SEMI-ANNUAL STATUS REPORT III

on

The Influence of Polarization on

Millimeter Wave Propagation through Rain

C. W. BOSTIAN, W. L. STUTZMAN,PARIS H. WILEY and R. E. MARSHALL

Submitted To: National Aeronautics and Space Administration

Washington, D. C.

NASA GRANT NUMBER NGR.47-004-091

Covering the Period i January j 1 - June 30, 1973

J u i y 1 1 9 7 3

Electrical Engineering Department

Virginia Polytechnic Institute and State University

Blacksburg,, Virginia 24061

https://ntrs.nasa.gov/search.jsp?R=19740002029 2018-06-29T01:07:37+00:00Z

PROJECT PERSONNEL

Dr. Charles W. Bostian, Principal Investigator

Dr. Warren L. Stutzman, Co-Principal Investigator

Dr. Paris H. Wiley, Faculty Associate

Mr. Robert E. Marshall, Graduate Research Assistant

TABLE OF CONTENTS

Page

1. Introduction 1

2. Narrative Summary of the Report Period 2

3. Theoretical Investigation 4

4. Data Processing 8

5. Data Presentation and Analysis 12

6. Literature Cited 90

7. Appendix: Considerations in RF System Design

for Cross Polarization Measurements 91

1. Introduction

This report describes the third six months of a continuing program

for the measurement and analysis of the depolarization and attenuation

that occur when millimeter wave radio signals propagate through rain.

Technical details covered in previous reports are repeated only as

necessary for clarity.

During the reporting period, progress was made in three major areas:

the processing of recorded 1972 data, acquisition and processing of a

large amount of 1973 data, and the development of a new theoretical

model to predict rain cross polarization and attenuation. Following

a brief narrative summary of the report period, each of these topics

will be described in more detail.

2. Narrative Summary of the Report Period

2.1 January

During January the primary concern was to develop an improved

theoretical model for rain depolarization. An effort to represent

rain scatter by Stevenson's 1953 low-frequency expansion for plane

wave scattering by a lossy oblate spheroid was abandoned when it

became apparent that Stevenson's approach would not converge fori

high permittivity scatterers like raindrops. Work continued on what

was to become a new depolarization model based on Oguchi's 1973 single

raindrop scattering functions.

2.2 February

The basic scattering model for depolarization prediction was

completed this month.

An effort was made to achieve optimum antenna alignment prior to

the 1973 thunderstorm season. Last year maximum cross polarization

isolation had occurred simultaneously on both channels, but now when

the isolation of one channel was maximized the isolation on the other

was not. Apparently one antenna is slightly out of adjustment and itsi

polarizations are not quite orthogonal (perhaps 89° apart instead of

90°) . The manufacturer could offer no suggestions beyond a trial-and-

error attempt at correction. As an interim solution the antennas were

aligned so that the residual + to - and - to + cross polarization levels

were about 25 dB and 50 dB respectively. Since our main interest is

in high rain rate data, this was an acceptable interim solution.

2.3 March

The data processing computer programs were modified to calculate

both the average cross polarization levels at each integer rain rate

and the standard deviation of the cross polarization data contributingi

to each average value. Subroutines were written to draw error bars

showing + and - one standard deviation on computer plots of average

cross polarization level versus rainfall rate. Computer routines were

developed to.extract attenuation values from recorded data and to

generate scatter plots of attenuation versus rain rates as well as

plots of average attenuation with error bars. A subroutine was written

to display average attenuation for each integer rain rate versus

average cross polarization-level for the same rain rate value. All

of these programs were used to generate the data presented in later

sections of this report.

Intense rainstorms were observed on March 16 and 17.

2.4 April

A new local oscillator (LO) for the receiver arrived and was

installed. Although initially unstable in frequency, it stabilizedi

after about a week's operation and remained stable for the rest of

April and May.

A storm occured on April 4.

2.5 May

The receiver calibration was checked on May 17 and found to be

unchanged.

An inspection of the raingauge network indicated that all gauges

but one were in good repair and working properly. The outer housing

of gauge //5 was replaced because of flaking paint.

Preparatory to setting up an on-line phase measuring system later

this year, a vector voltmeter was connected between the IF output ports

on the two receiver channels. With the instrument adjusted to read

0° phase difference between the incoming + channel and - channel co-

polarized signals, phase differences of 120° were measured between

the + channel direct signal and the - channel cross polarized signal

and 160° between the - channel direct signal and the + channel cross

polarized signal. In each case, the cross polarized signal was leading

the copolarized signal. These phase differences remained constant

over an observing period of about 10 minutes. While their absolute

values-are indicative only-of-slight component differences between

the two channels, the stability of the clear weather phase differences

indicates that measurements of rain-induced changes in differential

phase shift can be made.

Storms occurred on May 23, 26, 27, and 28. The storm on May 28

produced a peak rainfall rate of 217.4 mm/hour on one gauge for one

time interval between trips.

2.6 June

On June 3 a plexiglass roof was installed over the receiving

antenna. This will reduce the amount of water collected on the re-

flecting surface and the feed. Sufficient data are not yet available

to indicate whether or not the roof reduces the data scatter at lowi .

rain rates.

Frequency drift in the receiver LO again became a problem. An

oven to keep the LO at a constant temperature was installed on June 14

in an effort to end the drift.

3. Theoretical. Investigation

3.1 Introduction

During this report period a new method for calculating rain de-

polarization was developed. Called the scattering method, it was

described in detail in Interim Report I of this project, and the

information contained there will not be repeated. The scattering

method provides a means for calculating rain depolarization and

attenuation that is independent both of van de Hulst's equivalent

refractive index (van de Hulst, 1957) and of the differential

Attenuation-differential phase shift method (Watson and Arbabi,

1973). Since the important rain parameters are explicit rather

than implicit, it provides a rapid means for analyzing propagation

through rain under arbitrary drop shape and size distribution.

3.2 Discussion of Theoretical Models

There are presently three models for predicting the cross polar-

ization level at a given rainfall rate. The differential attenuation

method developed by Thomas (1971) was used to generate theoretical

curves for Semi-Annual Status Reports I and I_I_ of this project. UsingI

Oguchi's 1964 differential attenuation coefficients for this and similar

experiments, it usually predicts lower cross polarization levels (i...e.

greater isolations) than are measured. Substitution of Oguchi's revised

(1973) coefficients into the Thomas model leads to predicted cross

polarization generally higher than those measured.

The values of Oguchi's 1973 coefficients have been confirmed

independently by Watson;(private communication); hence, the difference

between measured cross polarization levels and Thomas's predictions

seem to lie largely in Thomas's neglect of differential phase shifts

and in the prevalent assumption that all raindrops are oblate spheroids.

Watson and Arbabi (1973) have shown that differential attenuation and

differential phase shift are equally important in determining the

received cross polarization ratio, and if one takes phase shift into

account, calculated cross polarization levels are significantly dif-

ferent from the Thomas model. This effect is illustrated in Figure 1.

If one assumes 100% oblate drops, the scattering model and Watson's

differential attenuation-differential phase shift model predict essen-

tially equivalent cross polarization levels. Figure 2 compares the

two models for a 1 km path with +45° linear polarization and uncanted

drops. However, there is meteorological evidence that the proportioni

of oblate drops in real rain is actually about 40% (see Interim Report I)

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effect of a 40% proportion of oblate drops is shown in Figure 2. The

theoretical curves which appear in the data presentations to follow

have been drawn for a. raindrop population which is 40% oblate.

3.3 The Question of Phase Shift

Although the scattering model and the differential attenuation-

differential phase shift models predict essentially the same cross

polarization level for a given raindrop population, the scattering

model and the van de Hulst (1957) approach disagree as to the exact

dependence of phase shift on polarization. For example, Figure 3

shows the differential phase shift at 19.3 GHz for a one kilometer

path. In the figure, differential phase shift by which the phase of

the received signal for a vertically polarized transmitted wave leads

the phase of the received signal for a horizontally polarized trans-

mitted wave. Shown for comparison are results of the scattering model,

of Oguchi (1973), and of Morrison, et. al. (1973). Although there are

no experimental data to support any of these curves, some differential

phase measurements will be made during 1973.

4. Data Processing

4.1 Introduction

The principal data processing efforts during this report period

were directed toward (1) the calculation of standard deviations for

measured cross polarization data and the presentation of error bars

on computer plots of average cross polarization level versus rainfall

rate, (2) the development of programs to extract and display attenuation

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4.2 Standard Deviations and Error Bars

The basic data reduction approach (see Semi-Anriual Status Report II )

has been to generate 15-second running averages of all data and to plot

selected running averages at one-second intervals over the course of a

storm. This leads to a scatter plot of cross polarization level (or

some other signal parameter) versus rainfall rate. The average cross

polarization level for each integer rainfall rate can then be determined

by collecting all of the cross polarization values corresponding to rain

rates greater than or equal to the integer value but less than the next

highest integer rain rate and averaging them. Suppose we identify all

of the cross polarization data points associated with rain rates R such

that 1 <_ R < I -f 1 where I is an integer. Let there be N.(I) of these

points. We can identify each point as X(I,J), J = 1, N(I). The average

cross polarization level corresponding to an integer rain rate of I mm/hr

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J=lN(I) - 1

- This procedure was '"used to calculate standard deviations for the

data in Section 5. To keep core requirements within reasonable bounds,

N(I) was limited to a maximum value of 70. The plotting subroutines

were modified to draw error bars extending from < X(I) > - S(I) to

11

< X(J) > + S(I) on plots of < X(I) > versus rainfall rate.

4.3 Attenuation Calculation and Display

Rain attenuation is more difficult to measure than rain-induced

cross polarization because attenuation measurement requires a clear

weather reference signal that is precisely known and has the same value

before and after a storm. So long as the cross-polarized signal stays

above the receiver threshold, the measured cross polarization level is

independent of transmitter power, antenna gains, path loss, and the

receiver gain and hence unaffected by variations in these quantities.

Because the reference signal depends strongly on all of these things,

measured values of attenuation must be interpreted with this fact in

mind.

A further difficulty peculiar to this project is that measured

attenuation values tend to be small for the short path, i.e., about

.124 dB per mm/hr. This means that a one dB error in estimating the

clear weather reference signal can introduce a significant percentage

error into attenuation computations.

For calculating attenuation it was decided to use as a reference

a 30-second average of the received signal level centered 60 seconds

after the beginning of a storm. For most storms this was satisfactory,

but for the occasional storm that begins with a "step function" of

rain, the rain rate was significant before the PB-440 computer began

recording-received signal levels.-- This meant that the reference-signal

was too low and that some calculated attenuations would come out negative,

Since negative attenuation is non-physical (rain does not amplify) the

IBM 370 data processing program was Instructed to scan the attenuation

12

data array for each storm and, if it found negative values, to add the

proper constant to make the minimum attenuation zero.

5. Data Presentation and Analysis

5.1 1972 Data

5.1.1 Introduction

Semi-annual Status Report II presented scatter plots and average

plots of measured cross polarization level versus rainfall rate for

six 1972 storms. At that time, scattering model predictions were not

available for comparison with experiment and attenuation values had not

been extracted from the measured data. For these reasons data from the

important 1972 storms are included in this report. In all the figures

which follow a * indicates an average of all data from both channels

corresponding to an integer value of rain rate and a A indicates a

value predicted by the scattering model. On cross polarization plot

a | indicates + to - polarization conversion and - to + conversion is

shown as a +. For attenuation plots + channel attenuation is shown as

a + and - channel attenuation is shown as a |. In cases where separate

averages from both channels appear on the same graph, points indicated

by a | are displaced 0.03 inches to the right of their correct location

to separate them from the + points.

Usually 5 figures are presented for each 1972 storm. These are

(1) average cross polarization level versus rain fate for each channel,

(2) average cross polarization level versus rain rate with both channels

together, (3) average attenuation versus rain rate for each channel,

(4) average attenuation versus rain rate with both channels together,

13

and (5) average attenuation versus average cross polarization level with

rain rate as a parameter. The last eliminates measurement errors due

to rain inhomogeneity and is perhaps the best experimental test of the

scattering model.

5.1.2 August 4, 1972

During this storm only the + receiver channel was working. Figure 4

illustrates the average measured cross polarization level as a function

of rainfall rate. The attenuation values measured are not displayed

because they were essentially uncorrelated with rainfall rate and cross

polarization level. This is illustrated by Figure 5 which plots average

values of attenuation versus average cross polarization levels.

5.1.3 August 17, 1972

Figure 6 illustrates the average cross polarization level for each

channel measured during this storm. In Figure 7 both channels are com-

bined to yield a composite average. Figure 8 is a plot of average

attenuation versus rain rate for each channel and Figure 9 indicates

the average attenuation resulting when data from both channels are

together. Figure 10 displays average attenuation versus average cross

polarization level.

5.1.4 September 29, 1972

Figures 11 through 15 display data from this storm.

5.1.5 October 27, 1972

Data from this storm appear in Figures 16 through 20.

5.1.6 November 13, 1972

Data from this storm are shown in Figures 21 through 25.

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36

5.1.7 November 14, 1972 !

Figures 26 through 30 present data from this storm.

5.2 1973 Data

5.2.1 Introduction

At the time of writing 7 storms have been observed in 1973 and two

of these brought the highest rain rates recorded thus far in the project.

Table 1 lists the important parameters of the 1973 storms; all are dis-

cussed in the following paragraphs except that of April 41. Data re-i

duction difficulties with that storm necessitate postponing it until

the next report. Because 'of the time delay involved in processing thei

data, storms which occurred after June 1, 1973, will appear in the nextI

report.

5.2.2 March 16, 1973

Data from this storm appear in Figures 31 through 36. Figure 31

is a scatter plot displaying 15 second running average cross polar-

ization levels taken at successive one second intervals. Data for rain

rates less than 10 mm/hr have been suppressed.

5;2.3 March 17, 1973

Figures 37 through 42 display data for this storm.

5.2.4 April 4, 1973

Plots from this storm were not available at the time of publication

^because of-computer.problems.

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55

5.2.5 May 23, 1973

Data for this storm appear in Figures 43 through 48. •.' I

5.2.6 May 26, 19731 : . •

Figures 49 through 54 present data from this storm.

; 5.2.7 May 27, 1973

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5.2.8 May 28, 1973

Figures 59 through 64 display data from this storm.

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5.3 Summary

Figure 65 is a cumulative scatter plot of all the 1973 depolar-' ' ' !

ization data presented in this report.I

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by channel for all of the 1972 and presently available 1973 data

respectively. Figures 68 and 69 present similar data for both channels

together. .l . • ' -

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values for each channel 'and Figures 72 and 73 illustrate the average

values resulting when data from both channels are combined.

Figures 74 and 75 are plots of average attenuation versus average• i

cross polarization level for 1972 and 1973 respectively.

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Figure 75. 1973 (six storms)iaverage attenuation versus average cross polarization level,

90

6. Literature Cited

1. C. W. Bostian and W. L. Stutzman, "The Influence of Polarization onMillimeter Wave Propagation Through Rain," Semi-Annual Status Report II,NASA Grant NGR-47-004-091, VPI&SU, Blacksburg, January 1973.

i .2. C. W. Bostian and W. L. Stutzman, "The Influence of Polarization on

I Millimeter Wave Propagation Through Rain," Semi-Annual Status Report I,! NASA Grant NGR-47-004-091, VPI&SU, Blacksburg, July 1972.

3. J. A.(Morrison, M. J. Cross, and T. S. Chu, "Rain-Induced DifferentialAttenuation and Differential Phase Shift at Microwave Frequencies,"BSTJ, Vol. 52, pp. 599-604, April 1973*. i

4. T. Oguchi, "Attenuation of Electromagnetic Waves Due to Rain with |! Distorted Drops," J. Radio Res. Lab. Japan, Vol. 11, pp. 19-44,

January 1964.

5. T. Oguchi, "Attenuation and Phase Rotation of Radio Waves Due to Rain:'• Calculations at 19.3 and 34.8 GHz." Radio Science, Vol. 8, pp. 31-38, !

January 1973.i '.

6. A. F. Stevenson, "Solution of Electromagnetic Scattering Problems asPower Series in the Ratio (Dimension' of Scatterer)/Wavelength," JAP,Vol. 24, pp. 1134-1142, September 1953.;

7. D. T. Thomas, "Cross Polarization Distortion in Microwave Radio Trans-mission Due to Rain." Radio Science. Vol.! 6, pp. 833-840, Odtober 1971.

8. -H..C. van de Hulst, Light Scattering by Small Particles, New York:John Wiley and Sons, Inc., 1957, pp. 28-34.

i9. P. A. Watson and M. Arbabi,' "Rainfall. Cross Polarization at Microwave

Frequencies." Proc. IEE (London). Vol. 120, April 1973.

10. P. H. Wiley, C. W. Bostian, and W. L. Stutzman, "The Influence ofPolarization on Millimeter Wave Propagation Through Rain," InterimReport I, NASA Grant NGR-47-004-091, VPI&SU, Blacksburg, June 1973.

91

7. Appendix; Considerations in RF System Design

for Cross Polarization Measurements

7.1 Introduction ;

:The RF system design, construction, and testing for this project

were done by R. E. Marshall. Mr. Marshall wrote an M.S. thesis about

the RF system and included in it both new information developed by this

project and information from widely scattered sources in the literature.

As Mr. Marshall's thesis will be of considerable interest to other

designers of depolarization experiments, most of it is reproduced in

the pages which follow,! It involves the design of a transmitter and: . ' i

receiver to meet the following criteria:

Transmitter '

11. Operating independent channels: 2(+45° and -45° polarization)

2. Switching capability: transmit or no transmit remotely selected

forieach channel!

3. Polarization isolation: > 40.0 dB

i

Receiver

1. Operating independent channels:. 2(+45° and -45° polarization)

2. Polarization isolation: > 40.0 dBt

3. Dynamic range: > 41.0 dB

4. Easily interpreted transfer function

! 92 . .

i SECTION 7.2 ' ,,

: RECEIVER

i '

7.2.1 Scope of Section. . . ; - - j

Presented in this section are typical receivers for attenuation

i • • ;and depolarization measurements, design calculations for the VPI&SU

17.65 GHz transmitter, and suggested improvements. \

! .7'.2.2 Typical Receivers

Once the path is defined, the receiver design is primary. Optimum

'receiver performance is obtained easily if one is not working under

the restriction of a specific received signal level. This restriction

fixes the upper bound of the dynamic range and forces the designer to

find hardware that will fit the situation. It is simpler to design theI ;

receiver and then provide the proper transmitted power.

Figure 2.1 is a block diagram of a receiver that will measurei

attenuation. Single conversion is used because it is economical,

introduces less noise, and is easily accomplished with available milli-

meter wave components. The RF amplifier and mixer are usually found

commercially in a single "black box." Noise associated with wide band-

widths and high noise figures is a major cause of shortened receiver

dynamic range. Bandwidths as low as 10 MHz are available with most

commercial millimeter wave mixer-preamplifiers; Typical noise figures

are from 9.0 to 10.0 dB. The local oscillator must be stable because

of the narrow bandwidth requirement. A 50% savings is'usually realized

when mechanical tuning is chosen over voltage tuning; voltage tuned

93

Detector Mixer

L. 0.

Figure 2.1. Attenuation Receiver

94

; oscillators also require more expensive and stable power supplies.| !

Detector choice is based mainly on the transmission mode used, but a

linear transfer function will result in easier data analysis.

Figure 2.2 is a block diagram of a dual channel receiver used for

cross-polarization measurements. The mixers, preamplifiers, local

oscillator, and detectors are identical to the components used in the

attenuation receiver. The local oscillator is shared by both channels,! ' . : . • • '

which greatly reduces cost and tuning difficulties. The relative cost, j

of the local oscillator allows the addition of another channel fori

about 70% of the cost of a single channel receiver.

Figure 2.3 is a block diagram of the VPI&SU 17.65 GHz, CW,

receiver. The isolators insure polarization isolation, and the

attenuators control local oscillator power to the mixers. The use of

uncalibrated attenuators here will save about $300.00 per attenuator.

7.2.3 Choosing Receiver Componentsi i

The elimination of cost and noise due to a transmission line

between a pre-amplifier and a mixer warranted the use of commercially

•available mixer-preamplifiers for the VPI&SU receiver. A balancedi

mixer with an orthomode coupling mechanism was chosen because of its

high isolation between the local oscillator port and the RF port.

Local oscillator noise suppression also prompted the use of a balanced

mixer , because local oscillator noise reduces the dynamic range of

the receiver..••—.

The mixer-preamplifier chosen waa an RHG MP015/2CI The specifi-

cations are listed below*

95

Detector

Detector

Mixer

PowerSplitter

Mixer

Figure 2.2. Depolarization Receiver

96

LOG

AMP

LOCAL

OSCILLATOR

LOG,

AMP

MIXER

MIXER

/r

Figure 2.3. VPI & SU Receiver

97

Gain: 25 dB ~ ,i

LO Injection: 0 to +3 dBm

! . LO to RF Isolation: 20 dB

Input VSWR: 3/1

IF: 30 MH*i • ,

IF Bandwidth: 10 MHz1 ;

: Noise Figure: 9.8 dB. : . ' ' : '

An ideal figure can now be placed on the receiver sensitivity.

This sensitivity will be a best case value and will be adjusted by• . I

local oscillator noise.i i

S - -174 dBm -f 10 log(BWXF) 7

S = CW sensitivity in dBm

BW e IF bandwidth in Mtiz"! | - • •*• i

F « noise figure expressed as a ratio

Diode conversion loss is included in the noise figure.i

S = -94.2 dBm',, • ' I ' • •

The orthomode coupling system does introduce the problem of ai

3 to 1 VSWR. The mixer must receive a 0 to +3 dBm local oscillator

signal above the reflected local oscillator power. Below is a cal-

culation of minimum LO power required for proper operation.

|p| a magnitude of the reflection coefficient

|p|_= (VSWR - I)/(VSWR + 1) •: 1/2i _ ' _ _

t+r -~1.0.s

t = transmitted power coefficientt

r = reflected power coefficient

98

1/4

3/4

PL_ = local oscillator power

P t = l m wLO

LO '

L

"** (minimum value)

8/3 raw (maximum value)

p (total) = 16/3 mw (maximum value)IA) ' ' . .

Since commercial specifications tend to reflect the best possible

values of VSWR, it is best to buy more LO power than is needed and then

put an attenuator in series with the mixer LO input. This also allows

the receiver to operate longer if the LO power decreases with age.

Receiver stability is almost completely dependent upon locali ' • i

oscillator stability with a fixed frequency CW receiver. The bandwidth

of the receiver is 10 MHz and if the LO drifts 5.0 MHz or more, an

attenuation measurement error of at least 3 dB would occur. Cross-

polarization data would! not be erroneous because each channel would

fade equally. The danger to cross-polarization measurements results

from a loss in dynamic range. As the LO drifts, the IF will drift out

of the bandwidth and the detector output will drop. This forces the

upper end of the dynamic range to move towards the lower end and! •

Jeopardizes the lower rainfall rate data. Below is a calculation of

required LO stability.

Frequency • 17/62 GHz

Stability- ±(BW/2) (100/P) - t y~|j - ± 6.0284%

This value of stability must be good over the temperature range

99

che LO will experience. If.temperatures are extreme, an environmental

chamber may be more economical than buying an LO that is stable over .1 t • ' , '

Che extreme temperature range.

Noise generated by the LO causes a reduction in the dynamic range

of the receiver, by causing a. detector output when no RF is applied to

che mixer. Noise problems can be minimized by choosing the most stable. . i

LO available and picking an IF bandwidth as small as possible for that

stability. This also aids in tuning because the detector output will

have a sharper maximum for the smaller bandwidths. The value for

allowable LO noise will vary depending on the type of detector used,• ' • . ; . i ' '' . ' i \

and for that reason, a more detailed discussion of LO noise will be1 ' i

presented after the section on detectors. Below is a list of LO

specifications defined to this point.

Frequency: 17.62 GHz . '

Stability: ± 0.0284% , '

Power: 4/3 to 8/3 mw (per channel)

Tuning: Mechanicalt iDetectors

As stated before, the dynamic range of the receiver must be greater

Chan 42 dB, and the transfer function should be as elementary as possible.

For these two reasons, the logarithmic amplifier is an ideal detector

for attenuation and cross-polarization measurements. Figure 2.4i

represents the input vs output characteristic of an RHG LST 3010 MAT

log amplifier chosen for the VPI&SU receiver. The linear D.C. output

vs input dBm is "tailor made" for attenuation or cross-polarization

100

. • o.

o1-1

oCMI

Oen

^—^

S

Oir>

OvO

oCO

oOlI

0)•rl

I

OP -Hsj- v-' HI «

H 005 O

cs

Q)M3 •00

CM

XfidlQO

101

measurements. Below is a list of the RHG log amplifier specifications.

Frequency: i 30 MHz ; .i i

Bandwidth: 10 MHz

• • •Input Impedance: 50 ft ;

Input VSWR: < 1.5-1

Dynamic range: > 80 dB

' • Log accuracy: i 1 dB over 80 dB range 1

It is now possible with the aid of Figure 2.4 to calculate the

maximum allowable LO noise. A dynamic range of 50 dB was used instead

of 42 to insure that all the data is observed. A 50 dB dynamic range

corresponds to a -50.0 dBm LO noise signal.

DETECTOR INPUT VSWR » 1.5 to 1 • '

t + r =• 1.0

t "- 24/25

25 —5p_ » noise power incident to the detector - -57- (10 ) mw

PND " ~49'8 dBm

IF gain - 25 dB

Pmn " -*9.& dBm - 25 dB - -74.9 dBm 5 local oscillatorMJLiU j • !

i> noise power.

j Since this is a two-channel receiver, the total allowable LO noise is

- 71.9 dBm. i ;:

8 "~ •Shurmer suggests the use of Gunn oscillators for local oscillators

' ! •when the IF is around 30 MHz because of their low noise properties.

•' ~

102

The Gunn oscillator is very economical as well because it can be

mechanically tuned and requires only a low DC voltage for power.

The local oscillator chosen for the V?I&SU receiver is an RHG

G01K131 mechanically tuned Gunn oscillator with the following specifi-

cations .

Frequency: 17.62 GHz

Stability: 15 to 35° C' . • . j- ' • '•

Noise: - 110 dB

Power: 25 mw

The output of the detector is 0.5 VDC when full LO power is allowed to

the mixer and no RF power is present at the mixer. This sets a lower

limit on the dynamic range of 68 dB. When the attenuators were set

for 4/3 of a milliwatt LO power to each channel, the detector output1

was 0.4 VDC corresponding to a 71.4 dB dynamic range.

The RHG mixers used have an LO to RF isolation of 20 dB. This

value is far short of the 42.0 dB polarization isolation needed as

predicted in Section 1. A 24 dB isolator was placed in each channeli

to insure the minimum polarization isolation.

PI - I + TX + M + A

PI » polarization isolation

I » isolator isolation « 24 dB

T => E plane tee isolation D 3.0 dBJ

MJ» mixer isolation. «?_20.0 dB

A_ o attenuator isolation - (0 to 25 dB) ::';-,

PI •> 47 dB (minimum value)

• 72 dB (maximum value)

103

The 24 dB isolator at the LO output is strictly for VSWR pro-

tection for the L.O., because a lossless reciprocal 3-port can never.

9be completely non-reflecting. ! ;

The two attenuators are used for adjusting local oscillator power

to the mixer. The local oscillator power is 25 raw and each channel

requires at least 4/3 mw for proper mixer operation. The amount of

attenuation required is given by A.-.

A Q = 10 log (y . •£> « 10 log (g=)

A Q = 9.75 dB.

Commercial uncalibrated attenuators for K band are available inu

0 to 25 dB varieties or higher, so a 0 to 25 dB uncalibrated attenuator

was chosen for each channel. Since LO power to the mixer is the

important parameter, the attenuator setting is made by monitoring

power from the attenuator. The use of uncalibrated attenuators saved

the project $600.00.

The E-plane tee splits the local oscillator power. It is the most

economical device available for that purpose, but it does cause the

phase of each output leg to be ,180° apart. Figure 3.6 represents an

E-plane tee and its scattering matrix. Ports 1 & 2 are the mixer legs

while port 3 is the local oscillator leg.

•i 1/2 1 -/2

a

— _ —,—. i

If ports 1 and 2 are matched then a. and a« are both zero.

104

2 - 2~

From these calculations it can be seen that the voltage at port 1 is

180° out of phase with the voltage at port 2. After filtering, the

mixer outputs are: ;

. " . i i

! a2eLOeRFC08([u)SIG " \0]t) (p°rt 1} i

a2eLOeSIGC08([uSIG '

ij> = TT for E-plane tee ;

Since the detectors respond to input dBm, the phase of the mixer output

is of no concern.

7.2.4 Suggested Improvementsj

The addition of a calibrated phase shifter in one channel would

allow the experimenter to set the clear weather phase difference between

channels to 0°. This would greatly simplify received signal phase

!measurements.

7.2.5 Final Receiver Specifications

Figure 2.5 is the transfer function for the VPI&SU 17.65 GHz

,receiver. Both channels are identically calibrated. Below is a list

of the final receiver specifications.

Dynamic Range: 71.4 dB

Channel Isolations > 60 dB

-St.

105

ao

u§

M0)

«W(0caVJH

a)o

c/]

CM

0)Hs00

106i

SECTION 7.3

, TRANSMITTERi

7.3.1 Scope of Section

Presented In this section are typical transmitters for atten-

uation and depolarization measurements, design calculations for the

VPI&SU 17.65 GHz transmitter, and suggested improvements.

7.3.2 Typical Transmitters1 % '

Figure 3.1 is a block diagram of a typical transmitter for atten-

uation measurements. The source should be stable in both frequency and• /

power. If the source is capable of delivering more than the requiredi '

power, an uncalibrated attenuator can be used to reduce the transmitter

output to the design level and to hold it there as the source ages. An

isolator should be used to protect the source from a high VSWR encoun-

tered during switching unless one is provided Internally with the

source., A directional coupler and a power meter will allow continuous

. monitoring of the output power. It is often convenient during antenna :i

alignment or receiver checks to shut down the transmitter power. Ai

waveguide switch and a matched load will allow the transmitter power to

be dissipated safely when so desired.

.Figure 3.2 is a block diagram of a typical transmitter for cross-;; - i

polarization measurements. The components are identical except for the

power splitting device. The VPI&SU 17.65 GHz transmitter is identical.: • . I . .*£-,to this except that a 3 dB coupler is used as the power splitting device

and an uncalibrated attenuator is placed In one channel to Insure equal

107

I

L

1s

. Kwitch

1L_ fc1 1 "Hr*1 1 Source

Figure 3.1. Attenuation Transmitter

.. i

108

SWITCH

POWER

SPLITTER

SWITCH

K

SOURCE

Figure 3.2. Depolarization Transmitter

109

V

Switch

Switch

Source

Figure 3.3. VPI&SU Transmitter

110

outputs.

7.3.3 Necessary Transmitter Potter and Stability

The power required depends upon! receiver saturation level, path ;

loss, antenna gain, and transmission line loss. Below Is a calcu-

lation of the required transmitter power for the VPI&SU 17.65 GHz

transmitter.

L = path loss - 21.98 + 20 log (r/X) 10

r = path length

X = wavelength

X - C/F = 0.017 m

L - 120.48 dB

PT = power transmitted - PR + L - G.-

PD = necessary receiver input power • -J.6.78 dBm ;&

G = total antenna gain - 90 dBAX

P_-•» 13.7 dBm or 23.44 tnw

For a two channel transmitter, the total output power must be 46.88 mw.

The stability of the source at the required power level is Just as

important as local oscillator stability. Below is a calculation of

required source stability for the VPI&SU 17.65 GHz source.

S = stability - ± (BW/2) (100/P)

BW 5 bandwidth - - - - - -

E = frequency :

S - ± (.01/2) (100/17.65) • ± 0.028Z

Ill

7.3.4 Component Selection ;

The source chosen is an RDL POOR(3) crystal oscillator and

varactor chain multiplier. The specifications are listed below.

Frequency: 17.65 GHz

Stability: ± 0.005%

Power Output: 70 mw minimum

Temperature: 0 to 50° C '

Spurious Noise: < - 40.0 dB

Mechanical waveguide switches were chosen because of their high

isolation between ports. The isolation must be as great or greater

than the polarization isolation of the receiver or erroneous cross-

polarization levels will be observed. The waveguide switches chosen

were Waveline 777-E solenoid operated, double pole-double throw, current

holding switches.

7.3.5 Transmission Line Components

A major concern in the design of the transmitter was the VSWR

Introduced by the waveguide switches during switching.

Figure 3.4 is a top view of a Waveline 777-E switch in the rest

position. For this explanation port 1 is the source power input, port

4 is matched, and port, 2 is the antenna feed. The 90° circular are

from part 1 to part 2 is 1.42 inches long. The dimension of the wave-

guide short wall is 0.311 inches. When the switch is activated,'part 1

Is fed to part 2 and part 4 is fed to part 3. As the cylinder moves,

part 1 is shorted for 1.11 inches of the 1.42 Inches of movement. If

112

oXoO

coV

•34J

O6400

(U•o

. oI

•»•

CO

0)

o O Oo jcs jao\ o> w

113

the cylinder moves at a uniform velocity for the 100 msec switching

time, the short will last 78 msec. The short is not perfect however,

and a VSWR reading in 78 msec is difficult to obtain. In order to get

an estimated value of the VSWR, a shorting plate was placed across partl

4 and a slotted line was placed in series with part 1. With the source

transmitting and the switch deactivated, the VSWR was 25.0. Under

operating conditions, the short will not be this good because of the

small clearance between the rotating cylinder and the four ports

1*1r i

P,,

„ VSWR-1 . 24 _ Q .VSWR+1 26 °' '

0: reflected power coefficient *> \g\ - 0.852

= power reflected «• r (source power) » 25 r « 21.3mw

Not only must the source be protected from this reflected power,

but the reflected power should not be coupled into the other channel.

Below is an analysis of a 3 dB coupler used as the power splitter.

Figure 3.5 represents a 3 dB coupler and its scattering matrix.2c = coupling ratio - 1/2 (for a 3 dB coupler)

a1 = /50

a. » 0 (part 2 terminated with a matched load)

a. a /21.3 mw (voltage reflected when switch activates)

bl-V.v

» , «

o i/1/r i//rI/ .2. .1.0. 0_

i/vT o o> . »

i

« :;0

/5O

/r

1 1-

114

I

0 I//2" I//2"

I//2" 0 0

0 0

Figure 3.5. 3dB Coupler and Scattering Matrirc

115

PDO = power reflected to source « 10.65 mwHo

When both switches are activated simultaneously, the reflected power

will be 21.2 mw.

50 r - 21.3

r = 0.426 - |p|2

|p | - 0.653 .

•+|P I.VSWR 4.76

Discussions with RDL technicians convinced project personnel that the

RDL POOK(3) source would withstand a 4.76 VSWR for 78 msec.

The 3 dB coupler also provides excellent channel isolation for

power reflected during switching.

0 1/»T 1//T

1/../5T. o o

0 0

lb = - (contains no reflections from part 3)2 -

& •b_ • 1 (contains no reflections from part 2)

The 40.0 dB coupler allows the source power to be continuously

monitored. The coupling ratio was carefully checked for accuracy at

17.65 GHz and was found to be 40.0 dB.

The attenuators used are un calibrated with a range of 0 to 25 dB.

The high values of attenuation were

attenuators were not available. The VSWR of each attenuator was 1.15

maximum.

never used, but lower value variable

116

7.3.6 Transmitter Performance

The VPI&SU 17.65 GHz transmitter was licensed in the spring of

1972 as a contract developmental station in the experimental radio

service. It was assigned the call letters KQ2XOC. In accordance with

FCC regulations, the transmitter source was provided with a remote

"on-off" circuit located adjacent to the PB-440.

Below are the transmitter specifications measured during the final

testing stage.

Frequency: 17.65 GHz ± 800 KHz

Power Output: 26 raw per channel

Isolation between channels: > 60 dB

VSWR: 1.1 (static condition)

The 26 mw value was the transmitter power required to produce a

2.5 volt receiver output. The calculated: value was 23.44 mw. One

source of error for this calculation is the actual antenna gain. The

antenna gain measured by the manufacturer was 44.5 dB as compared to

the estimated value used of 45.0 dB.

1.0 dbm is 1.25 mw

Actual required output power » 23.44+1.25 0 24.69 mw

7.3.7 Possible Transmitter Improvements

The addition of a phase shifter in one channel of the transmitter

would allow for easier phase measurements between cross-polarized

channels at the receiver. The phase shifter should be adjusted so that

the received clear weather phase difference is 0 degrees.

In the VPI&SU 17.65 GHz experiment, data is taken 100 msec after a

117

waveguide switch changes state. The possible VSWR of 25 has disappeared

in this time. For this reason, the use of the 3 dB coupler as a splitter

may be unwarranted. If the source is properly protected against a high

VSWR, an E-plane tee would be more economical to use. Figure 3.6 rep-

resents an E-plane tee and the scattering matrix for an ideal E-plane

tee. Part 3 is the source feed and parts 1 and 2 are the orthogonal

channel feeds.

channel 1 switching:

a-j = /2l73

a2'- 0

b2 » -2.68 or 7.23 mw

b2 « 3.26 or 10.65 mw

channel 2 switching

a. » 0

b-j » 2.68 or 7.23 mw ;; t

b3 » -3.26 or 10.65 mw j

channel 1 and 2 switching:

ax - /2l73

a " - /2l75

- 6.53 or 42.6 taw

118

1/2

1 1

1 1 -

/Z" -A 0

Figure 3.6. E-Plane T and Scattering Matrix

119

|p| " 0.925

VSWR -.l.+ .jpj •». 1.925 • 25.0

1 - IP! .075

As can be seen from the calculations above, the E-plane tee works

as well as the 3 dB coupler as a protection to the source when only one

switch is activated at a time, but the source sees the entire VSWR of •

25 when both switches are activated simultaneously. If the source is

internally isolated, the E-plane tee would be more economical to use

instead of the 3 dB coupler. If the source is not isolated at all, it

would be more economical to buy an E-plane tee and an Isolator instead

of a 3 dB coupler and an isolator.

120

7.4 LITERATURE CITED

1. Thomas, D. T., "Cross Polarization Distortion in Microwave RadioTransmission Due to Rain," Radio Science, vol. 6, no. 10,October, 1971, pp. 833-840. :

2. Watson, P. A., "Measurements of Linear Cross Polarization at11 GHz," Report to European Space Research Organization,Contract No. 1297/SL, U. of Bradford, Bradford, England,May, 1972.

3. Wiley, P. H., "Depolarization Effects of Rainfall On MillimeterWave Propagation," Ph.D., dissertation, Virginia PolytechnicInstitute and State University, Blacksburg, Virginia, 1973,pp. 1-90.

4. Bostian, C. W., and Stutzman, W. L., "The Influence of Polari-zation on Millimeter Wave Propagation Through Rain," Semi-Annual Status Report 1 to National Aeronautics and SpaceAdministration, Contract No. NGR-47-004-091, Washington, D. C.,July, 1972, p. 77.

5. Saad, T. S., "The Microwave Mixer," Sage Laboratories, Inc.,Natick, Massachusetts, 1966, pp. 15-16.

6. Ibid., pp. 17-18.

7. Ibid., p. 7.

8. Shunner, H. V., Microwave Semiconductor Devices. New York: JohnWiley and Sons, Inc., 1972, pp. 177-178.

9. Kerns, D. M., and Beatty, R. W., Basic Theory of WaveguideJunctions and Introductory Microwave Network Analysis, NewYork: Pergamon Press, 1967, p. 103.

10. Livingston, D. C., The Physics of Microwave Propagation, EnglewoodCliffs, New Jersey: Prentice-Hall, 1970, p. 9.

11. Thomas, H. E., Handbook of Microwave Techniques and Equipment.Englewood Cliffs, New Jersey: jPrentice-Hall, 1972, pp. 94-99.

12. -Bostian, C. W., and Stutzman, W. L,, op. cit.. pp. 7-8.