Tracking and Communications Development Division - R. H. Dietz IV. D. MICROWAVE RECEPTION AND CONVERSION SYSTEM 1. Rectenna Rationale: The problem of collecting the microwave power from the SPS and then changing this to a useful form for transmission to the electric utility grid system has been the subject of much attention in the past several years. The concept which has developed is that of a receiving antenna combined directly with a rectifying element. This combination is known as a rectenna. This rectenna concept is essentially a one step process of collection of microwave power and rectifying this power into direct current all in one unit. This concept has been chosen over several others because it offers the potential of being the most efficient and most reliable as well as having the lowest production costs. Some earlier concepts which have been investigated but have been rejected for various reasons are: a. Microwave (MW) conversion into heat--the heat would be used to run a heat engine of some type which could then be used to generate the desired electrical current. The drawback to this concept is mechanical complications and low overall efficiency. b. Microwave conversion to direct current (DC) (MW generator analogs) - concepts for use of traveling wave tube and Klystron analogs have been developed, but the early development of a magnetron analog led to the conclusion that the MW generator analogs were impractical. c. Microwave conversion to direct current (diode rectifiers) - thermionic diodes reached a stage of early development, however, component reliability proved to be a serious problem, and the outlook for efficient rectification was not good. Not until the introduction of the Gallium Arsenide Schottky barrier diode was there promise for an efficient, reliable device which could convert MW to DC. The next consideration in the rectenna concept is what is the best way to collect the microwave energy being transmitted from the SPS. The question of collection of the microwave energy has also been investigated over the years and various antenna designs have been evaluated. The general desired requirements for such an antenna are: (1) Appropriate power handling capability. (2) High reliability.
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