MICROWAVE SYSTEM PERFORMANCE SUMMARY G. D. Arndt - NASA/Johnson Space Center, Houston, Texas E. J. Nalos - Boeing Company, Seattle, Washington Introduction: The SPS microwave system as defined in the October 1978 Reference System Report, D0E/ER-0023, has a 1 Km diameter phased array antenna with a 10 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 1 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/parallei solar array configuration. The antenna is divided into 7220 mechanical subarrays, 10.4 meters x 10.4 meters on a side, having slotted waveguides as the radiating surface. Slotted waveguides were selected because of their high power handling capabilities and low I2R 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 rectenna 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 implemented 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. The ground rectenna converts the RF energy to DC electricity using halfwave dipoles feeding Schottky barrier diodes. Coherence of the incoming phase front needs to be maintained only over the area associated with a small group of dipoles. Physically the present rectenna configuration is a series of serrated panels perpendicular to incoming beam and covers approximately 75 square kilometers. 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 1-Km transmit antenna and 5 GW of DC output power from the rectenna is based upon a 23 KW/m2 heat dissipation limit in the antenna and a hypothetical 23 mW/cm2 peak power density limit in the ionosphere to prevent nonlinear 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 101,000 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 reduction 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
RkJQdWJsaXNoZXIy MTU5NjU0Mg==