that the microwave beam could be used successfully as a position reference in a control system in an automated helicopter which would keep itself positioned over the center of the bearm This progression of efforts established the rectenna device as a probable solution to the collection and rectification problem in a broad class of microwave power transmission applications, but much work remained to be done to make it a practical device in the context of the SSPS type of application. The opportunity to further evolve the rectenna device was largely the result of the interest of the Marshall Space Flight Center in applying microwave power transmission to the transfer of energy and power between satellites, (8) and the contractual effort supported at Raytheon Company. ' In a more recent time frame very substantial advances in overall system performance have been made. These advances include a certified overall transmission efficiency of 54% starting with the DC power applied to the microwave generator and ending with the DC power out of the rectenna at the receiving point. ' ' A particularly impressive demonstration was made at the Goldstone facility of the Jet Propulsion Laboratory. In this demonstration power was transmitted over a distance of 1. 6 kilometers and a DC power output of over 30 kilowatts was obtained at the receiving point. Table 1-1 presents a summary of the early chronology of the collection and rectification of microwave power. It will be noted that there was inter-' est in microwave power transmission prior to any capability of efficiently converting microwave power directly into DC power. ' ' TABLE 1-1 CHRONOLOGY OF COLLECTION & RECTIFICATION OF MICROWAVE POWER 1958 First interest in microwave power transmission 1958 No rectifiers available - turbine proposed and studied 1959-1962 Some government support of rectifier technology (1) Semiconductors at Purdue University (2) Magnetron analogue at Raytheon 1962 Semiconductor and close-spaced thermionic diode rectifiers made available. 1963 First power transmission using pyramidal horn and close-spaced thermionic diode rectifiers - 3 9% capture and rectification efficiency not practical for aerospace application. 1964 RADC microwave powered helicopter application demanded nondirective reception, light weight, high reliability. 1964 Rectenna concept developed to utilize many semiconductor rectifiers of small power handling capability to terminate many small apertures to provide non-directive reception and high reliability. 1968- Continued development of rectenna concept to format with high power Present handling capability, much higher capture and rectification efficiency, and potentially low production cost.
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