MICROWAVE POWER TRANSMISSION WORKSHOP SUMMARY John W. Freeman Rice University - Houston, Texas 77001 Introduction The Satellite Power System (SPS) Workshop on the Microwave Power Transmission System (MPTS) was held at the Johnson Space Center in Houston, Texas on January 15-18, 1980. The objectives of this workshop were to assess and critique the assumptions, methodologies and conclusions of the NASA SPS studies and to identify and assess critical issues and to make recommendations for follow-on work. The workshop review panel consisted of Dr. Robert C. Hansen, Prof. Bernard D. Steinberg, Prof. Aldo V. da Rosa, Mr. Harry Goldie, Dr. Paul Tallerico, Prof. William L. Wilson, Jr., and Dr. John W. Freeman. Presentations by NASA personnel and contractors were arranged by R. H. Dietz of NASA/JSC. The review panel assessment may be summarized as follows: Beam Forming and Control The present retrodirective phase control system has the following disadvantages: 1) inadequate provision has been made for security and anti-jamming protection; 2) ionosphere problems or other uplink disturbances or interruptions could lead to sudden and complete loss of function; 3) adequate long-term stability of the narrow band notch filter used for suppression of the power transmitter at 2.45 GHz is questionable in real world technology; and 4) aging and mistuning may lead to phase drift problems in the onboard circuitry. Additional potential problem areas are: 1) mutual coupling among the microwave amplifiers and 2) possible underestimation of the power in the far sidelobes due to position errors in the phase centers of the subarrays and power modules. Although there was not complete agreement, the panel tended to favor a closed-loop phase control system over the retrodirective approach. The onboard broadcast phase reference system presented by Rockwell in connection with the solid state sandwich configuration seems appealing because of its freedom from ionospheric variations and interruptions. Work should proceed on both closed loop and open loop systems. None of the phase control systems presented are clearly superior at this time. Microwave Amplifiers There is still no definite answer as to which choice is optimum for the microwave power amplifier devices. At this time the klystron looks most favorable, but either the solid state or magnetron source may look better later. The question of optimum power transmission voltage and amplifier size should be very carefully studied and re-examined. Some attempt should be made early to determine the maximum voltage which can be safely used in the SPS environment, as this has a significant bearing on many design decisions. If it is not possible to operate at 40-50 kV, then klystrons cannot be used. While ohmic and klystron efficiencies increase at higher voltage, reliability of power conditioning equipment and the klystrons decreases. Water cooling of the klystron looks troublesome. Noise and harmonic generation is a major problem with any of the power amplifiers being considered; however, it appears that the klystron will have better noise characteristics than the solid state or magnetron devices. Solid state devices have to overcome problems of noise, efficiency and high temperature operation before they can become viable contenders in the MPTS. As soon as reasonable solid state devices can be fabricated, an extensive test
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