Reintroduction of Microwave-powered Aircraft: The Canadian Sharp Program Generic improvement in beamed microwave technology and the standing need for a long endurance high altitude platform led to a revival of interest within NASA in microwave-powered platforms in 1978. Out of this interest came two microwave- powered airship studies from Wallops Flight Facility [17, 18]. These studies produced two outstanding technology advances. The first of these was a new thin-film, printed-circuit rectenna format which made its use in both air and space vehicles very attractive [17]. This format was later greatly improved upon and made ready for space use with the use of discretionary funding at LeRC [19], The second contribution was the conceptual design of an electronically steerable phased array composed of radiation modules similar to those for the SPS [18]. It was determined that a combination of an off-the-shelf microwave oven magnetron, a ferrite circulator, and a section of slotted waveguide array could become a building block for Earth-based transmitters for both space applications and for microwave-powered aircraft (Fig. 7). It was subsequently found that the design could be greatly simplified by adding additional external circuitry to the microwave oven magnetron to greatly increase its gain while locking its output phase to the phase of the driver [20]. The development of the new rectenna format remained unexploited experimentally in the USA, although it was studied in the context of a microwave-powered airplane for atmospheric surveillance [21], In 1981, however, the Canadian government embarked on the SHARP (Stationary High Altitude Relay Platform) program that in 1987 produced the first successful demonstration of the free flight of a microwave- powered aircraft; in this case, an airplane (Fig. 8). The Canadian team was successful in adding its own improvement to rectenna technology, a crossed polarized rectenna
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