solid state microwave transmitters and solar cells integrated onto the same substrate, larger transmitting antennas are possible. If each transmitter element is powered only by the solar cells in its immediate vicinity, then losses along power buses from the solar cells to the antenna are minimized. Furthermore, the design is simple, consisting of the same element repeated many times, as well as durable (since few single parts are critical to the overall operation of the SPS). For the study, the entire surface of the SPS is assumed to contain both solar cells and integrated transmitters. Thus, as the SPS diameter is increased, more power is squeezed into a tighter microwave beam. The SPS was sized so that the peak beam intensity at the Earth's surface is 30 mW/cm2. This figure is the same as that used in the previous SSI-sponsored study on SPS's built of lunar materials2, and is similar to that from the NASA/US DOE study (i.e., 23 mW/cm2). It has been suggested that higher values, such as 40 to 50 mW/cm2 may be acceptable3. Thus, a value of 30 mW/cm2 is in keeping with previously accepted intensities, while still allowing for some leeway in the design. Frequencies higher than 2.45 GHz lead to smaller, more feasible SPS's. However, frequencies higher than about 10 or 15 GHz are subject to attenuation in rain as well as clear air (although there are "windows" in clear air at 35 and 94 GHz)4>5’6. In addition, the efficiency of solid state microwave transmitters decreases with frequency. Therefore, 10 GHz was chosen as the baseline frequency for the study, although other frequencies were considered for comparison purposes. The nominal latitude for the study is 35°, in keeping with the NASA/US DOE reference design. The intensity of the power beam is dependent on the latitude, although it is not highly sensitive to it (at least, for latitudes below about 60"). Thirty-five degrees is not far removed from most of the world's population centers, so the resulting design will be practical for many regions. If the same design is mass produced to power many areas, the peak intensity at higher latitudes will be less than 30 mW/cm2, while the peak at the equator will be just under 40 mW/cm2. Alternatively, the design can be customized for each latitude. In the calculations for this study, the SPS-to-rectenna distance and the beam angle from vertical at the Earth's surface were corrected for latitude. Distances were thus somewhat greater than the geostationary altitude of 35,786 km. Beam angles from the vertical were slightly greater than the latitude. The energy distribution of a microwave beam consists of a central main lobe, where most of the energy is concentrated, as well as sidelobes. Sidelobes can be minimized by the proper choice of beam taper (i.e., a variation in the intensity of the beam across the face of the transmitting antenna), although the main lobe then becomes broader. Beam tapering
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