common microwave load, and an array of small aperture elements with independent microwave loads (rectenna concept). A comparison of these approaches is shown in Figure A-12. It was quickly decided that an array of solid-state diode rectifier elements, each combined with an individual dipole antenna and suitable harmonic filter, was the only option combining both high efficiency and low cost. The combination, receiving antenna and rectifier, came to be known as the rectenna. A simplified schematic of a rectenna element is shown in Figure A-13. RF-to-DC conversion efficiency has steadily increased from approximately 50 percent in May 1963 (ref. 20) to 82 percent with the JPL Goldstone tests (ref. 22) to 85 percent as a result of Raytheon's investigations into electronic and mechanical improvements in 1977 (ref. 24). Concepts other than the Raytheon developed concept have been investigated. Renssalaer Polytechnic Institute (RPI) has been investigating other types of receiving elements which have the potential of reducing mass manufacturing costs, as well as reducing the number of rectifying diodes, and in some cases, the number of receiving antenna elements. In addition to a continuing study of the reference system configuration, printed circuit dipoles, printed circuit yagis and conventional construction yagis have been investigated. The most promising to date is the printed circuit yagi which offers both the capability of increased gain and thus fewer elements, as well as a potential for reducing mass manufacturing costs. In addition to this effort RPI has performed an analysis of the sensitivities of series parallel combining the DC outputs from the rectenna elements. Alternate rectenna concepts were investigated by both Rockwell and Boeing. Rockwell's approach concentrated on stripline and bowtie dipole rectifiers. Analysis indicated that cost and high RF attenuation should be traded against reduced number of elements. The Boeing approach concentrated on (a) increasing the dipole spacing to reduce the total number required; (b) reducing structural cost by trading RF losses versus flat rectenna layout; and (c) increasing the antenna gain (reducing the number of diodes) with a
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