No ferrying vehicles • Energy can be transferred at the velocity of light • Direction of energy transfer can be rapidly changed • No loss of energy transfer in vacuum of space; little loss in Earth's atmosphere at 2.45 GHz under all meteorological conditions • The mass of the power converters at the terminals can be small • Energy transfer between points is independent of gravitational potential The applications that have so far been identified and make use of these unique properties are: • Space Applications o Solar Power Satellite with microwave power transmission to Earth o Low Earth orbit to geosynchronous orbit transportation system o Electrical power source for ‘orbiting industrial parks' • Terrestrial Applications o High altitude microwave-powered aircraft An interesting aspect of the space applications that give them an international flavour is that they take place in entirety or in part in the equatorial plane, for reasons having to do with the orbital mechanics of the Earth and satellites launched into low Earth orbit. The necessity to use the equatorial plane may be seen as advantageous or disadvantageous, depending on the point of view, but a common use of the facilities there facilitates a great cost reduction which becomes greater the more they are used. Each of the applications noted above has had a history of interest and study, and in some cases considerable amount of development effort. And it is the interaction between the technology and the potential applications that have largely shaped the technology. It may therefore be desirable to review some significant events and accomplishments of the past because they do indeed in many instances define the present status and validation of the technology. Visual demonstrations of complete beamed power systems are particularly important because they provide the most visible, most easily understood, and most reliable proof of the status of the technology. If they are also connected with an application the demonstration is even more significant. The first example, that of a microwave- powered helicopter, belongs in this latter category. The Microwave Power Helicopter Demonstration A tethered microwave-powered helicopter was the first experimentally-successful, application-oriented use of beamed microwave power (Fig. 1). It was successfully demonstrated by the US Air Force and the Raytheon Co. for the press and TV media in October 1964. In the subsequent month it was flown for ten continuous hours in the presence of an Air Force representative at an altitude of 60 feet, powered only by a microwave beam, to meet the test requirements of the contract. The frequency used was 2.45 GHz corresponding to a wavelength of 12.26 centimetres [3, 4]. The technology breakthrough that made this possible was the development of the ‘rectenna', a word derived from the words ‘rectifier' and ‘antenna'. This rectenna simultaneously absorbed the incident microwave energy impinging upon it and rectified it into DC power which was then applied directly to the electric motor driving the rotor. The rectenna is also well suited to aircraft applications in the sense that it can
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