Although many rectifier divices which were the analogs of various microwave generators were proposed, only the development of the rectifier analog of the magnetron was supported. This device proved to be impractical for reasons of a very basic physical nature. The point-contact semiconductor diode was earlier demonstrated to be an efficient converter of microwaves into dc power, but its power handling capability was so low as to cause it to be initially dismissed from serious consideration. Later, with the introduction of the "rectenna" concept, its true potential as a microwave rectifier was recognized. The limited but broad interest in microwave power rectification devices of all kinds that was initiated in the 1958 to 1962 time frame did not continue beyond that period. Residual interest was focused upon the Schottky-barrier diode because of its high demonstrated efficiency and its relationship to the rectenna concept. As a result there is today no broadly based microwave power rectification technology, and any approaches to the collection and rectification of microwave power must rely upon the semiconductor diode, whose power handling capability is limited. The chronology of the collection and rectification of microwave power is given in Figure 9-2 and major development programs are outlined in Figure 9-3. The introduction of the Gallium Arsenide Schottky-barrier diode proved very significant in terms of high efficiency and power handling capacity. The combination of this device with a harmonic filter to attenuate radiation of harmonics and to store energy for the rectification process led to the configuration shown in Figure 9-4. This was used in construction of a 4 foot diameter rectenna for 2 Marshall Space Flight Center, and in the recently completed 25m rectenna built for the Jet Propulsion Laboratory that demonstrated 82% efficiency at an output power level of 32 kW. Verification of rectenna element efficiency during this same program established a reference point on the curve of Figure 9-5. The variation of efficiency with frequency is estimated from the equivalent circuit, and is of value for system studies to establish a desirable MPTS operating frequency.
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