Microwave power transfer is a strong alternative possibility, especially when conducting cables are restricted to low voltage, high current configurations. Using a transmitting antenna size of 20 m and a receiving antenna of 30 m diameter, microwave beam transmission ranges of 5-10 km can be attained satisfactorily. End-to-end microwave transmission efficiency is around 53%, equivalent to that of a three-phase (3495 kg/line), 5 km, 200 V cable system, but with a considerably lower mass requirement. The steerable microwave beam produced by the phased array antenna provides significant advantages of system flexibility, incorporating features of continuous retrodirective targeting, and potential for producing multiple beams. Microwave transmission might be utilized with either a tethered system or a free flying reactor. In the latter case, however, the reactor could not be maintained in position close enough to the space station for high efficiency power reception (due to excessive station-keeping fuel consumption). Therefore tethered configuations are the leading option for nuclear reactor utilization. System characteristics for a typical microwave power configuration are summarized:
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