The Technologies Wireless Power Transmission Efforts to develop wireless power transmission (WPT) started with Hertz (1857- 94), and were pursued by Tesla (1856-1943). Tesla attempted to demonstrate global WPT with a tall tower on Long Island, New York, in the early 1900's but was not successful. WPT was not pursued again until low cost microwave generators were developed such as those used today in over 250 million microwave ovens worldwide. In 1964 Raytheon, a US defense contractor, successfully demonstrated a microwave-powered helicopter flying at a modest altitude. Canada developed the Stationary High-Altitude Relay Platform (SHARP) program as a low cost alternative to satellites for communication relay stations and surveillance of remote areas. SHARP was demonstrated in 1987 by the Canadian Department of Communications and proved that an aircraft could be maintained at a specified altitude indefinitely when powered by a controlled microwave beam. This application is an excellent example of dual use technologies. A microwave WPT demonstration was successfully completed in 1975 at the NASA Deep Space Antenna facility at Goldstone, California. In this demonstration, a microwave beam at a frequency of 2.45 GHz transmitted 30 kW over a distance of one mile to a receiving antenna. The microwaves were converted directly into de at an average efficiency of 82%. Japan has conducted its own test projects including WPT to an aircraft in 1992, from a rocket to a daughter payload in 1993. Kansai Electric recently conducted a new wireless power transmission test, similar to the one done in Goldstone (in the 60s), but using more power. Wireless power transmission is now a proven technology. It remains to be proven economically. The majority of technology developed for WPT systems has been at microwave frequencies, e.g., 2.45 GHz and 5.8 GHz, located in the Industrial - Scientific - Medical (ISM) band, thus formal permission to transmit at these frequencies may not be needed as long as interference with other systems outside this band is avoided. However, increasing demands for frequency slots close to these frequencies by cellular phone service providers makes it desirable to obtain a formal allocation of frequencies. Furthermore, without a formal allocation of frequency developers will be in the position of the wildcatter who is trying to raise money to drill for oil when he does have any oil leases. Power Relay Satellites In 1954 Kraft Ehricke proposed the concept of the power relay satellite. The power relay satellite (PRS) is designed to reflect a microwave beam from a selected microwave beam transmitter site to a specific location of a receiver separated by intercontinental distances. The configuration of a PRS is shown in Figure 6. The advantages of the PRS lie in the fact that the power beam can span oceans and deserts and the transmission efficiencies are much higher than those for high voltage transmission lines.
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