absorb energy over very large roll and pitch angles without significant loss in efficiency. The rectenna used in this demonstration was of the ‘string type' and used 4480 semiconductor ‘point-contact' diodes. The ratio of power output to weight was quite low because of the power handling capability of the diodes. However, there were shortly great improvements in diodes that were much more efficient and handled large amounts of power per unit of their mass. A non-tethered, beam-riding helicopter, but not microwave-powered, was successfully demonstrated in 1967 [4, 5]. With 400 kilowatts of continuous microwave power available, the technology was basically available for a high altitude helicopter platform. But in the meantime, communication satellites were coming into use and it was not until the early 1980s that the need for such platforms was again acknowledged. Certification of Overall DC to DC Efficiency of 54% in the Laboratory The next beamed power technology development was supported by NASA's Marshall Space Flight Center in the 1969 to 1974 period. The development was carried out in the ISM (Industrial, Scientific, Medical) band to 2.4-2.5 GHz because of the cost effectiveness of using the technology already existing there. The program was respon-
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