Power in the sky: Requirements for microwave wireless power beamers for powering high-altitude platforms

We will focus mainly on microwave beamers to provide the energy wirelessly to station-keep such platforms and for powering their payloads. Laser beamer systems are also in play but are limited by weather, beam safety, and current cost. The HAPPs are assumed to be equipped with rectennas to receive the wireless power beam and to convert it to electric power, although there are other beam-receiving power converters such as thermal absorbers. However, the rectennas are the lightest and most efficient collector-converters to date, being on the order of kW/kg and over 85% efficient and providing dc voltage output directly that can power electric motors for propulsion [4, pp. 1231]. Beamers in concept are simply transmitters and antennas, but they have very complex requirements involving and resulting from the power levels and the vehicle aerodynamics. The demonstration that provided proof that WPT could deliver electric power levels sufficient to power stratosphere vehicles with significant payloads was performed in 1975 at Goldstone in California´s Mojave desert, where the author provided the engineering design specifications and conducted [13], with JPL/ Caltech and Raytheon, tests sponsored by NASA that collected 34 kW dc output from a 23.84-m² rectenna array averaging 82.5% collection-conversion efficiency with S-Band (2.388 GHz) microwave power radiated from a 26-m diameter parabolic antenna equipped with a high-power klystron transmitter. The transmitter was located 1.55 km from the rectenna. More power could have been collected if resources had allowed a rectenna larger in area. However, this article is to concentrate on WPT beamers.