Microwave System Efficiencies - One of the earliest demonstrations that meaningful amounts of power could be transmitted via microwaves was done at Raytheon in May of 1963 (ref. 20). The collection efficiency was 87 percent; however, the RF-DC conversion efficiency was only about 50 percent. Overall efficiency was only 16 percent. As the experiments continued, more attention was devoted to increasing the efficiency of collection and conversion. The concept of a "rectenna" evolved (receiving antenna and rectifier) which exhibited low directionality for the collecting antenna, and high RF-to-DC conversion efficiencies. Early application of the rectenna concept to the SPS resulted in the addition of filters to attenuate the radiation of harmonics and to store energy for rectification. The rectifier was changed from a full - wave bridge using point-contact diodes to a single GaAs Schottky-barrier solidstate diode in a half-wave rectifier configuration. Conversion efficiencies of 80 percent were obtained (ref. 21). In 1975, the rectenna portion of the microwave system was tested at the JPL Goldstone facility (ref. 22). Microwave power was transmitted from an 85-foot antenna to an array of 270 rectenna dipole-diode elements over a distance of 1.6 km. Of the microwave power impinging upon the rectenna, over 82 percent was converted to DC power. A total of 30 kW was collected and dissipated into a lamp and resistive load. Since that time Raytheon has investigated mechanical and electronic improvements to the rectenna element which has resulted in demonstrated conversion efficiencies of 85 percent at certain power density levels (ref. 24). Current projections, assuming technology advancements, are for conversion efficiencies of approximately 90 percent in the time-frame needed for SPS. Coupled with this experimental concentration on improving the collection and conversion efficiency at the receiving end, a concept evaluation and technology assessment was being conducted on the transmit antenna. Major contributors toward transmit efficiency in the microwave system are: 1. Microwave power amplifiers (DC to RF converters) 2. Beam focusing and pointing (phase control)
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