In principle, efficiency may approach 100% because the receiving antenna element and the microwave radiation are coherent and polarized in an orderly manner. Hence, the effective conduction cycle of the diode rectifier circuit and the reactive energy storage combine to produce a very high efficiency. (Up to 87% conversion efficiency has been achieved in the laboratory.) A receiving antenna based on the principle of halfwave dipole rectification is fixed and does not have to be pointed precisely at the transmitting antenna; thus, its mechanical tolerances do not need to be severe. Furthermore, the density distribution of incoming microwave radiation need not be matched to the radiation pattern of the receiving antenna; therefore, a distorted incoming wavelength caused by non-uniform atmospheric conditions across the antenna does not reduce efficiency. The amount of microwave power received in local regions of the receiving antenna can be matched to the power handling capability of the microwave rectifiers. Any heat resulting from inefficient rectification in the diode circuit can be convected by the receiving antenna to ambient air, producing atmospheric heating which will be less than that over urban areas, because only about 15% of the incoming microwave radiation would be lost as waste heat. The low thermal pollution achievable by this process of rectification cannot be equaled by any known thermodynamic conversion process. The rectifying elements in the receiving antenna can be exposed to local weather conditions. The receiving antenna can be designed to be 80% transparent so the land underneath the antenna could be put to other uses. In the summer of 1975, tests of a 24-square-meter array of microwave rectifier elements were conducted at the NASA Venus antenna site at Goldstone, California, to demonstrate the effective performance of dipole rectification.7 The transmitting antenna, which consisted of an 86-foot-diameter dish antenna, was located about one mile from the receiving elements. At a radiated frequency of 2388 MHz, incident peak RF intensities up to 170mW/cm2 have yielded up to 30.4 kW of DC out power. An average conversion efficiency of 82% was obtained at the receiving arrays under these conditions. SSPS TRANSPORTATION, ASSEMBLY, AND MAINTENANCE The SSPS will require a space transportation system capable of placing a large mass of payload into synchronous orbit at the lowest possible cost. The cost of transportation, assembly and maintenance will have the most significant impact on the economic feasibility of the SSPS. Several approaches to achieving this objective are being investigated. It is highly likely that a two-stage transportation system will evolve, which will carry payloads first to low-earth orbit and
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