1. 2 Major Microwave Collector-Converter Technology Developments As a result of the early experience with the severe demands placed upon the receiving portion of a free-space microwave power transmission system and the discovery of the ability of the rectenna concept to cope with all of these demands, the history of microwave collector/converter technology is almost exclusively that of the development of the rectenna. The following general requirements are placed upon the collector/ conve rte r: • large aperture • high power handling capability • non-directive • high efficiency • ability to operate efficiently over a substantial frequency range • light weight • easy mechanical tolerances • ability to passively radiate any heat resulting from inefficient • operation • high rel lability • very long life • minimal radio frequency interference • low cost. The rectenna has been found to successfully meet all of these requirements, with the possible exception of radio frequency interference. RFI, however in the form of harmonic power, is a special problem that confronts both the transmitter and the receiver. Since the harmonic level must be down to such low levels to meet non-interference requirements and meeting it by wave filters would result in such higher cost and reduced efficiency, the proper solution may be to have an allocation of frequencies for the harmonics that are generated in the system. The rectenna has gone through a number of development stages whose nature was largely determined by the motivational influences of the period and the state of development of diodes. These stages are outlined with the aid of Table 1-2. The microwave powered helicopter application was the dominant early influence and was responsible for the initial development of two separate embodiments of the rectenna concept. The very first rectenna. Figure 1-3,which established its general properties made use of a rectenna element characterized by a halfwave dipole antenna terminated in a full-wave bridge. This development was based upon an early study of the solid-state diode as an efficient rectifier of microwave power by George (^4) and its adaptation as a rectifier of free-space radiation by Brown, et al. ^6) The rectenna elements were separated from each other by approximately one-half wavelength. Unfortunately, such a construction using the then existing point-contact diodes could not handle nearly enough power density to be used for the demonstration of a microwave-powered helicopter. A new configuration characterized by a dense compaction of diodes
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