technology [15] may allow for a collector surface that is the same size as the transmitting antenna. Alternatively, a smaller transmitting antenna can be used with a microwave lens [16] to increase its effective size. Conclusions The design of an SPS system involves choosing an appropriate frequency and aperture illumination function for power beaming. In order to make these choices, standards for peak beam intensity and human microwave exposure must be established. Strict human exposure limits, such as 0.01 to 0.1 mW/cm2 may be desirable, since people living near a rectenna site (which may have to be near consumers if transmission line losses are to be kept to a minimum) will be exposed to microwaves continuously. If peak beam intensities of a few hundred milliwatts per square centimeter are considered safe for the atmosphere, then a fourfold increase in frequency over the NASA/US DoE reference design (from 2.45 to 9.8 GHz) will allow for a reduction in rectenna and exclusion zone area while avoiding atmospheric and rain attenuation. A moderate amount of beam tapering (the n = 2, n = 3, or "ideal" cases) will allow for a further reduction in exclusion zone area, as well as the rectenna size needed to capture the amount of power in the main lobe of the untapered beam. In order to avoid excessive heating of the transmitting antenna, while keeping the rectenna size to a minimum, the n = 2 case is recommended for SPS systems in which high peak beam intensities can be accepted. If microwaves as intense as 10 mW/cm2 can be tolerated by humans on a continuous basis, then beam tapering is less strongly recommended. If peak beam intensities of only a few tens of milliwatts per square centimeter are considered safe for the atmosphere, and increased peak intensities at the transmitting antenna arc unacceptable, then it will be necessary to employ a larger number of smaller SPS’s. As seen in Table 3, increasing the frequency without tapering the beam allows for no change in total rectenna or exclusion zone area compared to the low frequency case, although the individual SPS units can be scaled down, making the system easier to finance. To decrease these areas, the n = 4 beam taper is recommended, especially for very strict (0.01 mW/cm2) microwave exposure safety thresholds. If capturing the entire main lobe is desired, or if a less strict safety threshold (e.g., 0.1 mW/cm2) is established, then the n = 2 or 3 cases are more feasible. Cases for n > 5 have larger exclusion zones, and cases for n > 6 have higher peak intensities at the transmitting antenna; therefore, these tapers are not recommended. REFERENCES [1] SETH D. Potter, MuraliN. Kadiramangalam, (1991) "Frequency Selection Issues for Microwave Power Transmission from Solar Power Satellites," Space Power, Volume
RkJQdWJsaXNoZXIy MTU5NjU0Mg==