MICROWAVE POWER TRANSMISSION DEMONSTRATION Efficient transmission of high power over long distances by an RF (microwave) link was recognized by NASA as a critical issue several years ago. Efficiencies on the order of 85 to 90 percent are needed for RF power generation, beam forming, and reception. An experiment program to explore these possibilities was initiated about two years ago. Results to date have been very encouraging. Efficient RF generation can be achieved by amplitrons. (The amplitron is a magnetron operated as an amplifier.) Common microwave oven magnetrons are over 60 percent efficient, and experiments with more efficient designs have exceeded 80 percent. Detailed analyses and studies of beam formation indicate that the required narrow beam is achievable through use of large phased arrays that can be electronically pointed with the required accuracy. The chart shows highlights of recent power transmission tests at Jet Propulsion Laboratory. A conventional communications transmitter was used as the beam source. A receiving antenna of the type required for power transmission systems was mounted on a tower more than a mile away. This antenna consists of a phased array of dipoles, with each dipole coupled to the D.C. load through a diode rectifier and capacitive filter. Since rectification to D.C. occurs at each dipole, the antenna is not highly directional and therefore can be of low-cost stationary construction. The tests demonstrated a receiver efficiency of 82.5 percent in terms of D.C. power delivered to the load referenced to RF power intercepted by the antenna. Over 30 kilowatts of D.C. power output was achieved.
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