Early investigators of the use of microwaves for power transmission in the 1957-1960 time frame resorted to the conversion of microwave energy at the receiving point into heat, which was used either directly or to run a heat engine. However, this approach leads to many mechanical complications and in any event can provide an overall efficiency of at most 30%. These considerations led immediately to the desire for an efficient electronic device that would convert the microwave power directly into dc power. The earliest known rectifier development projects in which the end use was intended for power purposes rather than information were those supported by two contracts from the laboratories of the U.S. Air Force at Wright Field. One of these contracts was awarded to Purdue University to examine broadly the development of devices to rectify microwave power. The other was awarded to the Raytheon Company for the study of a rectifier device that was the rectifier analog of the magnetron. The findings of these two investigations were important background in determining the course of subsequent investigations and in attempts to develop and operate complete systems making use of microwave power transmission. Rectifiers may be classified in several different ways. One division of classification is into solid-state and electron-tube devices. Another division would be into microwave-tube analog devices and diode rectifiers. Still another classification would be into low-impedance devices and high-impedance. Microwave-tube analog devices are characterized by low-current and high-voltage output, whereas diode rectifiers of both the solid-state and electron-tube types tend to be low impedance devices. There was considerable interest from private and industrial organizations in addition to the limited interest of the Department of Defense in the technology of microwave power rectification in the 1958 to 1962 time period. This interest is well documented in Okress, "Microwave Power Engineering", Volume I . Figure 9-1 summarizes a number of these concepts and their state of development. One of these concepts, the close spaced thermionic diode rectifier reached a state of development in which it could be used as a rectifier in the first known demonstration of microwave power transmission. However, it had serious reliability and life problems.
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