impact on other users of the EM spectrum that may be operating at nearby frequencies. Both copper and silver exhibit attractive features for use in the reflector. To implement a frequency selective surface, a metallized pattern is deposited on the reflector substrate material. This pattern, in conjunction with a substrate material that appears transparent to microwaves at the operating frequency constitutes a spatial filter, so that only the desired beam frequency' will be reflected. Doping the substrate material with a high resistivity substance to provide a de path will counter any potential spacecraft charging that may occur. Resistive materials can be selected that will have no influence on the performance of the reflector while providing a sufficient path for bleeding off any potential charges. Beaming Efficiency Beaming efficiency depends on the sizes of the transmitting antenna, the orbiting reflector, and the receiving antenna. The larger the antenna, the smaller the beam width it produces. For efficient beaming, each antenna and reflector must be sufficiently large to focus the beam into its associated reflector or rectenna at the beaming distance. If one antenna is more costly than the other (e.g., the rectenna in orbit compared to the transmitting antenna on the ground), the diameter of one antenna can be increased and the other decreased as long as their product is not changed. For equal transmitting and receiving antennae apertures, energy will be beamed from one antenna to the other with only about 60% efficiency, partly because the receiving antenna will only catch the energy out to the half-power beam width of the transmitted beam, and partly because some of the transmitted energy falls outside the main beam as sidelobes. 100% microwave beaming efficiency can be approached by making the antennas sufficiently large. However, 95% efficiency requires increasing each antenna diameter by 34% and their areas by 80%. Therefore, size-versus-efficiency is clearly an important tradeoff. In PRS applications where a reflector is used to relay the power from the transmitting antenna to the receiving antenna, the efficiency between the transmitting antenna and the reflector (the "up-path"), and again between the reflector and rectenna (the "down-path") must be evaluated. The North-South dimensions of the transmitting antenna and the rectenna must be increased because the line-of-sight for each of these antennas to the orbiting reflector is tilted from the vertical. Although this size increase requires more area on the ground (by the inverse of the sine of the elevation angle), it does not necessarily increase the active area of the antennas, which could be built so that individual sections of the rectenna are facing the beam.
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