latter applications are of especial international interest because all system elements must be located in the equatorial plane for economic reasons. Introduction Currently, there is a revival of interest in using beams of electromagnetic energy to transfer power to terrestrial and space applications. The revival of interest is the product of both improved technology for beam power transmission and a recognition that current conventional technology is inadequate to provide large amounts of electric power and energy in space and to provide adequate transportation in space. An example of applications made possible by the improved technology was the recent Canadian government sponsorship of a microwave-powered airplane that was successfully flown for the international media and invited guests in the fall of 1987 in Ottawa, Canada [1]. This was made possible by the sponsorship within the USA of the thin-film format for the rectenna that improved the ratio of DC power output to weight by a factor of nearly ten over its previous format. The same thin film rectenna that exhibits 85% overall efficiency is well adapted to space use, as well. An example of a need not being fulfilled by existing technology is a low-cost form of transportation from low earth orbit to geosynchronous orbit. The present approach using conventional chemical rockets is not suitable for this purpose because of the very large amounts of propellant that must be brought up from the Earth to LEO for even a modest payload in GEO. Electric propulsion has the potential to reduce the needed propellant mass by a factor of ten or more but requires a low mass source of electric power to do so. A microwave beam acting through the low specific mass rectenna carried on the vehicle would be such a source [2]. The revival of interest in beamed power transmission is global in nature. This global interest is motivated in part by a growing realization that beam power transmission technology is an integral part of the Solar Power Satellite system which may some day contribute significantly to a reduction in CO2 emissions on the earth that result from burning fossil fuels. The general interest in beamed power includes the use of many portions of the electromagnetic spectrum. However, this article will focus on the technology that exists at 2.45 GHz and some of the applications of that technology. This technology which includes key components at the receiving and transmitting ends is much further advanced at that frequency and serves as a ‘standard' against which technology at other frequencies can be compared. In addition there are a number of inherent advantages at that frequency. First, if transmission through the earth's atmosphere is involved, as it often is, the attenuation of the power is minimal in even very heavy rainstorms. This is extremely important in the case of the Solar Power Satellite concept which must perform reliably around the clock. Secondly, it is in the centre of the 2.4 to 2.5 GHz ISM (Industrial Scientific Medical) band which is reserved for those activities and which may be used for experimental work on power transmission systems. It also makes good use of components developed for some of the applications of the ISM band. This includes the microwave oven magnetron which can be used almost directly for Earth-based installations and in redesigned form for use in space. At this time it may be useful to review the unique properties of beamed power because it is these properties that will determine its applications: • No mass required between source of energy and point of consumption No wires
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