PRELIMINARY EXAMINATION

OF 20 GHz G/T MEASUREMENTS OF

EARTH TERMINALS

D.F. Wait

W.C. Daywitt

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

March 1983

NATIONAL BUREAU OF STANDARDS

LIBRARY MAY 10 1983

NBSIR 83-1686

PRELIMINARY EXAMINATION

OF 20 GHz G/T MEASUREMENTS OF

EARTH TERMINALS

y

D.F. Wait

W.C. Daywitt

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

March 1983

Prepared for: U.S. Army Communications Electronics Command

Fort Monmouth, New Jersey

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

1. OBJECTIVE 1

2. BACKGROUND 1 2.1 Available Techniques 2

2.2 The Existing NBS Precision G/T Measurement Program 2 2.2.1 The Radio Star Measurement Method 2 2.2.2 NBS G/T Measurement System 4

2. 2. 2.1 Instrumentation 4

2. 2. 2. 2 Measurement Procedure 6

2.2.3 Accuracy Considerations 7 2.3 NBS Automated Noise Measurements 8

3. MEASUREMENTS AT 20 GHZ 14

3.1 Measurements of Small Earth Terminals 16

3.1.1 Using the Sun or Moon as Known Sources 17

3. 1.1.1 Solar Star-Shape Factor 17

3. 1.1. 2 Lunar Radio Emission 17

3. 1.1. 3 Lunar Star-Shape Factor 20

3. 1.1. 4 Water Vapor and Molecular Oxygen 20

3. 1.1. 5 Atmospheric Refraction 22

3. 1.1. 6 Diffusive Attenuation 22

3. 1.1. 7 Estimated Measurement Accuracy 24

3.1.2 Using a Calibrated Comparison Antenna 24

3. 1.2.1 Measurement Method 26

3. 1.2. 2 Estimated Measurement Accuracy 28

3.2 Measurements of Large Earth Terminals 31

4. RECOMMENDATI ONS AND CONCLUSIONS 33

ACKNOWLEDGMENTS 33

REFERENCES 33

i i i

Preliminary Examination of 20 GHz G/T

Measurements of Earth Terminals

David F. Wait and William C. Daywitt

National Bureau of Standards

Boulder, Colorado 80303

Three basic measurement techniques and the associated

measuring systems are examined to determine which are most likely

to meet the needs for measuring the figure of merit (G/T) of

future 20 GHz satellite systems: use of the Sun as a known

source, use of the Sun as an intercomparison source with a calibrated reference terminal, and the use of a satellite signal as an intercomparison source. It is shown that the method of using the

Sun as a known source is not very accurate (about 1.5 dB uncertainty), but that using the Sun as a transfer source is useful

(0.3 dB to 0.5 dB, depending on measuring system) for Earth terminals with antenna diameter less than 1.8 m (6 ft). For Earth

terminals with antenna diameters greater than 1.8 m (6 ft), the

Sun cannot be used as a transfer source for technical reasons, but

a satellite signal can be used as a transfer source.

Key words: automated noise measurements; diffusive attenuation;

figure of merit (G/T); lunar star-shape factor; millimeter wave;

Moon flux; noise equivalent flux; noise measurement; satellite

communications; six-port; Sun flux.

1. OBJECTIVE

The objectives of this initial effort were to (1) determine which

measurement techniques and systems are most likely to meet the needs for

future 20 GHz satellite systems, (2) estimate the accuracy and repeatability

associated with these techniques, and (3) produce estimates of the cost and

time required to develop, evaluate, and implement the required measurement

systems and techniques.

2. BACKGROUND

One of the more popular specifications of antenna system noise is the

figure of merit, usually denoted as G/T [1-11]. Figure of merit is defined to

be the ratio of the antenna receive gain to the system noise temperature

(referred to as the antenna output port). Figure of merit has the advantage

of being a relatively easy noise parameter to measure (for most antenna

systems). It can be shown that the figure of merit does not depend on which

internal reference plane is selected as being the output port of the

antenna. Thus, one can say that the figure of merit is the ratio of the

system gain to the system noise temperature [7].

The figure of merit (G/T) has some shortcomings for the precise characterization of the noise of an Earth terminal because it neither characterizes

1
the hardware, nor the hardware plus atmosphere. This is because the atmospheric effects are excluded from the antenna gain (G) part of G/T, but are included in the system temperature part (T). So, in addition to using the

measurement data to calculate G/T, one should also calculate the noise

equivalent flux (NEF). NEF characterizes the hardware only. It is a measure

of the noise performance of the Earth terminal analogous to effective input

noise temperature for an amplifier [12]. NEF is the magnitude of an ideal

white, random noise flux density [W nT^ Hz

-

-*-] incident normal to the aperture

of a noiseless equivalent Earth terminal operating in a lossless atmosphere

such that the output noise power equals the output noise power of the actual

Earth terminal.

2.1 Available Techniques

The noise that originates within or enters into a microwave communication

antenna system is a basic characteri stic of the system. There are four

methods used to make system noise measurements in antenna systems. The first

method is to make precision noise and gain (or loss) measurements for the

various components of the system and combine these results in an appropriate

manner to obtain an overall system noise. In evaluating the characteristics

of the individual components, three keys to the overall accuracy for the

system noise measurement are the accuracy of measuring (a) the antenna gain

[13-17] (NBS calibration services use near field [18-20] or extrapolation [21]

techniques), (b) the antenna noise [22], and (c) the amplifier noise [23-25].

A second method is useful for large antenna systems that can "see" a radio star such as Cassiopeia A, the Sun, or the Moon. The known noise from

the radio star can be used as a reference to measure the system noise

[3,17,22,27]. A third method is to use the signal of known intensity from a satellite. This satellite signal can be used in a way not too different than

that of a radio star [4]. Lastly, a modified gain comparison technique can be

used where one antenna is basically used to calibrate the strength of some radio source in order to then calibrate a second antenna.

2.2 The Existing NBS Precision G/T Measurement Program

The National Bureau of Standards (NBS) has been making precision noise

measurements of antenna systems using radio sources for about ten years

[1,12]. Initially, the radio star Cassiopeia A (Cas A) was used [2,28-29],

but recently we have begun using the Moon [30] and have made rather extensive

preliminary investigations of using the Sun. The uncertainty of the noise

measurements in X-band is typically between five and ten percent for systems

with antenna gains between 51 dB and 65 db. In general, this method is

applicable when (a) the antenna system can "see" the radio source with a signal -to-noise ratio of better than 0.2 (0.8 dB), (b) the half-power

beamwidth (HPBW) of the antenna radiation pattern is greater than the diameter

of the radio source being used, and (c) the antenna can be pointed at the

radio source to a resolution of better than 10% of the HPBW.

2.2.1 The Radio Star Measurement Method

To determine G/T using radio sources, one measures the ratio (Y) of the

output noise power when the Earth terminal antenna is pointed at a radio

2
"star" to the output noise power when the antenna is pointed to the nearby

cold sky. The noise power out of the antenna system when the antenna is

pointed to a radio source is the power at the output of the antenna times the

gain (g) from the antenna output to the system output. Thus,

Power out (on source) = gk (aT + T) B,

where AT is the temperature rise due to the radio source at the antenna output

port, and T is the system temperature expressed relative to the antenna output

port, and

Power out (on sky) = g k T B.

The ratio of these two measurable powers is,

Y = (AT + T)/T. (1)

The temperature of the cold sky is included in T. The temperature rise AT

O 1 caused by the star depends on its flux density, S(W m Hz ) , and on the

effective area of the antenna A

0 (m2 ),

AT= (1/2)k SA

e /k, (2)

where < is the correction factor near unity that accounts for the atmospheric

loss, star shape, antenna pointing, polarization effects, and instrumentation

effects; and k is Boltzmann's constant, (k = 1.38045 x 10" 23J/K). The factor

1/2 in (2) accounts for the fact that only one polarization of radiation can

be received from a star at any one time. To introduce the receive gain G, one

uses the following relationship.

where c is the veloci

A

e

= c

2 G/(4 tt f2

) (3)

ty of light (2.99793 xlO^ m/s), f is the frequency; so

G/T = (Y-l ) (8 tt k f

2

)/(< c

2 S) , (4)

or expressed in decibels above one inverse kelvin.

G/T (dB/K) = 10 log (G/T). (5)

3
By replacing T by (NEF A

e )/(2k) + T$ky (A

e /Aeo ), where T

sk is the noise power

originating from the atmospheric losses plus the three kelvin cosmic

background temperature, and Aeo

is the antenna effective area at the antenna

aperture (i.e., no resistive antenna losses included), then the Y-factor (1)

becomes

Y = (< S + 2 k Tsky /A

eo + NEF ) / ( 2 k Tsky /A

eo + NEF). (6)

Boltzmann's constant, k, and the antenna effective area, Aeo , are used to

convert T$ky to a power density expressed in watts/meter2

. Rearranging (6) we

can obtain

NEF = k S/(Y-1) - 2 k Tsky /A

eo

. (7)

2.2.2 NBS G/T Measurement System

To measure the system noise in a large antenna system, the NBS developed

the Earth Terminal Measuring System (ETMS) [12,31-34], The ETMS is a computer-aided measuring system that calculates the pointing angle for the

antenna. It also contains a very accurate noise power ratio measuring system

(0.1% uncertainty) to measure the noise from the antenna system as the "radio

star" drifts through the antenna pattern in five equally spaced declination

cuts. The resulting antenna-star response is fit to a three dimensional

Gaussian curve.

2. 2. 2.1 Instrumentation

The ETMS is an automated measurement system. A simplified block diagram

of the ETMS is shown in figure 1.

The ETMS contains nine subsystems: (1) A calculator which provides

computation capability, a means of controlling each of the remaining subsystems under automatic sequence control, a means of storing the measurement

results on magnetic tape in order to rework the data at a later time, and a keyboard to control the measurement procedures or to enter program modifications. (2) An external cassette which allows redundant recording of measurement data. (3) An NBS type IV self-balancing power meter used to measure

noise power. (4) A programmable voltmeter. (5) A multiplexer which connects

the digital voltmeter to various measurement points of interest. (6) A digital clock to provide required time information to determine current star

coordinates. (7) Dual solid state noise source to provide a stable reference

signal needed to eliminate the effects of gain fluctuations in the Earth

terminal. (8) An rf control unit which provides signal conditioning, system

test signals, and the precision circuits which allow the calculator to control

the various measuring instruments. (9) A coaxial switch under computer

control which allows the input of the rf control unit to be connected to any

of three down converters of the Earth terminal.

4
Figure 1. Earth Terminal Measuring System block diagram

5

CALCULATOR
2. 2. 2. 2 Measurement Procedure

The measurement procedure of the ETMS is designed to measure G/T, NEF,

and to estimate the antenna gain. The measurement procedure contains the nine

steps listed in Table 1.

TABLE 1. Steps in the Measurement Procedure.

STEP PURPOSE

(PRELIMINARIES)

1 Prepare daily data tapes, one for each measurement day

2 Validate hardware and tapes before shipping to antenna site

(DATA ACQUISITION)

3 Validate hardware after arrival

4 Determine antenna offsets, sky profile

5 Collect data

(DATA ANALYSIS)

6 Split multiple frequency data files into single frequency data

files

7 Replace isolated bad data points

8 Obtain best fit parameters for data sets

9 Calculate G/T, etc., and plot results

Many of these steps are self-explanatory. In step 4, sky profile refers

to a measurement of the sky temperature versus antenna elevation along the

star trajectory. The sky profile is used as an additional check on the atmospheric loss.

In step 5, the measurement data are collected for G/T and NEF. A

measurement set contains six cuts. For a cut, the antenna is pointed to a computed coordinate position; then a string of power measurements (typically

30) relative to a reference "noise add" signal are taken 6 seconds apart as Cas A or the Moon drift through. One cut is taken on the cold sky about 2 degrees away (in declination) from Cas A or the Moon. The remaining cuts are spaced equidistant throughout the main beam of the antenna pattern. For the

string of power measurements, the ETMS is sequentially connected to the out6
puts of one, two, or three different down converters so that the information

for one, two, or three frequencies are collected within one measurement set.

The data are stored and G/T, NEF, HPBW's, and the updated antenna point

offsets are calculated and printed out.

After all the data have been taken, the remaining steps may be done after

leaving the measurement site. Any isolated bad data points are removed from

the data sets. Then each data set is least squares fitted to a three-dimensional Gaussian curve and the values for G/T, NEF, and HPBW are calculated

using the precision fit results. The last step is to plot and tabulate all of

the results as a function of antenna elevation.

2.2.3 Accuracy Considerations

For the measurement of system noise, the noise from a radio star is used

as a reference signal. To obtain an accurate measurement, careful attention

is given to atmospheric effects, antenna pointing, accurate noise power

measurements, adequate characterization of the radio source, and careful

calculation of the "star shape" correction factor.

Typically, the uncertainty in the accuracy of the flux of the radio

"star" dominates the total measurement uncertainty unless the antenna system

is just barely able to observe the radio "star" (i.e., signal -to-noise ratio

of less than about 0.1) [2].

The atmospheric effects fall roughly into three categories: absorptive

attenuation, diffusive attenuation, and refractive attenuation. The absorptive attenuation is small enough that simple approximate algorithms for the

losses, based only on local temperature, humidity, and the site elevation can

be used [35]. However, the diffusive attenuation is rather troublesome

because of the scarcity of experimental and theoretical information to predict

its magnitude and its uncertainty [36-37]. These atmospheric effects are important sources of measurement uncertainty.

Most recently, the NBS measurements have been improved by including a more accurate procedure for measuring the increase of noise caused by the

radio "star," and by developing an algorithm which enables the use of the Moon

as a precise noise source for broad beam antennas. The ETMS measures the

noise power ratio from the antenna system as the "radio star" drifts though

the antenna pattern in five equally spaced declination cuts. The resulting

antenna-star response is fitted to a three-dimensional Gaussian curve. The

accuracy of the antenna system noise ratio measurement is directly related to

the accuracy with which the amplitude of this three-dimensional Gaussian curve

can be fit, and to how adequately the Gaussian curve represents the true starantenna convolution output. The fitting procedure is complicated by the time

varying radiation coming from the sky. This variation is caused by changes in

the atmospheric conditions along the path of the antenna beam.

There are three major sources of error associated with the ETMS measuring

system. First is the accuracy with which the noise power ratio from the Earth

terminal is measured; second is the accuracy with which the measurement system

can respond to the change in power as the radio star drifts through the

antenna pattern; and last is the degree of immunity of the measurement to the

gain instability of the Earth terminal. In the ETMS, the power is measured

7
with an MBS type IV power meter [38] which measures the ratio of stable noise

powers to an accuracy of better than 0.1%. The ETMS responds to changes in

power level as the star drifts through to an accuracy of better than 0.2%.

The effects of gain variations in the Earth terminal under test are suppressed

by utilizing a noise adding technique [39]. The stability of the noise source over the 30 minutes required to make a complete measurement set is better than

0.6% [40].

2.3 NBS Automated Noise Measurements

Some of the measurements discussed in this report require a calibrated

noise source. At the present time, no noise calibration service exists at 20

GHz. If it proves necessary to establish a 20 GHz service, it would

incorporate the automated radiometer currently under construction, and would

be implemented in a way similar to the 94 GHz section of radiometer which is now entering the final phases of construction. To understand this better, a short overview of the NBS automated noise measurements, and particularly the

94 GHz section of the automated noise radiometer is presented.

The National Bureau of Standards uses two different automated noise

radiometers which are used for noise calibration services. One has been used

to calibrate noise sources in WR15 waveguide (56 GHz to 64 GHz) since 1972

[41]. The other, still under construction, is scheduled in the next five

years to measure coaxial noise sources in the frequency range between 30 MHz

and 12 GHz, and waveguide noise sources in WR284 (2.6 GHz to 3.95 GHz), WR90

(8.2 GHz to 12.4 GHz), and WR10 (94 GHz to 95 GHz). Only the 30 MHz and 60

MHz segments of the coaxial radiometer are currently being used in calibration

services [42].

Both of these radiometers, like the ETMS, are systems developed around

the NBS type IV self-balancing rf power meter [38]. The radiometers incorporating this power meter measure the ratio of stable noise powers to an uncer- tainty of less than ±0.1%. The proof of this measurement accuracy is

accomplished in stages. First, an initial verification of measurement

accuracy; then, a validation that the measurement accuracy has not changed.

The measurement accuracy depends on the accuracy of the power meter and on the

linearity of the amplifiers in the radiometer. Each of these sources of

uncertainty are studied separately, and then the overall uncertainty is

exami ned.

For example, in the WR15 radiometer, the initial verification of measure- ment accuracy was examined using a matched precision rotary vane WR15

attenuator at the front end of the radiometer. This attenuator was con- structed and adjusted so that it followed very closely the theoretical

dependence on the attenuator vane angle [43]. By comparing the measured

changes at the output of the radiometer with the known changes of power (and

correcting for the noise originating within the radiometer), as shown in

figure 2, an upper bound for the linearity of the radiometer was determined.

That is, the physical principles of changing the power into the radiometer

(rotary vane attenuator), and that of measuring the power out of the

radiometer ( type IV power meter) are independent. Thus, within the error

that one tracks the other, they both must be known. Accidental error subtractions can be eliminated by changing the initial source power level.

8
Figure 2. Error in measuring known WR15 noise source.

9
The goal of the NBS is to transfer the bulk of the noise measurement and

calibration services onto automated noise comparison radiometers within the

next five years. These systems provide improved measurement control and

require less maintenance. They also provide continuous frequency coverage and

enable the NBS to offer noise measurement services for the much needed satellite communication frequencies that heretofore were not available. Further,

calibration time is reduced; in some cases, 8 to 10 frequencies can be calibrated in the time formerly required for one frequency.

A block diagram of the automated radiometer is shown in figure 3. A more detailed schematic of the radiometer, as it exists now, is shown in figure

4. On the right hand side of figure 4 are shown the 30 MHz and 60 MHz

sections that are currently being used in calibrations. On the left hand side

is the 2-4 GHz section of the radiometer, and in the center is the 94 GHz

section of the radiometer. If a 20 GHz noise calibration service is

implemented, then a 20 GHz section similar to the 94 GHz section would be

added.

The 30 MHz and 60 MHz noise calibrations have a typical calibration

uncertainty of about 1%, which is comparable to the uncertainty of the retired

manual calibration system [44]. The measurement procedure for this service

uses three solid-state noise check standards (which are switched into the

measurement sequence automatically), and one cryogenic check standard. Two of

the solid state noise sources have permanently connected attenuators so that

the output of the solid state noise sources are 11,050 K, 5,755 K, and 3,055 K respectively, and the output of the cryogenic check standard is 77.1 K.

Significant with regard to their function as check standards is the

measurement repeatability. The measurement repeatability depends on the

resolution of the radiometer, and on the long term stability of the check

standard sources. Over six months, the system/source repeatability has been

11,050 K + 20 K, 5,755 ± 10 K, 3055 K ± 10 K, and 77.1 ± 1 K. Because the

check standards have outputs that span the normal calibration amplitude range,

the linearity of the radiometer is automatically assured during the calibration cycle.

Other than at 30 MHz and 60 MHz sections, each front end begins with a six-port reflectometer as shown in figure 5. This allows for an automated

compensation of the mismatch difference between the standard noise source, and

the source being calibrated. Another feature is the use of cryogenic noise

standards. Although this results in poorer relative radiometer resolution, it

has the advantages of being easier to maintain and to use so that it becomes

practical to use the primary standard directly in the calibration procedure.

This also allows calibrations at any arbitrary frequency in the calibration

range, rather than being restricted to preferred frequencies.

An important feature of the automated radiometer is the relative ease in

extending calibrations into new frequency bands. Heretofore, frequency extensions were major undertakings. This situation has changed because of several

different developments. First, many of the validation procedures are automated, and the computer programs and the IF section of the radiometer are common to all of the different frequency bands. Thus, the effort to validate

the system at a new frequency is minimal. Secondly, when the coaxial standard

10
94 GHZ RADIOMETER

RADIOMETER TABLE

94 GHZ FRONT END -<— I.F. /DETECTION

* ALTERNATE FRONT ENDS —

L

tttttttt

TABLE CONTROLLER/INTERFACE

ON TABLE

SCANNER

SWITCHES

DIGITAL VOLTMETER

PROGRAMMABLE ATTENUATORS

POWER METER MANIFOLD

POWER SUPPLIES

A

GPIB (IEEE 488 BUS)

MINI COMPUTER

30/60 MHZ

360 - 500 MHZ

500 - 1000 MHZ

1 - 2 GHZ

2-4 GHZ

4-8 GHZ

8-12 GHZ

Figure 3. Block diagram of the automated noise radiometer.

11
PROQAMMABLE

ATTEN.

EEE 488 BUS

Figure 4. Current configuration of the automated radiometer.

12
AUTOMATED NOISE RADIOMETER

94 GHZ FRONT END

Figure 5. 94 GHz front end including six-port refl ectometer
is completed, physical noise standards will exist for all frequencies below 18

GHz (although the standard might need to be evaluated at the new frequency).

Finally, for frequencies above 18 GHZ, a new type of primary standard has been

developed which makes extensions to new frequency ranges relatively easy. The

new standard consists of a millimeter wave horn "looking" at a cryogenic

absorber, as shown in figure 6. Since the analysis used for one frequency

range is easily extended to a different frequency range, and because the same evaluated cryogenic absorber can also be used for different frequency ranges,

new standards can sometimes be created merely by constructing new horns.

To provide a new 20 GHz noise calibration service then requires three

parts: a new horn noise standard, a 20 GHz front end, and a system evaluation.

3. MEASUREMENTS AT 20 GHZ

The characteristics of the Earth terminals that are being considered in

this examination are shown in Table 2. For the 0.3 m (1 ft), 0.6 m (2 ft),

1.8 m (6 ft), and 2.4 m (8 ft) diameter antennas, the temperature rise at the

output of the antenna due to the Sun or the Moon are listed for those cases where it is practical to correct for the finite extent of the source (referred

to as the star shape correction factor) [2,28,30]. Table 2 indicates that the

signal produced by the Moon is too small compared with T

$ys except, perhaps,

for the 2.4 m antenna. Further, Table 2 indicates that one cannot correct for

the finite size of the Sun or Moon for antennas with diameters greater than

2.4 m, and only the Moon can be used with an 2.4 m diameter antenna. This

last conclusion is rather important because it precludes the use of the Sun even as a transfer source and implies that only a satellite signal may be

used. Because this is important and not immediately obvious, it is discussed

in more detail before continuing on.

TABLE 2. Earth Terminals being Considered.

It is assumed that the system temperature is 1000 K and the aperture

efficiency is 55% for each antenna system.

Ant. Diam G/T G HPBW | SUN 1 I MOON 1 m (ft) (dB) (dB) (deg) T(ant) Y_ T(ant) _Y 0.31 (1) 3.5 34 3.38 46 K 1.05 1 K 1.001

0.61 (2) 9.5 40 1.69 180 K 1.18 4 K 1.004

1.83 (6) 19.1 49 0.56 1380 K 2.38 31 K 1.031

2.44 (8) 21.6 52 0.42 - - 48 K 1.053

6.10 (20) 29.5 60 0.17 - - - -

13.72 (45) 36.6 67 0.08 - - - -

14
Level

Indicator

Brass

mi

Low

Loss

Foam

Vacuum

Ln2

Al

0.02* SS

Cu

Si C

Lossy

Foam

Figure 6. 94 GHz Cryogenic Noise Standard.

15
To obtain a star shape factor, one has to know or assume the brightness

distribution of the source and the antenna pattern, calculate the sourceantenna convolution, estimate the errors caused by the limitations in the

distribution/pattern assumptions, and obtain a practical algorithm to

represent this convolution integral for different antenna HPBW's, the frequency ranges desired, and source conditions (i.e., secular decay. Moon phase,

Moon-Earth distance, etc.). In practice, the star-antenna convolution calculation results are not very dependent on the antenna pattern assumed nor the

details of the star brightness distribution as long as the source is smaller

than the antenna HPBW. That is, assuming that the antenna pattern is Gaussian

does not give a significantly different result than using an actual measured

antenna pattern; and assuming some appropriate, simple brightness distribution

(e.g., a disk distribution with the "best" diameter and brightness) does not

give a significantly different result than using the actual measured distributi on

This situation changes dramatically as the size of the source approaches

and exceeds the HPBW of the antenna. Instead of the star shape correction

factor depending on neither the details of the antenna pattern nor the star

brightness distribution, it begins to depend heavily on both. For example,

for a point source the star shape factor is unity and independent of the

antenna pattern. For an ideal disk source equal to the HPBW of the antenna

pattern, the star shape factor is near 0.7 and a 1% error in estimating the

HPBW causes a 0.5% error in the star shape factor. For a disk source equal to

twice the HPBW, the star shape factor is only 0.3 and a 1% error in estimating

the HPBW causes a 1.6% error in the star shape factor. In principle, if one had an accurately measured antenna pattern and a detailed brightness map for

the source, the appropriate star shape correction factor could be calculated. But this shifts the measurement from being an easy measurement to

being a major undertaking. The antenna would have to be constructed or mounted so that the necessary measurements are feasible, and an adequate farfield source or adequate near-field measurement facility available.

Further, if the source size approaches or is larger than the HPBW, a new and very significant problem is encountered. Namely, (4) and (7) become

inappropriate as k (because of the star shape correction factor) becomes

nearly inversely proportional to the receiver gain. The physics of the situation is that the magnitude of the signal "on star" (see eq 2) begins to depend

only on the brightness temperature of the source and not on the shape of the

antenna pattern. That is, if the antenna were defocused, the boresight gain

would decrease, but the pattern widens so that the total radiation collected

by the antenna is nearly the same (assuming that the brightness temperature of

extended source is uniform). As k becomes inversely proportional to G, the G

dependence drops out, and the measurement error becomes unbounded (because NEF

and G/T depend directly on G).

3.1 Measurements of Small Earth Terminals

In this section the accuracy and repeatability associated with two potential measurement techniques are estimated, (1) using the Sun or Moon as a known source, and (2) using the Sun or Moon as a transfer source. One could

also use a satellite as a transfer source, as will be discussed in the next

section for measuring G/T for large Earth terminals, and will not be discussed

separately here.

16
3.1.1

Using the Sun or Moon as Known Sources

Figure 7 [45] shows a portion of a graph describing the magnitudes of the

various sources of radiation in the solar atmosphere that contribute to the

total solar radio emission. Superimposed on this graph are hash marks representing monthly-averaged measurements [46] of the solar output by the Air

Force Geophysical Laboratory (AFGL) from January 1979 to May 1982 at 5, 8.8,

and 15.4 GHz. It is clear that, as the frequency increases, the solar output

is increasingly better approximated by the quiet-sun component alone. At 15.4

GHz the quiet-sun graph gives the solar output as 580 sfu (1 solar flux unit = 10 _22 W/m2 /Hz/steradian) ± 60 sfu between the S.S. (sunspot) maximum and

minimum curves. The average of the AFGL measurements for the period indicated

is 566 sfu with about the same spread as the graph between max and min S.S.

activity. The difference between the two averages (580 and 566) is 14 sfu, so it is reasonable to expect the average graph value to approximate the true

solar output to within ±74 sfu (60+14). It is also reasonable to expect no larger error at 20 GHz. Therefore, it appears safe to assume that the solar

output can be approximated by

853 + 74 sfu

at 20 GHz, where 853 sfu is the average graph value at 20 GHz.

3.

1.1.1

Solar Star-Shape Factor

At 20 GHz most of the solar radio emission comes from the region of the

chromosphere [47] that is within 1% of the solar photosphere. This implies

that to first order the brightness-temperature is approximately uniform across

the solar disk, and appears to come from a circular disk with the same apparent diameter as the photosphere. For this model the star-shape factor is

k

2

= (1 - exp(-x2 ))/x 2 (8)

where

x

2 = (In 2) ( d/HPBW

)

2 (9)

and where d is the optical angular diameter of the Sun subtended at the

antenna site. The error in the preceding approximation has not, as yet, been

analyzed and will be taken as zero for the present. The value for d can be

easily obtained [48] to less than ±0.06%. The error in the HPBW depends on the technique used in viewing the Sun, and on the antenna. A graph of the

resulting errors in k

2

is shown in figure 8.

3. 1.1.Lunar Radio Emission

The lunar output equations developed for the 1 to 10 GHz frequency range

[30] were reexamined for 20 GHz. The resulting lunar flux density is

17
Solar

Flux

(W/m

2

/Hz)

>-17

1 10

Frequency (GHz)

100

Figure 7. Solar radio emission.

18
%

ERROR

IN

Az

Jkz ERROR FOR THE SUN

% ERROR IN 6m

Figure 8. The k

2 error for the Sun.

19
S ( 10 ) = 7.349 f

2 T d2

where, S is the flux density in janskys (10

-26 W/m2 /Hz/steradian), f is the

frequency in GHz, T is the average lunar brightness temperature in kelvins,

and d is the angular lunar diameter in degrees subtended at the antenna

site. T and d are calculated from

T = 206.8 (1 - 0.1648 cos(<j> - 35)) (11)

and

d = 0.5182/ c/60.268 - 0.0166 sin(el) (12)

<j> is the lunar phase angle in degrees, and is related to the phase F by the

equation

(j) = arccos(l - 2F). (13)

F can be obtained [48] to an uncertainty of +2% by a simple parabolic fit over

a three-day period. The Earth-Moon separation (c) distance and is obtainable

to within +0.02% by the same procedure. The error in S due to these various

parameters is given in Table 3.

3. 1.1. 3 Lunar Star-Shape Factor

The 1 to 10 GHz analysis [30] leads to the following star-shape factor

for the Moon:

k

2 = (1 - exp(-x2 )/x

2

) + 0.65%, (14)

where

x

2

= 0.6455 ( d/HPBW) 2

. (15)

The 0.65% is the minimum error in k

2 due to the approximate nature of the disk

model, and the constant 0.6455 was chosen to minimize this error. The apparent angular diameter (d) of the Moon subtended at the antenna site and can be

obtained from (12) to within ±0.1%. Including this error with the 0.65% error

leads to the k

2

-error-versus-HPBW error shown in figure 9. The graph was prepared for an eight-foot antenna (assuming a 0.43 degree HPBW).

3. 1.1. 4 Water Vapor and Molecular Oxygen

There is attenuation of radio waves at 20 GHz caused by the absorption

due to water vapor and due to molecular oxygen. The first water-vapor

resonance line lies at 1.35 cm (22.2 GHz), and is close enough to 20 GHz to

20
Jk z ERROR FOR THE MOON & AN 8' ANTENNA

(eH=0.43,,Jm=052')

% ERROR IN 6m

Figure 9. The k ^ error for the Moon and a 2.4 m antenna.

21
cause serious errors in the algorithms developed for the 1 to 10 GHz system.

These algorithms were therefore modified by approximating a collision term in

TABLE 3. Systematic Lunar Flux Density Errors at 20 GHz.

Source of Error Magnitude Resulting % Error in S

Variation in the Solar

Insul ation 3.34% + 1% 4.34

Neglect of Higher

Harmonics in the

lunar phase. 0.61% 0.61%

To 5% 5%

VTo 6.6% 1.09%

COS <t> 0.04 0

5% 0.50%

d 0.1% 0.2%

Quadrature Sum 6.76% (0.28 dB

)

Linear Sum 11.7% (0.48 dB)

the denominator of the absorption coefficients that was ignored in the 1 to 10

GHz version (to save computation time). With the addition of this term, the

total error in the atmospheric efficiency "a" due to gaseous absorption was recalculated and appears in figure 10 as a function of antenna elevation

angle.

3. 1.1. 5 Atmospheric Refraction

The decrease in radio refractivity with altitude causes a spreading in

the wavefront of any radiation coming through the atmosphere, leading to a decreased flux density and, consequently, an effective attenuation. However,

there is no significant difference in this attenuation between 10 and 20

GHz. The present algorithms are, therefore, sufficient to account for this

effect.

3. 1.1. 6 Diffusive Attenuation

Tropospheric turbulence causes a diffusion of rays, and a distortion of

the resulting wavefront over the antenna aperture. Both effects effectively

22
%

ERROR

Figure 10. Total oig error vs. antenna elevation angle.

23
attenuate the signal received through the antenna waveguide lead, and may be

as significant as the gaseous attenuation itself.

The algorithms developed for the 1 to 10 GHz range were based upon some work by Yokoi , et. al., [37] which, in turn, was based upon the theory of

Booker and Gordon [49]. This theory assumed a correlation function for the

turbulence whose value is incapable of being extrapolated from the measurement

frequencies of Yokoi to the 20 GHz of present interest, and since there is no available data at 20 GHz for transatmospheric attenuation, some other approach

is needed.

The theory of wave propagation in a turbulent medium has been extensively

developed [50], and with the numerous clear-air turbulence measurements, it

appears possible to obtain a useful estimate of the effective attenuation at

20 GHz. However, this task will require an additional effort which could not

be accomplished with the current funding, so reliable estimates are not available for this report.

3. 1.1. 7 Estimated Measurement Accuracy

The best accuracy attainable using the Sun or the Moon directly to measure G/T of small Earth terminals is an uncertainty of about +40% (+1.5

dB). Table 4 summarizes the sources and contributions of the measurement

errors. Because (1) the measurement techniques are elaborate and time con- suming, (2) the conditions for meeting the measurement conditions needed for

Table 4 are rather stringent and, thus, expensive in time and money to produce, and (3) the resulting errors are large, the direct use of the Sun or Moon for the primary measurement of G/T is not very attractive and will not be

further considered in this report. However, the recommendation (later in this

report) is to use the Sun as a known source for a secondary measurement of

G/T. From Table 4, it can be seen that the atmospheric effects are the source

of the large error. Thus, if in the measurement of G/T, using the Sun as a transfer source, the data are also used to see what G/T would have been if the

Sun had been used as a known source, then information concerning the atmospheric loss can be presented. But more important, for "good" measuring con- ditions, the secondary measurements of G/T can be used to alert the operator

if something happened to the calibration of the reference antenna system.

3.1.2 Using a Calibrated Comparison Antenna

An experimental arrangement to measure the system noise in an Earth

terminal is shown in figure 11. The calibrated Earth terminal can be similar

to the uncalibrated terminal except that a calibrated directional coupler and

noise add reference source are inserted between a 0.6 m calibrated antenna and

the preamplifier of the Earth terminal. The output of the system under test

and the output of the calibrated system are connected to the ETMS. The ETMS

switches alternately between the two terminals. The gain drift in the calibrated Earth terminal is compensated for by injecting the noise from a stable

noise source into the front end of the system. The gain drift in the Earth

terminal under test is compensated for by comparing its output with that of

the calibrated Earth terminal.

24
TABLE 4. Estimated Accuracy in using the Sun or Moon.

The measurements are assumed to be made at 20 GHZ, at an antenna elevation of 15 degrees, a site elevation of 0.15 km, ambient temperature 80° F,

dew point 46° F, and 0.1% noise power measurements. It is assumed that the

earth terminals under test are gain stable within 0.5% over five minutes, or that it is equipped with a directional coupler at the front end of the

terminal so that a noise add technique can be used to correct for the gain

instabil ity.

I ERROR CONTRIBUTION TO G/T

Source Sun Sun Sun Moon

Antenna Diameter 0.3 m 0.6 m 1.8 m 2.4 m

Parameter

1. Flux 8.7 % 8.7 % 8.7 % 8.6 %

2. Y-factor (+0.1 %) 2.3 % 0.7 % 0.2 % 2.2 %

3. ET stability (+0.5 %) 11.5 % 3.3 % 0.9 % 11.0 %

4. Atmospheric absorption 29.0 % 29.0 % 29.0 % 29.0 %

5. Diffusive attenuation 30.0 ? % 30.0 ? % 30.0 ? % 30.0 ? %

6. Refractive attenuation 0.1 % 0.1 % 0.1 % 0.1 %

7. "Star" shape 0.1 % 0.1 % 0.2 % 4.4 %

8. Antenna point(+0% HPBW) 2.6 % 2.6 % 2.6 % 2.6 %

9. Antenna polarization 0.4 % 0.4 % 0.4 % 0.4 %

Linear sum 84.7

(2.6

%

dB)

74.9

(2.4

%

dB)

72.1

(2.4

%

dB)

88.3

(2.8

%

dB)

Quadratic sum 44.3

(1.6

%

dB)

42.8

(1.5

%

dB)

42.7

(1.5

%

dB)

44.4

(1.6

%

dB)

25
3. 1.2.1 Measurement Method.

Both the test antenna and the calibrated antenna are pointed at the Sun

(or Moon), then at the cold sky at the same elevation and the appropriate Yf actors (see (1)) are established for each Earth terminal. The respective

G/T's are related to the associated Y-factors. Sun flux (or Moon flux), and

atmospheric parameters, star shape factor, antenna polarization effects, etc.

as implied in (4). Taking the ratio of the G/T for the unknown to the G/T for

the calibrated system, one obtains.

<G/T) X

= <Y X

- l)/(Ystd - 1) <G/T) std (16)

where the subscript x refers to the Earth terminal under test, and the subscript std refers to the calibrated Earth terminal. Equation (16) is approximate in that the "star" shape factors, the antenna pointing, and the antenna

polarization might differ slightly.

Using (16), ( G/T)

x is known when (G/T) td is determined. The antenna

gain and the noise injected into the calibrated terminal by the noise add source are assumed to be calibrated by NBS. The system noise, T$td , which

depends on the atmospheric conditions present at the field site, is determined

using the calibrated injected noise, T

g

. With the antenna of the calibrated

Earth terminal pointed at the appropriate elevation angle, the ratio of the

power out of the terminal with the noise add source on to when it is off is,

or

Y

' ' Tstd

+ V/Tstd < 17 >

Tstd V«Y

' - !>• < 18 >

Using G from a prior NBS calibration divided by T$td using (18) provides

(G/T) std needed in (16).

Similarly, using (7) instead of (4) one obtains

NEF

X

~ (Y

x

- l)/(Y std - 1) (NEF std + ^

s \^y/^eo ~ ^sky^eo,x

where T

sky

is estimated using the temperature, humidity, antenna pointing, and

site altitude information [35]; and Aeo

is estimated on the basis of the

antenna diameter. Without a noise add technique available on the Earth terminal being measured, one can not make a direct measurement of J

s ^y or Aeo

.

26
Figure 11. Experimental setup for G/T comparison measurement.

27

g/j

comparison

system
3. 1.2. 2 Estimated Measurement Accuracy

The estimate of the accuracy of measuring G/T of a small Earth terminal

depends on many factors. The optimum choice depends, in part, on the size of

the antenna system under test, and the cost-accuracy trade off that suits the

needs. For a test measuring system that minimizes the measurement error, G/T

can be measured to about +0.3 dB. The sources of error, and their contribution to the total error for this "best" G/T measurement system are shown in

Table 5. From Table 5, the error that dominates is the uncertainty with

which the G/T of the reference Earth terminal is known. Thus, from a measurement point of view, it pays to put the measurement effort into calibrating

this reference terminal. In particular, Table 5 assumed the use of a 0.1%

power ratio measurement so that (Y^^ - 1) error contributed 1% error to

G/T. This table suggests that the use of a 0.5% power ratio measurement might

not significantly degrade the measurement accuracy for the 1.2 m (4 ft) and

1.8 m (6 ft) Earth terminals.

The measurement errors in Table 5 assume that the noise source in the

measuring system is calibrated against a WR42 primary noise standard by the

NBS, which, at the present time, does not exist.

Without a calibration service, probably the best one could do would be to assume the output of an argon gas discharge noise source. Our experience has

been that for properly designed, properly cared for argon gas discharge noise

sources, most are within 15% of the predicted values, but not always. The

tubes do age and do have a finite usable lifetime. Without a good comparison

radiometer, it would be difficult to identify which tube is going bad. To use

an uncalibrated noise source in a measuring system is too serious a compromise

to the measurement and, therefore, is not recommended.

Table 6 shows the impact on the quadrature measurement error for several

measurement system options. The first line in Table 6 corresponds to the system assumed for Table 5. From Table 6 we see that by using a 0.5% power meter

in measuring G/T instead of the 0.1% meter assumed in Table 5 does not

increase the error very much for the 2 foot or 6 foot Earth terminals. For

the 1 foot Earth terminal, the 0.5% power meter causes the measurement error

to increase to about 0.59 dB uncertainty.

Also shown in Table 6 is the resulting error in measuring G/T depending

on different uncertainties for the noise source used to measure (G/T) $tC|. The

most accurate measurements of G/T (±0.3 dB) assume the noise source used to

determine (G/T) $tcj was calibrated with an uncertainty of 2% (assuming a calibration service based on a yet unavailable WR42 primary noise standard).

The accuracy of measuring G/T degrades about 0.05 dB if the noise source was calibrated with an interim standard to an uncertainty of 4% (a calibration

service based on using the existing WR62 primary noise standard with a calibrated transition section to WR42 waveguide).

28
TABLE 5. "Best" G/T Accuracy for Small Terminals.

This table lists errors anticipated in measuring G/T using the comparison

method with a measurement system which minimizes the resultant error. The errors listed are those associated with the uncertainty in the parameter

indicated. It is assumed that the reference terminal has a 0.6 m (2 ft) calibrated antenna, both antennas are pointed at the same source within 10% of

their half-power beamwidths, the system temperatures are 1000 K, and the ETMS1 uses a noise source calibrated within 2% against a WR42 primary noise

standard and measures noise power ratio with an uncertainty of 0.1%.

Source Sun Sun Sun

antenna diameter 0.3 m 0.6 m 1.8 m

parameter error in G/T

-< X

1 I—* 2.3 % 0.7 % 0.2 %

^ Y std

" ^ 0.7 % 0.7 % 0.7 %

< G' T >std 5.0 % 5.0 % 5.0 %

"star" shape (unknown) 0.1 % 0.1 % 0.2 %

"star" shape (calibrated) 0.1 % 0.1 % 0.1 %

2.6 % 2.6 % 2.6 %

antenna point calibrated 2.6 % 2.6 % 2.6%

antenna polarization 0.4 % 0.4 % 0.4 %

Linear error 13.8 % 12.2 % 12.1 %

(0.56dB) (0.50dB) (0.49dB)

Quadrature error 6.7 % 6.3 % 6.3 %

(0.28dB) (0.27dB) (0.26dB)

29
TABLE 6. Measurement Accuracy Options for small terminals.

This table lists the total quadratic errors anticipated in

measuring G/T using different measuring system specifications.

The measurement system options are (a) to use an NBS type IV power

meter with 0.1% uncertainty, or a commercial power bridge with

0.5% uncertainty, and (b) to use a noise source in the measuring

system calibrated with an accuracy of 2% using a new WR42 primary

noise standard, or a noise source calibrated with an accuracy of

4% using an existing WR62 primary noise standard with a calibrated

transition section. In all cases, it is assumed that the refer- ence Earth terminal has a 0.6 m diameter antenna that can be

pointed to within 10% of its HPBW and a 1000 K system noise. All

measuring systems use a comparison method with the Sun as the

transfer source.

(Source) Sun Sun Sun

(Antenna diameter of unit under test) 0.3 m 0.6 m 2.4 m

noise pwr meas noise calbr quadratic error to G/T

accuracy accuracy

0.1% 2% 0.28 dB 0.27 dB 0.26 dB

0.1% 4% 0.35 dB 0.33 dB 0.33 dB

0.5% 2% 0.51 dB 0.32 dB 0.29 dB

0.5% 4% 0.55 dB 0.37 dB 0.35 dB

30
3.2 Measurements of Large Earth Terminals

Neither the. Sun nor Moon can be used as a transfer source for antennas

larger than 2.4 m (8 ft). As the antenna size gets larger and larger, the

output from the antenna depends less and less on the antenna gain, and more

and more on only the brightness temperature of the source. At 20 GHz, the

HPBW of the 2.4 m antenna is smaller than the Moon diameter and the sensitivity of the measurement using the Moon as a transfer source is marginal.

Thus, for the larger antennas one is forced to use a satellite signal for the

transfer source. The satellite signal provides better signal to noise, and it

is polarized. The polarization match between the satellite signal and the

receiving antennas are important, but it has been assumed that this is not a significant problem. If it is, this is better checked separately using techniques not addressed in this report.

The estimated uncertainty in measuring G/T for large Earth terminals is

about 0.4 or 0.5 dB as noted in Table 7. It is assumed that a satellite

signal is used for the measurement transfer signal. From Table 7, one can see that the error contributions from the various sources are about the same magnitude, implying that the measurement assumptions for this table are near "optimum." Thus, for example, it is clear that the expense of using a 0.1%

power bridge is justified if the satellite signal to noise is 10:1 for a 13.7 m (45 ft) Earth terminal as assumed (because a 0.5% power bridge would

cause 5 times the errors listed for (Y - 1) and (Y $td - 1), and the resulting

18% error for (Y $td - 1) would completely dominate the error budget). On the

other hand, if satellite signal-to-noise ratios of 50:1 are readily available,

then a 0.5% power bridge would be more "optimum". The question marks next to some of the error listings are a reminder that the estimate of the magnitude

of the diffusive attenuation is rather uncertain. More theoretical work on the diffusive attenuation problem is a must for the G/T measurements of large

Earth terminals.

31
TABLE 7. G/T Comparison Accuracy for Large Terminals.

This table lists typical errors anticipated in measuring G/T using the

comparison method. It is assumed that the source used in the comparison is a satellite and the signal to noise available for a 13.7 m (45 ft) Earth terminal is 10:1. The errors listed are those associated with the uncertainty in

the parameter indicated. It is assumed that the Earth terminal being tested

has a directional coupler in front of the preamplifier where a noise add

signal can be injected in order to correct for changes in gain. It is also

assumed that the calibration terminal has an 2.4 m calibrated antenna, the

system temperature is 1000 K, the noise source in the reference Earth terminal

is calibrated to an uncertainty of 2%, the antenna can be pointed to the

satellite within 10% of its HPBW, and the power is measured with an uncer- tainty of 0.1%.

antenna diameter 2.4 m 6.1 m 13.;1 m

parameter error in G/T

(Y

x

- 1) 3.6 % 0.6 % 0.1 %

^std " ^ 3.6 % 3.6 % 3.6 %

(G/T) std 5.0 % 5.0 % 5.0 %

diffusive attenuation 0. % 3.0 %? 10.0 %?

antenna point unknown 2.6 % 2.6 % 2.6 %

antenna point calibrated 2.6 % 2.6 % 2.6 %

antenna polarization 0.4 % 0.4 % 0.4 %

Linear error 17.8 % 17.8 %? 24.3 %?

(0.7 dB) (0.7 dB) (0.9 dB)

Quadrature error 8.0 % 7.8 %? 12.3 %?

(0.3 dB) (0.3 dB) (0.5 dB)

32
4. RECOMMENDATIONS AND CONCLUSIONS

In this section, what measurement technique and system are most likely to

meet the needs for future 20 GHz satellite systems are considered.

For measuring G/T of small Earth terminals, two different systems should

be considered; (1) an Earth Terminal Measuring System (ETMS-1) constructed

similarly to the existing ETMS shown schematically in figure 1 (with measurement accuracy of 0.3 dB or better using a gain comparison technique with the

Sun/Moon as the transfer source), and (2) an Earth Terminal Measuring System

(ETMS-la) similar in purpose to the ETMS-1 but simplified by using a 0.5%

power ratio measurement. If the 0.5 dB measurement uncertainty for the 0.3 m

(1 ft) terminal is adequate, the simpler ETMS-la is preferred (about 0.35 dB

uncertainty for the 0.6 m (2 ft) and 1.8 m (6 ft) systems). A potential

advantage of the ETMS-la is that an existing computer within the reference

Earth terminal might be utilized (provided it has an IEEE 488 port that can be

used as a controller, some means of entering the G/T comparison program

exists, and hard copy output is available).

Due to the uncertainty (see Table 4) and complexity, it is not recom- mended to use the Sun or Moon directly for the primary measurement of G/T.

However, this method is recommended as a secondary measurement. That is, by

using the known flux value of the Sun, and reusing the the Y-factor data

obtained in the comparison measurement, an independent measurement of G/T is

available. From this redundant measurement one can either deduce the atmospheric loss, or verify that the reference terminal is holding calibration.

Furthermore, as a secondary measurement, the elaborate star drift measurements

and the sophisticated data fitting routines are avoided, albeit additional

theoretical work concerning the diffusive attenuation by the atmosphere is

needed. A bonus for completing the diffusive attenuation theory is that the

calculation of NEF (discussed in section 2) can also be implemented without

any great effort.

For measuring G/T of large Earth terminals (antenna diameters greater

than 1.8 m (6 ft), it is necessary to use a G/T comparison technique which

utilizes a satellite signal as the transfer source. Unless signal -to-noise

ratios substantially greater than 10:1 are available for the 13.7 m (45 ft)

diameter antenna systems, then one needs to use a measuring system, ETMS-2

which incorporates an uncertainty of 0.1% accuracy noise-power-ratio meter.

ACKNOWLEDGMENTS

The authors wish to acknowledge the helpful discussions of John Wakefield

and the financial support of the U.S. Army Communications Electronics Command,

Fort Monmouth, New Jersey.

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37
NBS-1 14A (REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 83-1686

2. Performing Organ. Report No. 3. Publ ication Date

March 1983

4. TITLE AND SUBTITLE

Preliminary examination of 20 GHz G/T Measurements of Earth Terminals

5. AUTHOR(S)

D. F. Wait and W. C. Daywitt

6 . PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7 . Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

U.S. Army Communications Electronics Command

Fort Monmouth, New Jersey

10.

SUPPLEMENTARY NOTES

2) Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnifi cant bibliography or literature survey, mention it here)

Three basic measurement techniques and the associated measuring systems are examined to determine which are most likely to meet the needs of measuring the

figure of merit (G/T) for future 20 GHz satellite systems: use of the Sun as a known source, use of the Sun as an intercomparison source with a calibrated

reference terminal, and the use of a satellite signal as an intercomparison source.

It is shown that the method of using the Sun as a known source is not very accurate

(about 1.5 dB uncertainty), but that using the Sun as a transfer source is useful

(0.3 dB to 0.5 dB, depending on measuring system) for Earth terminals with antenna

diameter less than 1.8 m (6 ft). For Earth terminals with antenna diameters greater

than 1.8 m (6 ft), the Sun cannot be used as a transfer source for technical reasons,

but a satellite signal can be used as a transfer source.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) automated noise measurements; diffusive attenuation; figure of merit (G/T);

lunar star-shape factor; millimeter wave; Moon flux; noise equivalent flux;

noise measurement; satellite communications; six-port; Sun flux

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

XXl Uni imited

| | For Official Distribution. Do Not Release to NTIS

3] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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