There is no inherent reason why dc-RF converters cannot also be designed to be capable of bi-directional power transfer. IMPATT diode oscillators can rectify RF and klystrons or magnetrons can be operated in reverse (Okress. ed.. 1968). The direction of de current flow is the same but the voltage polarity is reversed between generation and reception. Obviously, the operating power level dynamic range must be considered in the converter design and properly phasing the harmonics is a muse Not enough attention has been given to this possibility. An investment in developing the dual-mode technology may pay off in lower system costs and with a system engineered to better serve customer needs. Multifunctional uses of the WPT equipment are desirable so that revenues can be generated while the total system is being constructed. For example, using the ground array elements as radars for orbital debris tracking might yield income from the fleets of LEO telecommunications satellites. Charging spacecraft batteries during eclipse periods may be profitable. Propelling small payloads to orbit may be a future business (Benford and Dickinson. 1995). Using the elements to power aerostats serving as surrogate satellites for telecommunications and observations might generate income. The output RF from the high-power transmitters might be used to sinter ceramics or mediate hazardous waste, for example. A notional concept for a multiuse tourist hotel aboard a microwave beam-powered aerostat is shown in Fig. 2. CONCLUSIONS AND RECOMMENDATIONS In addition to the standard wireless power transmission system engineering issues, there are three key issues that continue to need addressing. They are beam safety, frequency allocation, and affordability. Public education about the control of radiating microwaves, safety hazard awareness and visible beam location indicators may help to overcome the public perception of fear of frying. Frequency allocation issues will ultimately be resolved by international cooperation when potential economic benefits are more evident The affordability of large beamed power systems will be aided by multiple uses of the equipment during the buildup phase and the ability io move power in both beam directions. ACKNOWLEDGMENT The research described in this paper was carried out at the Jet Propulsion Laboratory. California Institute of Technology, under a contract with the National Aeronautics and Space Administration. REFERENCES Benford. J. and Dickinson. R.. 1995. “Space Propulsion and Power Beaming Using Millimeter Systems.** Paper 25. SPIE 2557. San Diego. Bigness. J.. 1995. “FDA’s New Rule: Don’t Blind Pilots With Las Vegas Glitz/* Wail Street Journal. Brown. W.C., 1981. ‘The Solar Power Satellite as a Source of Base Load Electrical Power/* IEEE Trans, on Power Apparatus and Systems. Vol. PAS-100. No. 6. p. 2768. Okress. E.C.. ed.. 1968. Chapter 4 Rectification, in Microwave Power Engineering. Academic Press. NY and London, pp. 273-340.
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