/s signals, hereafter referred to as "phase difference" or A <(> data. This variable

was also split into a low-pass recording (from to

5 c/s) and a band-pass recording (from.. 04 to 15 c/s).

Although it was not possible to calibrate this record in

terms of absolute phase difference, the time variations

in phase difference were recorded relative to an arbitrary reference in the same way as were the single

frequency phase variables.

3-
3. The amplitudes of the 9.2 and 9.4 Gc/s signals. These records have a pass band extending

from to about 15 c/s. Since the purpose of this

part of the experiment was to study amplitude

changes at one frequency relative to those occurring on the other, no attempt was made to

calibrate these data in absolute units of received

signal power. Hence, they are discussed here

in terms of decibels relative to an arbitrary

zero.

4. Refractivity variations at the low-level refractometers at both terminals, and the variations in

refractivity difference between the refractometers

located 6 and 43 meters above ground at Powell

Point.

5. Wind speed and direction at each terminal.

6. Wet and dry bulb temperature and barometric

pressure at each terminal.

The phase and amplitude data were recorded continuously in

analog form on magnetic tape, with occasional real-time monitoring

on paper strip charts. The wind speed and direction and refractivity

were recorded on relatively slow-moving paper strip charts to indicate long-term changes. The temperature and pressure were logged

at half-hour intervals throughout each recording period.

3. Phase Variation Data

3.1. General

In general, the single-frequency phase ( <j> ) data were very similar to those obtained in previous experiments. The gross, hour-to-hour

and day-to-day variations caused by long-term changes in the mean

refractive index along the path covered many hundreds of phase degrees.

This is illustrated in figures 2, 3, and 4, which show the 9.4 and 9.2 Gc/s

phase records sampled at 30-minute intervals and plotted versus time.

The phase ordinate here is expressed in terms of changes in the apparent

radio path length both in parts per million and in wavelengths at the two

radio frequencies (i. e. , 1^ = 360 phase degrees). (Only the first half

4-
of the second run, October 30 - November 1, is plotted here. Equipment trouble at the middle of the run resulted in loss of continuity, and

hence precluded showing the long-term variations for the entire run. However, the spectrum and distribution analysis described below contain

samples taken over the entire recording period. )

The corresponding long-term changes in surface ref ractivity*

measured at both terminals along with the vertical gradient variations

at Powell Point are plotted in figures 5, 6, and 7.

Some correlation between the grossest features of the phase and

refractivity can be seen, but in general, the correlation is poorer than

in the case of shorter, over-land paths used previously [ Thompson and

Janes, 19^9; Thompson, Janes, and Kirkpatrick, 19&0] . This is not

surprising, since the refractivity data were taken on land and were

influenced heavily by local terrain, vegetation, etc. Hence they do not

accurately represent the mean refractivity along the over-water path.

The long-term variations in the vertical refractivity gradient at Powell

Point are plotted in the lower part of these graphs. Similar long-term

plots of the phase difference (A <|>) data are shown in figures 8, 9, and

10.

In interpreting these data, it is helpful to consider a highly oversimplified model of the propagation path and medium. In this model, the

received radio energy is confined to an infinitely thin ray path between

the two antennas with no multipath permitted. At any given instant the

phase of the received signal at radio frequency f relative to that emerging from the transmitting antenna would be given by:

#J . 360fL[ 1 + N (t) x 10" ] 4>f

(t) = =

*The relationship between the refractivity, N, and the refractive

index, n, is given by:

6 N = (n - 1) x 10

In other words, a change of 1 N-unit represents a change of about 1 part

per mill ion in n.

-5-
where

<t>r( t ) ~ phase-of-arrival in degrees

N(t) = refractivity averaged over the line path

L = path length

C = velocity of propagation in a vacuum

<J> (t) then would be a linear function of N(t), and the derivative

d<|>

dN

would be proportional to f. Similarly, the phase difference between

two signals at f and f is given by

360 (f - f ) L(l + Nxl0" ) *<|>(t) = +r

(t) - <|>r

(t) =

and the derivative — would be proportional to f - f - . As N(t)

dN 1 L

varies with time, <J> (t), ()> (t) and A <|>(t) would (under this model) be

1 2

perfectly correlated with N(t), and the variations in A <j> would be

smaller than those in <j>,. bv the ratio

fl" f

2

f

l

Even in this simple model, it would be very difficult to determine N(t)

so as to provide a corrector for the (j> and A § variations. (The N

data taken at the path terminals are obviously a poor estimate of N in

-6-
this case. ) However, it would be possible to use a single frequency phase

record to compensate for the correlated part of the phase difference variations .

The observed data behave somewhat in the manner predicted by

the use of this model, but they also differ from it in some important

respects. For example, the long-term variations in single-frequency

phase, A, at 9.2 and 9.4 Gc/s shown in figures 2, 3, and 4, are obviously well correlated with each other. Furthermore, the corresponding variations in phase difference between the 9.4 and 9.2 Gc/s signals

(i. e. , A A) are also well-correlated with either of the single frequency

phase records, as can be seen by comparison of these figures with

figures 8, 9, and 10. The lower graph in each of the latter figures is

simply

V f2 A* (t) = AA(t)- — A (t). R ±

x ,IX

It shows the extent to which the effects of long-term changes in A A

could be reduced by subtracting from them the scaled-down A variations and hence removing the A A variations caused by the difference

in radio frequencies.

It should be pointed out that

A A (t) = — A (t) - A (t) R f

l fl f

2

and hence is directly proportional to the difference in radio path lengths

that would be observed at f and f .

3.2. Power Spectra of Phase Data

The relationships among these phase and phase difference data are

illustrated in a more systematic way in the power density spectra of

single frequency phase and phase difference shown in figures 11, 12, and

-7-
13. (In each figure, the lower frequency portion was computed from the

data for the entire run, digitized at one-minute intervals; the higher

frequency portion is the median of 12 to 15 individual spectra computed

from three-minute data samples digitized at the rate of 30 points per

second. ) The purpose of the power spectrum is to indicate how much

of the mean square fluctuation (variance) of a given variable is contributed by a particular band of fluctuation frequencies. Since the ordinate

is given in units of variance (degrees squared) per unit bandwidth, the

variance contributed by any band of spectral frequencies can be computed simply by integrating that portion of the spectral density curve.

Of particular interest here are the relative magnitudes of the. $

and A § spectra. At the lowest fluctuation frequencies (below 10 c/s),

the phase difference, A cj>, is approximately three orders of magnitude

smaller in spectral density (i. e. , variance) than the single frequency

phase, <J>. This is in accord with the simplified picture given above,

i. e. ,

f - f 2

variance of A6 = ( ; ] x [variance of <±> 1 f

l /

f

l

where

f

l "

f2 \

Z

/ 9.4 - 9.2 \

2

A c -4

\

V 9.4

= 4.5 x 10

However, the A <j> spectrum increases relative to the cj> spectrum as the spectral (fluctuation) frequency increases until the two spectra cross

in the neighborhood of 1 c/s. Beyond 1 c/s, the A cf> spectrum tends to

assume a value twice that of the (f> spectrum as the fluctuation frequency

increases.

From these spectra and the above simple model we can conclude

the following: at the lowest spectral frequencies, § and (j> are so

1 2

well correlated that the principle difference between them is the slight

difference in magnitude caused by the difference in radio frequencies.

Consequently their difference, A <j>, is well-correlated with both $

and 4> , but reduced in magnitude by 1 2

-8-
fl" f

2 . h'h . -qr (or ~v '•

With increasing spectral frequency, the correlation of § 12and <})

reduces, causing a relative increase in A 4> fluctuations. Above about

1 c/s, the correlation of 6. and <j> is apparently zero, since the

1 2

spectral density of A (J) is approximately twice the spectral density of

<f> or cj> . At this point it might be helpful to recall the following re1 2

lationship:

var A <j> = var <f> + var A. - 2 r n/ var 6. var 4> f

l 2

f

l f 2

where "var" indicates a variance or mean squared fluctuation, and

r = correlation coefficient between d> and 6

f f

1 2

The fact cb and (j> do not remain correlated throughout this

1 2

range of fluctuation frequencies suggests that the simplified single ray

path model described above is not very realisitic. Actually, the radio

energy sent between the Mistram Central Site and Powell Point travels

over many slightly differing paths simultaneously. These include sea

reflections as well as refraction and scattering from atmospheric irregularities. According to scatter propagation theory [ Wheelon, 1959] , the

amplitude of the energy scattered in a given direction from an atmospheric irregularity is a function of (among other things) the radio frequency. The received signals, then, are the vector sum of a number of

contributing sources, and the resultant phase of the signal at f contains

some rapidly fluctuating components due to scattering which are statistically independent of the corresponding phase fluctuations at f _ . An interesting sidelight of the spectrum analysis is the appearance

of two pronounced spikes in the <\> spectrum during the run of October 3

to November 1 (figure 12). These appear at about 0. 55 and 2.4 c/s and

are apparently caused by small resonant vibration of the antenna tower

at Powell Point. The wind velocity data for each run are plotted in

figures 14, 15, and 16, and it should be noted that the winds were at their

maximum at Powell Point during this run.

-9-
3.3. Comparison of Power Spectra with Theory

E. C. Barrows [ 1 9&4] of CRPL has computed theoretical power

spectra of 4> and A <j> and these are shown in figure 13a along with R

a derived spectrum of A cj> and corresponding experimental spectra of

<j) and A cj) from the November 2-4 run. (A <j> was not recorded directly R

in this experiment and hence an experimental A <b spectrum could not R

be obtained. However, figures 8, 9, and 10 show a rough estimate of

the long term variations of this parameter obtained from the cj> data

as described in Section 3. 1. ) The theoretical c|> and A <J> spectra R

were computed from forward scatter theory using the appropriate path

length and antenna parameters and assuming (a) a mean cross-path

wind velocity component of 2 m/sec; (b) Taylor's hypothesis relating

time and space fluctuations in the turbulent atmosphere; (c) the absence

of surface-reflected multipath; and (d) a slope of f for the <j> spectrum

(chosen as a convenient approximation of the observed slopes). The

theoretical spectrum of A § was produced by first deducing the cospectrum of cj> and <f> from the relative magnitudes of the theoretical <|> 1 Ct and A <J> spectra and then using this cospectral function to compute the R

A cf> spectrum.

There appears to be good general agreement between the experimental and theoretical cj> and A 4> spectra (the placement of the theoretical spectra on the vertical scale is arbitrary). The agreement could

probably he improved by assuming a steeper slope for the <j> spectrum,

e.g., f instead of f . This agreement is somewhat surprising in

view of the fact that the theory assumed no multipath (for the sake of

simplicity). Since the path geometry offers ample opportunity for strong

sea multipath effects, (as evidenced by the fading data discussed below)

the influence of multipath on the cj> and A cj) spectra in this range of

spectral frequencies may not be great.

One interesting discrepancy between theory and experiment occurs

at the high-frequency end of the spectra. The theoretical <j) spectrum

shows a decided steepening in slope above about 0. 8 c/s caused by

antenna aperture smoothing. The corresponding experimental curve

shows some steepening but fails to follow the theoretical curve at the

highest frequencies plotted. The higher spectral density of the data here

may be due to measurement and data processing noise. The latter dominated the spectrum above 5 c/s and may have influenced the spectrum

somewhat between 1 and 5 c/s.

- 10-
3.4. Cumulative Distributions of Phase Data

In order to provide an estimate of the probability that the variations in phase difference will exceed a given range during a given time

period, cumulative distributions were obtained for each of the threeminute samples used in the spectrum analysis described above, and

also for 30-minute periods spaced as nearly as possible at 4-hour intervals throughout each run.

The three-minute distributions are shown plotted on Gaussian probability paper in figures 17 through 22. (In these and subsequent distribution graphs, the times noted refer to the beginning of the sample. ) Of

the 35 samples shown, 34 have 1-99% ranges of 5 degrees or more, while

25 have 5-95% ranges of 5 degrees or more. It should be borne in mind

that these distributions are taken from a band-pass recording with lower

cutoff at 0. 04 c/s, and consequently, the ranges of variation shown here

are systematically somewhat smaller than the actual ranges that would

have been obtained in the absence of filtering.

To illustrate the general appearance of the short-term variations

in phase difference, two examples of chart records are shown in figure

23. They represent the "quietest" and "noisiest" of the band-pass

samples used in the distribution analysis. In figure 24, two examples

of longer-term variations are shown. These are 30 minute samples

transcribed from the low-pass record and were chosen primarily to

coincide with two of the three differential fading samples shown in

figure 28. It should be noted from these samples that phase difference

changes on the order of 2 degrees occurred within a period of one

second, and that changes twice that size occurred within a minute. (The

very large sweeps of the pen extending over nearly the width of the chart

in the 1813-1843 sample in figure 24 are caused by the automatic 180

scale-shift feature employed in all the low-pass phase recording circuits).

The thirty-minute cumulative distributions are shown in figures 25,

26, and 27. They display a wide variety of slopes, with 1-99% ranges

extending from less than 2 degrees to more than 60 degrees. It should

be pointed out here that this is not an unbiased random sample of phase

difference data. As will be explained in the next section, the signal

levels sometimes went below the operating threshold of the equipment

during periods of deep and violent fading, resulting in loss of both phase

and amplitude data. Since the phase difference variations were often

observed to be unusually large during periods of weak signal, it is probable that these sample distributions are somewhat biased in that they

fail to represent the worst propagation conditions encountered.

-11-
4. Signal Amplitude Data

The time variations in the amplitude of the 9. 4 and 9. 2 Gc/s

signals were recorded continuously on magnetic tape. The differential amplitude, or fading of one signal relative to the other, was subsequently generated in analog form in the laboratory.

Three examples of chart transcriptions of the 9- 4 Gc/s amplitude function along with the corresponding differential amplitude are

shown in figure 28. It will be seen at once that the amplitude data are

characterized by long periods of strong signals with negligible fading,

interrupted by periods of significant fading lasting from several

minutes to several hours. The long-term fading characteristics of the

9.4 Gc/s signal for each run are shown in figure 29. Here the medians

of half-hour periods are plotted versus time, along with vertical bars

indicating the 10% to 90% fading ranges taken from cumulative distributions of selected half-hour periods. In the lower portion of each

graph, the 10-90% fading range of the differential amplitude is shown for

the same half -hour periods. The cumulative distributions of amplitude

and differential amplitude for the selected periods are shown in figures

30 through 41. To illustrate the wide variety of fading conditions represented in these cumulative distributions, figure 42 shows transcriptions

of the differential amplitude variations for three of the periods. They

were chosen to represent both quiet and active fading periods. For

example, the 0230 to 0300 sample shows a 10 db fade of one signal relative to the other occurring within one minute, while the fading during

the entire 1727 to 1757 sample covered only about 0.2 db. The 1813 to

1843 sample is especially interesting because it contains both quiet and

active differential fading within a relatively short period of time. Comparing it to the phase difference transcription for the same period in

figure 24, there was apparently good time correlation between the magnitude of the differential amplitude and phase difference variations.

However, with the possible exception of the deep fading periods, some

of which were necessarily excluded from this analysis, this was not

true in general, i. e. , the observed variances of phase difference and

differential amplitude were found to be essentially uncorr elated.

From figures 28 and 29 and from the sample data transcriptions,

it can be seen that the ranges of short-term (i. e. , minute -to -minute)

variations in both amplitude and differential amplitude are very small

(on the order of 1 db) during strong signal periods. A general decrease

in signal strength was invariably accompanied by a pronounced increase

in the fading range of both signals and of their decibel difference. The

-12-
periods of weak signal, then, are of primary interest in the present study.

The minimum signal level at which the phase and amplitude measurement

equipment will operate was approximately 2 db below the maximum

signal strength observed. When the signal dropped below this threshold,

both phase and amplitude data were lost for periods of a few seconds to a few minutes. Unfortunately, this occurred occasionally during all

three runs, with the results that the fading ranges and cumulative distributions shown here do not include some of the worst fading conditions.

(About 13% of all half-hour periods contained at least momentary losses

of data believed to be caused by deep fading. ) The half-hour samples

for the distribution analysis were spaced as nearly as possible at 4-hour

intervals, but considerable deviation from this rule was sometimes , required to avoid these occasional sections of lost data. The bias that

would inevitably result from shunning periods of lost data (i. e. , deepest

fading) was partially compensated for by the arbitrary inclusion of three

or four additional signal amplitude samples from each run, deliberately

chosen to represent periods of deep fading (but still avoiding periods of

lost data, of course). This compensation may have been reasonably

successful, since 15% of the resulting samples have peak-to-peak differential fading ranges in excess of 10 db, while visual inspection of the

differential fading records for all three runs indicates 17% of all halfhour periods have peak-to-peak ranges greater than 10 db.

The fading periods do not appear to be correlated with the refractive index data (figures5, 6, 7), wind velocity (figures 14, 15, 16), phase

data, tidal fluctuations (figure 43), or time of day. (However, some of

the fading periods did coincide with the passage of rain showers across

the path. ) The most readily-suspected cause of this deep fading is sea

multipath, i. e. , the interference of direct and strong sea- reflected

signal components. The phase relationship of the two components could

easily be changed from reinforcement to cancellation by a very moderate

change in the refractive index gradient between the sea surface and the

minimum height of the direct ray. If this were the case, one might

expect some correlation between the fading and the refractive index

data in figures 5, 6, and 7. However, the latter data were obtained at

the path terminals, several hundred meters from the shore in both

cases, and hence should not be expected to be closely correlated with

refractive index gradient variations over the sea surface.

Another possible (but in this case less likely) cause of fading is

one described by Dutton [19&1] which requires a vertical refractive

index profile such as would produce a low level, surface-based radio

-13-
"duct". If the upper limit of this duct lies above one path terminal and

below the other, it is possible that the radio waves will be refracted in

such a way that the receiving terminal will lie in a "shadow zone" with

respect to the transmitter, with resulting deep fading.

Simultaneous measurements of signal amplitude on this path and

the refractive index structure (on the path rather than at the land terminals) from the sea surface to a height of about 100 meters would be veryvaluable in studying the propagation phenomena involved.

5. Sea Reflection Screen

Since it appeared likely that reflections from the sea surface would

have an important influence on phase and amplitude variations, a screen

was designed and constructed by Pan American World Airways to be

placed in the water at the nominal reflection point to partially block off

sea reflections. (Bussey [ 19^0] had successfully used such a screen on

a much shorter path. ) The screen actually consisted of two sections

mounted in the same vertical plane with a 6 meter horizontal separation

between them. Each section was a square 6 meters on a side, covered

with galvanized hardware cloth having a grid spacing of 0. 5 cm.

The screen was put into position at about noon on November 2,

and remained there throughout the November 2-4 run. Although the

data from this run contain unusually long periods during which the

signals exhibited very little fading, they also contain periods during

which some of the deepest and most rapid fading occurred. Keeping

in mind the usual differences in data gathered in successive runs under

"identical conditions" (i. e. , nothing permitted to change but the weather! ), one cannot say that the data reveal any evidence that the screen produced

a significant effect in the data. Apparently, if such effects exist, they

are sufficiently subtle to require a more extensive experimental study to

detect them.

6. Conclusions

The conclusions drawn from the results of this experiment can be

summarized as follows:

-14-
6.1. Phase Variations

Although the phase variations in the 9.4 and 9. 2 Gc/s signals are

well-correlated at low fluctuation frequencies, the correlation deteriorates toward the high frequency end of the power spectrum and is

essentially zero above about 1 cycle per second. Hence virtually all

of the phase difference variation at low spectral frequencies is attributable to the difference in radio frequencies and is well- correlated with

both the 9.4 and 9.2 Gc/s phase variables. However, the loss of correlation at higher spectral frequencies is indicated by the relative

increase in the phase difference spectral density at the higher frequencies.

A cumulative distribution analysis of the phase data indicates that

in 34 out of 35 cases the phase difference varied by more than 5 degrees

during a 3-minute period, and that changes in phase difference on the

order of 20 degrees occurred within a period of 1 second.

The variance of phase difference showed no correlation with time

of day, tide, or atmospheric refractive index and wind data gathered

at the path terminals.

6.2. Signal Amplitude Variations

A wide variety of fading conditions were observed, in terms of

both the variations in signal amplitude at either frequency and the variations in the differential amplitude. The data indicate that a 10 db

change in differential amplitude can occur within a period of one minute.

They also indicate a 0. 17 probability that during a given half -hour period,

the differential amplitude of the two signals will vary by more than 10 db.

As in the case of the phase data, the differential fading range

showed no correlation with time of day, tide, wind velocity or refractive

index data.

A specially-designed screen placed at the nominal sea reflection

point in an effort to eliminate multipath interference from the sea

surface failed to produce any clearly identifiable effect in the fading

data.

15-
7. References

Barrows, E. C. (December, 1964), Private Communication.

Bussey, Howard E. (Dec. 1950), Reflected ray suppression, Proc

IRE ^^ No. .12, 1453.

Dutton, E. J. (June 16, 1961), On the climatology of ground-based radio

ducts and associated fading regions, NBS Tech. Note 96.

Herbstreit, J. W. , and M. C. Thompson, Jr. (July 1956), Measurements of the phase of signals received over transmission paths

with electrical lengths varying as a result of atmospheric turbulence, IRE Trans. AP-4, 352-358.

Norton, K. A., J. W. Herbstreit, H. B. Janes, K. O. Hornberg,

C. F. Peterson, A. F. Barghausen, W. E. Johnson, P. I. Wells,

M. C. Thompson, Jr. , M„ J. Vetter, and A. "W. Kirkpatrick

(1956), An experimental study of phase variations in line-ofsight microwave transmissions, NBS Monograph 33.

Thompson, M. C. , Jr. , and H. B. Janes (Feb. 1959), Measurements

of phase stability over a low-level tropospheric path, J. Res.

NBS 63D (Radio Prop.), No. 1, 45-51.

Thompson, M. C. , Jr. , H. B. Janes, and A. W. Kirkpatrick (Jan. 1960)

An analysis of time variations in tropospheric refractive index and

apparent radio path length, J. Geophys. Res. 65, No. 1, 193-201.

Wheelon, Albert D. (Apr. 1959), Radio-wave scattering by tropospheric

irregularities, J. Res. NBS 63D (Radio Prop. ), No. 2, 205-233.

-16-
8. Appendix

8. 1. Phase and Amplitude Measurement Equipment

The equipment used for two-frequency phase and amplitude

measurements in this experiment can be described briefly as follows:

The outputs of two frequency- stabilized c-w klystrons are combined,

passed through a circulator and onto an antenna. The signal received

at the remote antenna is divided between two transponders; each transponder retransmits its received frequency with a small offset in frequency. The transponder signals are received at the first antenna and

passed through the circulator into a mixer where they are mixed with a

small portion of the output of the stabilized klystrons. The output of the

mixer is amplified by an IF amplifier for each transmitter transponder

pair. The output of each IF is sent to a phase meter and amplitude

detector. The IF signals produced at the transponder are sent to the

control end by a 4. 9 Gc/s FM link where they are used as phase referencesfor the phase meters. The IF signals are fed into mixers and the

difference frequencies drive a phase meter to provide the phase difference information. The IF signals from the transmitter end provide the

amplitude variation information.

Referring to the block diagram in figure 44, a 10 Mc/s secondary

frequency standard is used as a frequency reference for stabilizing the

klystrons at the control end of the path. The oscillator stays within

a few parts in 10 over a wide temperature range and ages at a rate of

3 parts in 10 per day. A times 2 multiplier raises the frequency to

2 00 Mc/s and the power level to about 0. 5 watt. The 2 00 Mc/s is fed

to the harmonic generators. A 20 db crossguide coupler supplies L.O.

power from the 9.2 Gc/s klystron (K ) to a mixer (X , 1N23D); the output of the harmonic generator (H ) goes through a ferrite isolator to the

mixer. The output of the mixer goes to a 500 kc/s phase-lock circuit

which locks the klystron ±500 kc/s away from the 9-2 Gc/s output of

the harmonic generator.

The 9. 4 Gc/s klystron is locked ± 93 kc/s away from the 9. 4 Gc/s

output of the harmonic generator (H ?

) by the same technique. A hybrid

tee is used to combine the output of the two klystrons and provide some

isolation between them. Ferrite isolators are used between each klystron

and the hybrid tee to provide additional isolation. The combined output

goes to the antennas through a ferrite circulator; the circulator isolates

the incoming from the outgoing signal on the antenna. A mixer (X ) mixes

-17-
the incoming signal with a small portion of the klystron outputs and provides the difference signals between the outgoing and incoming signals.

At the other end of the path, the signals coming off of the antenna are

split by a hybrid tee; each output of the hybrid tee goes through a circulator (C_ and C ) to a mixer (X and X ). The 9. 2 Gc/s klystron

2 3 la 2a

(K ) is phase -locked to the incoming 9. 2 Gc/s signal ± 50 kc/s by

let way of C and X and the 9.4 Gc/s klystron (K ) is phase -locked to

l* la ^a

the incoming 9.4 Gc/s signal ± 80 kc/s by way of C and X . Because

j ^a

of the means used to take the phase difference, the klystrons must be

both either higher or lower in frequency than their incoming signal.

The outputs of the klystrons (K and K ) are added in the hybrid tee

and go to the antenna. The 80 kc/s and 5T) kc/s signals are amplified

in narrow band amplifiers and multiplexed on to a 4. 9 Gc/s FM transmitter. The 4. 9 Gc/s FM receiver at the control end separates the

80 kc/s and 50 kc/s signals. The output of mixer X at the control end

goes to 50 kc/s and 80 kc/s narrow-band amplifie.rs. The two 50 kc/s

signals go to a phase meter -which provides the 9. 2 Gc/s range variation

measurement, and the 80 kc/s signals go to a phase meter to provide

the 9.4 range variation measurement. The 50 kc/s and 80 kc/s signals

from the control end go to a mixer, and the two signals from the FM

receiver go to a mixer; the two 30 kc/s difference signals go to a third

phase meter which provides the range difference variation measurement.

Because the 9.4 and 9. 2 Gc/s signals are each transmitted over the

path twice (once in each direction), the phase variations actually measured

are twice as large as would be the case if the path were only traversed

once and the incoming phase were compared to a local phase reference.

In this report, all phase data have been reduced by the factor of two to

represent "one -way "phase variations. The output of the 50 kc/s and

80 kc/s narrow band amplifier at the control end goes to amplitude

detectors for signal amplitude variation measurements.

-18-
ELEUTHERA ISLAND

B.W.I.

-19-
o

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-20-
PHASE VARIATIONS OF

9.4 AND 9.2 Gc/S SIGNALS

MISTRAM CENTRAL -POWELL POINT PATH OCT 30-31, 1962

^<#94Gc/s)

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IN PATH CHANGELENGTH

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-21-
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REFRACTIVITY VARIATIONS

OCTOBER 20-22, 1962

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Figure 5

-23-
REFRACTIVITY VARIATIONS

OCTOBER 30 -NOVEMBER I, 1962

CO

1200 OCT. 30

Figure 6

-24-
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NOVEMBER 2-4, 1962

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-35-
CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2 Gc/s

PHASE DIFFERENCE VARIATIONS

Ml ST RAM CENTRAL - POWELL POINT PATH

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(3-MIN. SAMPLES; PASSBAND: 0.04-15 c/s)

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-36-

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PHASE DIFFERENCE VARIATIONS

MISTRAM CENTRAL- POWELL POINT PATH

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-37-

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PHASE DIFFERENCE VARIATIONS

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-38-

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PHASE DIFFERENCE VARIATIONS

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CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2 Gc/s

PHASE DIFFERENCE VARIATIONS

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NOVEMBER 2-4, 1962

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-40-

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PHASE DIFFERENCE VARIATIONS

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-46-
EXAMPLES OF AMPLITUDE AND DIFFERENTIAL AMPLITUDE VARIATIONS

MISTRAM CENTRAL- POWELL POINT PATH

UPPER CURVES = DIFFERENTIAL AMPLITUDE (9.2-9.4 Gc/S)

LOWER CURVES = AMPLITUDE AT 9.4 GC/S

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Figure 28

-47-
SIGNAL AMPLITUDE VARIATIONS

MISTRAM CENTRAL- POWELL POINT PATH 1962

(BASED ON 30 MINUTE SAMPLES)

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..... LEVEL EXCEEDED 10% OF TIME -- MEDIAN

1 — LEVEL EXCEEDED 90% OF TIME

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Figure 29

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CUMULATIVE DISTRIBUTIONS OF 9.4Gc/s SIGNAL AMPLITUDE

MISTRAM CENTRAL -POWELL POINT PATH

OCTOBER 20-22, 1962

(FROM 30-MIN. SAMPLES)

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Figure 30

99.9 99.99

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MISTRAM CENTRAL - POWELL POINT PATH

OCTOBER 20-22, 1962

(FROM 30-MIN. SAMPLES)

99.9 99.99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 31

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CUMULATIVE DISTRIBUTIONS OF 9.4 GC/S SIGNAL AMPLITUDE

MISTRAM CENTRAL - POWELL POINT PATH

OCTOBER 30 -NOVEMBER I, 1962

(FROM 30-MIN. SAMPLES)

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Figure 32

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CUMULATIVE DISTRIBUTIONS OF 9.4 GC/S SIGNAL AMPLITUDE

MISTRAM CENTRAL - POWELL POINT PATH

OCTOBER 30 -NOVEMBER I, 1962

(FROM 30-MIN. SAMPLES)

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OCT. 31

NOV 1 —0125 — 0555

10 30 50 70 90 99 99.9 99.99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 33

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CUMULATIVE DISTRIBUTIONS OF 9.4 Gc/s SIGNAL AMPLITUDE

MISTRAM CENTRAL -POWELL POINT PATH

NOVEMBER 2-4, 1962

(FROM 30-MIN. SAMPLES)

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Figure 34

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CUMULATIVE DISTRIBUTIONS OF 9.4 Gc/s SIGNAL AMPLITUDE

MISTRAM CENTRAL -POWELL POINT PATH

NOVEMBER 2-4, 1962

(FROM 30-MIN. SAMPLES)

- -..

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NOV. 3

1 727 2137

NOV. 4 —

10 30 50 70 90 99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 3 5

99.9 99.99

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CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2 GC/s

DIFFERENTIAL AMPLITUDE

MISTRAM CENTRAL- POWELL POINT PATH

OCTOBER 20-22, 1962

(FROM 30-MIN. SAMPLES STARTING AT TIME NOTED)

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99.9 99,99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 36

-55-
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CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2 GC/S

DIFFERENTIAL AMPLITUDE

MISTRAM CENTRAL- POWELL POINT PATH

OCTOBER 20-22, 1962

(FROM 30-MIN. SAMPLES STARTING AT TIME NOTED)

10 30 50 70 90 99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 37

-56-

99.9 99.99
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CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2 Gc/S

DIFFERENTIAL AMPLITUDE

MISTRAM CENTRAL - POWELL POINT PATH

OCTOBER 30- NOVEMBER I, 1962

(FROM 30-MIN. SAMPLES STARTING AT TIME NOTED)

\

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OCT. 30

1635 - — 1813 "-- 2135 "V.X

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IN 9.2 Gc/s SIGNAL \

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OCT. 30

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OCT. 31 — 0130 — 0557

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0.01 0.1, I 10 30 50 70 90 99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 38

99.9 99.99

-57-
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CUMULATIVE DISTRIBUTIONS OF 9.4 AND 92 Gc/s

DIFFERENTIAL AMPLITUDE

MISTRAM CENTRAL- POWELL POINT PATH

OCTOBER 30- NOVEMBER I, 1962

(FROM 30-MIN. SAMPLES STARTING AT TIME NOTED)

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1532 - 1750 — 1834 —

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IN 9.2 Gc/s SIGNAL

n=-=——- i r - — - —

—- OCT. 31 2130

NOV. 1 —0125 —0555 —0944

aoi o.i 10 30 50 70 90 99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 39

99.9 99.99

-58-
en

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CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2. GC/s

DIFFERENTIAL AMPLITUDE

MISTRAM CENTRAL- POWELL POINT PATH

NOVEMBER 2-4, 1962

(FROM 30-MIN. SAMPLES STARTING AT TIME NOTED)

t~"~"-

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NOV. 2 1000 ioto

1743 2222

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RELATIVE INCREASE

IN 9.2 Gc/s SIGNAL \.

---

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NOV. 3 — 0055

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0.01 0.1 10 30 50 70 90 99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 40

99.9 99.99

-59-
CUMULATIVE DISTRIBUTIONS OF 9.4 AND 9.2 Gc/S

DIFFERENTIAL AMPLITUDE

MISTRAM CENTRAL- POWELL POINT PATH

NOVEMBER 2-4, 1962

(FROM 30-MIN. SAMPLES STARTING ATTIME NOTED)

CO

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RELATIVE INCREASE

IN 9.2 Gc/s SIGNAL

V

0.01 0.1 I 10 30 50 70 90 99 PERCENT TIME ORDINATE WAS EXCEEDED

Figure 41

999 99.99

-60-
VARIATIONS IN DIFFERENTIAL AMPLITUDE OF 9.2 AND 9.4 GC/S SIGNALS

MISTRAM CENTRAL -POWELL POINT PATH

0230-0300, OCT. 22, 1962

1813-1843, OCT. 30, 1962

SANBORN /R&cxyuU<itj PtZA*napape/t.

lOdb

1727-1757, NOV. 3, 1962

Figure 42

-61-
10

12

00

12

00

12

1

OCT

20

1

OCT

21

1

OCT

22

2

00

12

00

12

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30

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