Assuming that 1 GW is to be delivered to the utility grid, that the transmitter frequency is 2.45 GHz, and that the peak power at the receiver is 300 w/m*2, then the nomograph in the SPS System Definition Study (6, vol. 2, p. 175) determines: Rectenna Diameter = 4 km Space Antenna » 2.5 km The lens is a Fresnel lens made of layers of wire mesh. It is flat on the side toward the SPS and ridged on the side toward Earth. The index of refraction at the first and second surfaces of the lens is selected to minimize reflections. The index of refraction is varied smoothly from front to back and from center to edge within the lens by using different wire diameters. The smallest wire diameter used is 0.125 mm. The near focus of the lens is 2.1 km from the lens center. Figure 5.4-1 shows the general arrangement of the lens. The transmission pattern at the lens was assumed to be a lOdB gaussian taper. The transmitting pattern required at the focus to achieve this has not been determined. The simplifying assumption that the waves will travel in straight lines in the interior of the lens was made. The lens is 99.95% composed of lunar materials, as shown in Table 5.4-1. Figure 5.4-2 shows how several lens parameters vary with distance from the center of the lens. Using data from Jasik (7, p 14-34), the theoretical transmission efficiency of the lens was found to be 97,12%. Some attendees at the 1985 Princeton Conference on Space Manufacturing questioned this result, claiming that recent experiments show much lower transmission efficiencies for microwave lenses. However, Richard Dickinson of JPL reports (17) that the experiments cited all deal with flat (e.g. Fresnel zone plate) lenses, many operating at multiple frequencies. These lenses use diffraction to redirect the microwave beam; the lens proposed here uses refraction. Dickinson believes that a single—frequency lens with 3- dimensional structure would indeed have better transmission than flat multifrequency lenses, though he is unconvinced that efficiencies as high as 97% are attainable. TABLE 5.4-1 MASS STATEMENT FOR 1 GW SPS MICROWAVE LENS
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