Stationary High Altitude Relay Platform

SHARP, short for Stationary High Altitude Relay Platform, was an experimental aircraft using beam-powered propulsion designed by the Communications Research Centre Canada (CRC) and built by the University of Toronto Institute for Aerospace Studies (UTIAS) during the 1980s. SHARP used microwaves to provide energy from a ground station that powered electric motors spinning propellers to keep the aircraft aloft. The power was also used for the onboard electronics. SHARP could remain aloft indefinitely, and was intended to be used as a sort of low-altitude communications satellite for smaller geographical areas.

The concept of using beamed power for aircraft propulsion was invented almost single-handedly by William C. Brown. After joining Raytheon in the 1940s, Brown started work on improving their magnetron products. This led to the development of the crossed-field amplifier, a simple, reliable and highly efficient microwave amplifier. He later worked with colleagues to develop the rectenna, which receives microwaves (the (an)"tenna") and converts them directly to DC power (the "rect"ifier).

Brown now had a system that could convert input power to microwaves with up to 70% efficiency and convert it back to electric power with 70% efficiency, resulting in an overall efficiency of about 50%. Brown looked for applications of the technology, working on both solar power satellites (SPS) and the High Altitude Powered Platform (HAPP) concept. This research went as far as flying a model helicopter using beamed power in 1965.

NASA also invested in the rectenna concept as part of their SPS work. This was tested in a ground-to-ground experiment in 1975, and as part of this they developed lightweight versions of the rectenna. In 1982, Brown and James Trimer (of NASA) announced a new version of the rectenna using printed circuit techniques that reduced the weight by ten times. This made aircraft applications much more attractive.

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