Overall efficiency, n, is shown in Figure 12-4 for the amplitron configuration. These approximations emphasize the frequency region 1 GHz to 5 GHz. Although preliminary studies examined performance to higher frequencies, the rain attenuation data showed that the region of interest should not extend much above 3 GHz. Also, in this range the atmospheric attenuation presented above is the dominant propagation factor; lower frequencies would have to consider the effect of Faraday rotation (l/f^ dependence) on the efficiency of the rectenna which is designed to a linear polarization, or the rectenna would have to be designed for dual polarization at added expense. Cost factors for all system elements are dependent on weight, area or power level, and extrapolations were made from nominal designs at either 5 GW or 10 GW ground output power. A summary of these is given in Figure 12-5, where a range of values is given for the power source and orbital operations costs and for microwave orbital and ground systems. Frequency is taken as 2.45 GHz. It is assumed that peripheral land out to a level of 0. 1 mW/cm power density is purchased for safety reasons. The power source characteristics enter the search for a desirable MPTS design because its weight and cost reflect the impact of MPTS efficiency, or lack of it, as noted above. The key parameters are specific cost in dollars per kilowatt and specific weight in kilograms per kilowatt (or grams per watt), where the reference power is that delivered in orbit to MPTS. The use of ground delivered power often is used as a normalization factor but that approach mixes the power source and MPTS parameters, and leads to confusion in the optimization process. Also, the recommended approach permits a direct comparison of SPS power source characteristics with those for ground based systems. The candidate technologies for the power source - solar photovoltaic, solar thermal, and nuclear - have been studied in decreasing detail for space application in the order given The suggested group of parameters evolved over the course of the study as a composite set representing the widest range
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