program should be initiated for determining failure mechanisms and radiation sensitivity. Cooling, and maximum allowed temperature are critical to the design of a solid state MPTS. Cost may turn out to be a serious problem for the solid state devices because of the large quantities that will be required. Injection-locked magnetrons may offer substantial promise from a cost point of view, however, substantial work needs to be done at the device level in the area of noise reduction and improved efficiency. The panel recommends further work in this area. In the efficiency budgets produced so far, only the most optimistic predicted values have been used for the estimates of DC to rf conversion. A more conservative approach would be to use demonstrated efficiencies, with the variabilities of loading and performance included. Radiating Elements The principal problem the panel identified in this area is related to materials. Aluminum looks attractive except for its bad thermal expansion characteristics. Work should be initiated to see if there are any manufacturing or design techniques which would ameliorate this. Although low CTE composites were mentioned frequently during the Workshop, there was no evidence presented which would indicate that these materials would in any way be suitable for microwave circuitry on the SPS. Obvious problems which come to mind include outgassing from the epoxies, conductor adhesion problems, and fabrication techniques. At the present time, these materials are a complete unknown, and should not be relied upon too heavily in the SPS design. Problems of I2R losses should also be addressed early so that potential later snags can be avoided. The multipacting problem was mentioned frequently. Although this phenomenon is fairly well understood, there does not seem to be enough data at the present time to be able to predict if it will be a problem in the MPTS radiator. Finally, the problem of harmonic interaction with the radiating structure needs to be addressed. It will not be feasible to place filters, circulators, or much else between the power amplifier and the radiating element without introducing unacceptable losses. Thus, harmonic supression on the SPS itself must be achieved with the design of the radiating elements. The Rectenna The major problems which the panel sees are those of weather protection, parts count, and harmonic re-radiation. The demonstrated efficiencies at Goldstone have shown that the basic concept is reasonable, but have not answered the question of scaling this approach to SPS power levels and larger mass produced arrays. Regarding harmonic generation and re-radiation, the amount of harmonic suppression possible with any economically reasonable filter placed on 1010 individual elements does not seem to be sufficient to limit the harmonic signals to an acceptable level. The only logical approach is to look for ways to lower the number of individual receiving elements, so that more care can be exercised in their design and construction. Some form of weather protection or radome will be needed over all of the active elements in the rectenna. The rectenna is presently a major cost factor in the total SPS system. As such, it should be subjected to careful cost effectiveness sensitivity studies which might point towards a slightly less efficient system, but with substantial
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