The design of the microwave system for the solar power satellite is described. Design modifications recommended include changes in phase control to the power module level, a reduction in allowable amplitude jitter, the use of metal matrix waveguides, and sequences for startup/shutdown procedures. Investigations into reshaping the beam pattern to improve overall rectenna collection efficiency and improve sidelobe control are surveyed

Arndt, G. D.

Nalos, E. J.

English

NASA Technical Reports Server

MICROWAVE SYSTEM PERFORMANCE SUMMARY
G. D. Arndt - NASA/Johnson Space Center, Houston, Texas
E. J. Nalos - Boeing Company, Seattle, Washington
I 82' 22724
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Introduction: The SPS microwave system as defined in the October 1978 Reference
System Report, DOE/ER-O023, has a l Km diameter phased array antenna with a lO
dB gaussian taper illumination which focuses the beam at the center of the ground
antenna/rectifying system (rectenna). The power beam has approximately 88% of
its energy within a 5 Km radius from the rectenna boresight, with a resultant
beam width of 1.2 arc-minutes. Mechanical alignment of the l Km antenna is
maintained within one arc-minute while electronic alignment has a 1.8 arc-second
accuracy. The DC-RF power converters within the antenna are 70 KW klystrons fed
by 40 KV power lines from a series/parallel solar array configuration. The an-
tenna is divided into 7220 mechanical subarrays, IO.4 meters x I0.4 meters on a
side, having slotted waveguides as the radiating surface. Slotted waveguides
were selected because of their high power handling capabilities and low I_R losses.
The klystrons will be phase controlled at the individual tube level through the
use of a retrodirective pilot beam signal transmitted from the center of the rec-
tenna and phase conjugated in receivers in each power module. An onboard phase
reference signal is distributed through the antenna to provide the same reference
in each conjugating receiver. The reference phase distribution system is imple-
mented in the form of a four level tree structure with electronic compensation
for minimizing phase shifts due to unequal path lengths from the center of the
transmit antenna to each phase control receiver. The uplink pilot beam signal
has a double sideband, suppressed carrier with code modulation to provide link
security and anti-jamming protection from radio frequency interference.
._ he ground rectenna converts the RF energy to DC electricity using halfwave di-
poles feeding Schottky barrier diodes. Coherence of the incoming phase front
needs to be maintained only over the area associated with a small group of di-
poles. Physically the present rectenna configuration is a series of serrated
panels perpendicular to incoming beam and covers approximately 75 square kilo-
meters. A 75-80% optical transparency of the panels allows other utilization of
the area beneath the rectenna if so desired.
The SPS sizing of the l-Km transmit antenna and 5 GW of DC output power from the
rectenna is based upon a 23 KW/m 2 heat dissipation limit in the antenna and a
hypothetical 23 mW/cm 2 peak power density limit in the ionosphere to prevent non-
linear heating. System sizing tradeoffs given in another paper in this session
indicate the ionospheric limit is a critical design and costing parameter. This
limit may be revised upward pending the completion of the Department of Energy
Environmental assessment studies on ionospheric heating.
Recent Study Results: Several changes in the microwave system are now recommended
as a result of recent NASA and contractor studies. These modifications to the
reference system documented in the aforementioned October 1978 DOE/NASA report
include:
• Phase control to the power module (tube) level. It is recommended that
phase conjugation be performed at each of the lOl,O00 power modules rather than
at the 7,220 subarrays. The advantages of phase control at the tube level is a
reduction in the antenna and subarray mechanical tilt requirements (or a reduc-
tion in scattered microwave power if the same tilt requirements are maintained)
and a reduction in the effects of distributed phase errors within the subarrays.
The disadvantage is increased costs due to the 94,000 additional phase control
- receivers. In the May 1979 SPS Microwave Symposium in Washington, D. C. it was
reported that an overall cost savings could be achieved (i.e., the cost benefits
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of less scattered power were greater than the additional receiver costs) if the
phase control receivers were less than $600 each. A later Boeing Aerospace Com-
pany study indicates that these receivers can be built for less than $600 in high
volume quantities. There is also an environmental advantage in phase controlling
at the power module level in that the grating lobes incident upon the earth are
reduced in amplitude and in quantity. Figures l and 2 show the locations and
amplitudes of the grating lobes from a single 5 GWSPSsystem with phase control
to the power module level. Recent simulation results indicate the off-axis
grating lobes may be considerably reduced from the data shownin the figures.
The location jitter or the error in path length from the pilot beamtransmitter
to each radiating slot in the antenna is reduced by going to the smaller antenna
size associated with an individual tube rather than to the larger subarray. This
location jitter, which appearsas a phase error, scatters 6MW of power at the
tube level as comparedto 87 MWat the subarray level.
• A reduction in allowable amplitude jitter. The reference SPS system has
a ±l dB amplitude jitter across the surface of each subarray or power module.
Analysis results indicate that power transfer efficiency (88% for the reference
system) is relatively insensitive to amplitude jitter. However the voltage and
amplitude regulations for the high efficiency, high gain klystron tubes have to
be maintained to approximately I% for satisfactory operation. Therefore a ±1%
amplitude tolerance is recommended for the antenna error parameter. This change
will not affect the microwave transmission efficiency budget.
• Metal matrix waveguides. The SPS reference system has aluminum for the
subarray distribution and radiating waveguides. Because of thermal distortion
problems a graphite/aluminum metal matrix composite is now being developed as a
possible replacement for the aluminum.
The antenna structural members are composed of a high-temperature graphite plastic
material for rigidity. The antenna primary structure has a I040 meter x I040
meter x lO0 meter pentahedral truss configuration which suppors a secondary struc-
ture. This secondary structure provides a base for mounting and aligning the
transmitter subarrays. Both the primary and secondary structures must maintain
a high degree of stability over wide operating temperature fluctuations to pre-
serve the three arc-minute flatness requirement, hence the need for low coeffic-
ient of thermal expansion materials.
• Startup/Shutdown Procedure. The satellite will have to shut down 87
times per year due to solar eclipses by the earth. In addition there will be
eclipses by the moon and other SPS, as well as scheduled shutdowns for mainten-
ance. A number of possible sequences for energizing/deenergizing the microwave
system were investigated. Three sequences provided satisfactory performance in
that the resultant sidelobe levels during startup/shutdown were lower than the
steady-state levels present during normal operations. These three sequences
were: random, incoherent phasing, and concentric rings-center to edge. Thus no
microwave radiation problems are anticipated during startup or shutdown operations,
either scheduled or unscheduled.
Shaped Beam Synthesis: Studies into reshaping the beam pattern to improve over-
all rectenna collection efficiency and to provide additional means of sidelobe
control were undertaken. These studies included: (1) Adding phase reversal at
the klystron input as a first step towards a continuously variable phase distri-
bution across the antenna surface. The results showed that reshaped beam patterns
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into "squared" main beams are possible with both reverse and continuous phase
tapers. However the penalty is an increased antenna size or a larger rectenna.
(2) Adding suppressor rings to the antenna for reducing the first few sidelobe
levels. Results indicate a 5 dB reduction in sidelobe levels at the expense of
a loss in rectenna collection efficiency. Larger antennas or rectennas are
again needed to retain the 88% rectenna collection efficiency. (3) Quadratic
phase tapers. The analyses showed a decrease in on-axis power density (i.e., a
squared beam) with a corresponding loss in beam transfer efficiency, dependent
upon the amount of phase taper introduced. (4) Multiple antenna beams. The
transmission of multiple beams from a single antenna is possible by spatially
modulating the illumination function. Results for a simple two-beam SPS system
were as predicted except for a small residual central lobe. Elimination of the
central peak is a goal for future studies in this area.
Summary: The characteristics and error parameters of the updated microwave
transmission system may be summarized as follows:
Frequency
Output Power to Power Grid
Transmit Array Size
Power Radiated from Transmit Array
MPTS Efficiency
Array Aperture Illumination
Peak Microwave Power Density in
Ionosphere
Phase Control
Waveguide material
Error Budget:
Total RMS phase error per power
module
Amplitude tolerance per power
module
Failure rate of DC-RF power con-
verter tubes
Antenna mechanical alignment
Subarray mechanical alignment
- 2.45 GHz
- 5 GW (DC)
- l Km Diameter
- 6.72 GW
- 63% (DC/RF Input to RF/DC Output)
- lO step, truncated gaussian ampli-
tude distribution with lO dB edge
taper
- 23 mW/cm 2
- to power module level
- metal matrix composite
= lOo
= ±1%
= 2% (a maximum of 2% failed at any
one time)
- one arc-minute
- three arc-minutes
The relative importance of these electrical and mechanical tolerances upon scat-
tered microwave power (extra power not incident upon the rectenna) is summarized
in Figure 3.
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Figure 3. Reduction in Microwave Power due tO Electrical and Mechanical Errors
(Power Module Level Phase Control)
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Microwave system performance summary

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