Harmonics of X-band (8.4 GHz) spacecraft transmitter systems can be used to measure the performance characteristics of higher frequency deep space communication links. The Galileo X-band transmitter system was measured at the second, third, and fourth harmonics (16.8, 25.2, and 33.6 GHz, respectively). The effective isotropic radiated power was determined to be approximately 52 dBm at 16.8 GHz, 47 dBm at 25.2 GHz, and  25 dBm at 33.6 GHz. These values can be useful, depending upon the sensitivity of the Earth-based receiving system and the distance of the spacecraft from Earth

Manshadi, F.

Stanton, P. H.

English

NASA Technical Reports Server

N87-17963
TDA Progress Report 42-88 October- December 1986
Harmonic Measurements of the Galileo Spacecraft
X-Band Transmitter System
P. H. Stanton and F. Manshadi
Radio Frequency and MicrowaveSubsystemsSection
Harmonics of X-band ( 8.4-GHz ) spacecraft transmitter systems can be used to measure
the performance characteristics of higher frequency deep space communications links.
The Galileo X-band transmitter system was measured at the second, third, and fourth
harmonics (16.8 GHz, 25.2 GHz, and 33.6 GHz, respectively). The effective isotropic
radiated power was determined to be approximately 52 dBm at 16.8 GHz, 47 dBm at
25.2 GHz, and 25 dBm at 33.6 GHz. These values can be useful depending upon the
sensitivity of the Earth-based receiving system and the distance of the spacecraft from
Earth.
I. Introduction
There is a need to accurately measure the performance of
the Ka-band (33.6 GHz) deep space communications link
between a spacecraft and the Deep Space Network (DSN).
This must be done prior to making a commitment to use
Ka-band as a primary deep space communications frequency.
Harmonics of the current X-band (8.4 GHz) spacecraft trans-
mitter system can be used as "beacons" to enable simultane-
ous relative measurements of Ka-band and X-band link per-
formance with high differential precision.
The Galileo X-band transmitter system was selected for
evaluation at the second, third, and fourth harmonics because
a spare transmitter and feed system were available. The objec-
tive of the evaluation was to determine the effective radiated
power provided by the harmonics of the Galileo X-band trans-
mitter system. During September and October 1985, measure-
ments were made to determine the amplitude and phase pat-
terns of the Galileo X-band antenna feed and to compute the
maximum effective radiated power and radiation patterns of
the overall Galileo antenna and transmitter system at 16.8 GHz
(second harmonic), 25.2 GHz (third harmonic), and 33.6 GHz
(fourth harmonic).
II. Measurement of Amplitude and Phase
Patterns
The transmission line from the traveling wave tube ampli-
fier (TWTA) output to the antenna feed is of sufficient size to
pass higher order modes of the X-band transmitter's harmonics.
The transmitted power in each of these higher order modes
will be extremely dependent on the configuration of the
transmission line. For future use with the actual Galileo
spacecraft, the geometrical dependence associated with higher
order modes required the measurement of an accurate copy of
the Galileo X-band transmission system on an antenna pattern
148
https://ntrs.nasa.gov/search.jsp?R=19870008530 2020-03-20T12:38:34+00:00Z
range. To help compensate for the small dimensional differ-
ences between the flight hardware and this test hardware, the
operating frequency was swept from 8395 MHz to 8425 MHz
to establish a pattern envelope useful in the selection of
representative frequencies for the final measurements. The
drive power of the TWTA was varied above and below the
normal operating power to reflect the differences between
TWTAs. The frequencies of 8415 MHz and 8420 MHz and
normal operating power were then chosen for detailed
measurements.
A test unit support structure was fabricated and the RF
equivalent of the Galileo flight X-band hardware, from the
TWTA to the antenna feed, was assembled on it. The near-
field antenna facility was instrumented and tested as a far-
field range for 16.8 GHz, 25.2 GHz, and 33.6 GHz. A sche-
matic diagram of the test chamber setup is shown in Fig. 1.
The test unit was mounted on a rotatable platform with the
phase center of the feed antenna on the axis of rotation.
The patterns were measured by sliding the probe antenna
along a track located above the feed. Data were recorded at
various angles of feed rotation with increments of 30 degrees.
These data were stored on a digital recorder and later cor-
rected for distance variations and test probe patterns. Far-
field amplitude patterns were generated, along with the
absolute power reference, so that they could be integrated to
obtain the total available harmonic power.
The phase patterns of the Galileo X-band feed antenna
were needed to complete the data required to compute the
effective radiated power of the overall antenna/transmitter
system. These phase and amplitude patterns were recorded
simultaneously.
III. Calculations of Antenna Radiated Power
The overall antenna effective isotropic radiated power
(EIRP) was predicted using the feed pattern measurements.
To effectively use the Galileo Ka-band signal for link compari-
son with X-band, an EIRP of at least 30 dBm is considered
necessary for satisfactory reception on Earth. The EIRP is
predicted by computation of the overall antenna normalized
gain pattern (with respect to the feed) and the feed power
measurement. Then the EIRP calculation is modified by the
spacecraft antenna reflector mesh loss as well as the subre-
flector dichroic loss at the frequencies of interest.
The geometry of the Galileo high-gain antenna is shown in
Fig. 2. Both the main reflector and the subreflector are shaped
surfaces. The feed pattern was measured (as described in
previous sections) over 12 azimuthal cuts (phi-cuts) for small
but uneven increments in polar angle (theta). These data were
first reduced to obtain the feed pattern for every 0.5 degree
in theta direction over all the phi-cuts. Then the Cassegrainian
reflector system was replaced with an equivalent paraboloid
('Ref. 1). This yields a good approximation for the overall
antenna pattern while saving considerable time and computer
costs. For the equivalent paraboloid, the dish diameter is
the same as the main reflector diameter (D = 188.8 in. or
479.55 cm) and the effective focal distance was calculated to
be F e = 195.22 in. or 495.86 cm. The far-field pattern of the
paraboloid was computed using a modified version of the
physical optics/Jacobi-Bessel polynomial expansion program. 1
This program was developed for circularly symmetric feed
patterns; however, for this case, the feed pattern for harmonic
frequencies is not circularly symmetric. Therefore, the PATRN
subroutine of Rahmat-Samii (Footnote 1) was modified to
generate the feed pattern, at any point required by the pro-
gram, via a two-dimensional interpolation of the feed mea-
sured data. Using this program, the far-field patterns of the
Galileo antenna for second, third, and fourth harmonics of
8415 and 8420 MHz were computed. The phi = 0 cut patterns
for the above frequencies are shown in Figs. 3 through 8.
These patterns represent the normalized antenna gain relative
to the feed gain, i.e., they are computed by assuming zero
gain for the feed boresight. It can be seen that the higher the
frequency is, the more lobes are generated near the boresight.
This is due to the interaction of a larger number of propagat-
ing modes at higher frequencies.
To find the feed radiated power, its RF pattern was mea-
sured relative to a known reference antenna. This was accom-
plished by moving a probe in front of the reference and the
test (Galileo feed) antennas. In this measurement, the far-field
patterns of both antennas, as well as the input power of the
reference horn, were found. Taking into account the distance
between the probe and the reference and test antennas, the
overall EIRP can be determined. The results are shown in
Table 1. Considering the 30-dB minimum EIRP requirement, it
is evident that the fourth harmonic output of the antenna
would not be useful at a distance greater than 1 AU, but the
second and third harmonic signals should provide more than
enough power for accurate measurements.
IV. Losses
The reflector mesh of the Galileo high-gain antenna is
identical to the tracking and data relay satellite (TDRSS)
antenna built by the Harris Corporation. For the TDRSS
antenna, the peak gain loss (compared to a solid reflector),
1Rahmat.Samii, Y., Offset Parabolic Reflector Computer Program for
Analysis of Satellite Communication Antennas, JPLdocument D-1203,
Jet Propulsion Laboratory, Pasadena, CA, Dec. 1983 (JPL internal
document).
149
as a function of frequency, is computed and shown in Fig. 9. 2
It can be seen that the mesh loss for the second and third
harmonics of the Galileo transmitter (16.8 GHz and 2';.2 GHz)
are approximately 0.5 dB and 1.22 dB, respectively; these
losses would not reduce the EIRP of the antenna significantly.
Test samples of dichroic subrefiector material were mea-
sured in the laboratory and found to have reflection loss of
between 3 and 6 dB, between 18 and 40 GHz. Phase shift as
a function of incident angle was not measured. Making a gross
2Rahmat-Samii, Y., and Lee, S. W., Vector Diffraction Analysis of
Mesh Reflector Antennas for Space and Ground Applications, JPL
document D-1573, Jet Propulsion Laboratory, Pasadena, CA, May 14,
1984 (JPL internal document).
simplification by ignoring the phase, the dichroic reflected
power loss is included in the final EIRP given in Table 1.
V. Conclusions
The second, third, and fourth harmonic effective radiated
power of Galileo's X-band transmission system was computed
based on measurements of the transmitter and teed system
without main or subreflector. The results of this study indi-
cate useful EIRPs at the second and third harmonics. The
fourth harmonic EIRP is too low to be considered useful at
the distance planned for the Galileo mission. The results are
sufficiently encouraging to consider this approach for other
missions such as Magellan, where the distance from Earth will
vary from 0.3 AU to 1.7 AU.
Acknowledgment
The authors would like to acknowledge M. Gatti and G. Redenbaugh for the experi-
ment setup and collecting and processing the measurement data; Y. Rahmat-Samii for the
information on mesh loss; R. Clauss for intormation on dichroic subreflector loss; and
F. Ott and N. Bixel for providing the TWTA.
References
1. Hanan, P. W., "Microwave Antennas Derived from the Cassegrain Telescope," IEEE
Trans. Antennas and Propagation, Vol. AP-9, pp. 140-153, March 1961.
2. Agrawal, V. D., and Imbriale, W. A., "Design of a Dichroic Cassegrain Subreflector,"
IEEE Trans. Antennas and Propagation, Vol. AP-27, No. 4, pp. 466-473, July 1979.
150
Table 1. Effective isotropic radiated power (EIRP)
Frequency
EIRP w/o Mesh and EIRP w/Mesh
Mesh or Subreflector and Subreflector
Subreflector
Loss, dB Loss, dB
Loss, dB
33.660
33.680
25.245
25.260
16.830
16.840
8415
32.0 5.1 to 8.1 26.9 to 23.9
32.0 5.1 to 8.1 26.9 to 23.9
54.4 4.2 to 7.2 50.2 to 47.2
52.0 4.2 to 7.2 47.8 to 44.8
57.7 3.5 to 6.5 54.2 to 51.2
56.7 3.5 to 6.5 53.2 to 50.2
96.3 0 96.3
151
REF CH.
HARMONIC
MIXER
TEST CH, RECEIVER
1
DIGITALRECORDER
MIXER I
(REF) L
ANTENNA
TEST UNIT
i HP11517AMIXERHARMONIC ]_ X-BANDHIGHVOLTAGE
,os_o_s ' 1- ±FREQUENCY ROTATING
MULTIPLIER) I _'J VARIABLE
PLATFOR_ _ ATTEN
COUPLER
ISOLATOR
Fig. 1. Near-field facility setup for far-field radiation pattern measurements
TWT
SUPPORT
EQUIPMENT
GENERATOR
(X-BAND)
FREQUENCY
SYNTHESIZER
152
S-BAND F_
X-BAND PHASE CENTER
Fig. 2. Galileo HGA configuration
153
40
35
30
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q;
w 20
z
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40
35
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I I I I I I I
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THETA, deg
2.0
Fig. 3. Far-field patterns for _ = 0 degree plane at second harmonic
of 8415 MHz
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uJ
4O
351
30
25
20
15
10
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2.0
Fig. 4. Far-field patterns for _ = Odegree plane at second harmonic
of 8420 MHz
154
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36
29
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THETA, deg
Fig. 5. Far-field patterns for (_ = 0 degree plane at third harmonic
of 8415 MHz
2
.<
50
43
36
29
22
15
8
1
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I I I I I I [ 1
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THETA, d_
Fig. 6. Far-field patterns for _ = 0 degree plane at third harmonic
of 8420 MHz
155
LU
I--
,<
50
43
36
29
22
15
8
1
-6
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I I I I I I I
LU
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<
50 / I I I [ I I I /
43
36
29 -
22
15
8
1
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-13
-20
-2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0
THETA, deg
Fig. 7. Far-field patterns for _ = 0 degree plane at fourth harmonic
of 8415 MHz
F-
z
(_9
,<
50 I I I I I I I |
43
36
22 -
1 -
,,'>, ,-13
-20
50 I I I I I I f
43
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29
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15
8
1
-6
-13
-20
-2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0
THETA, deg
Fig. 8. Far-field patterns for _ = 0 degree plane at fourth harmonic
of 8420 MHz
156
==
Z
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(.9
-2 i I I I i I J I i I i
0 5 10 15 20 25
FREQUENCY, GHz
Fig. 9. Boresight gain lossvs frequency for TRDSS mesh
parameters
3O
157


Harmonic measurements of the Galileo spacecraft X-band transmitter system

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