of rain and good operating efficiencies for the microwave components. However, there is one reason for considering a higher frequency such as 3.0 GHz. Since the effective gain of the transmit antenna is proportional to X? (the wavelength squared), a 44% increase in gain is achieved for the same 1 km array by going to 3.0 GHz rather than 2.45 GHz. Such a change in frequency would require approval by the ICCR (International Radio Consultative Committee) which meets again in 1979. IV.A.2(e) ANTENNA APERTURE ILLUMINATION In order to achieve a high transmission efficiency, the transmit antenna must have an aperture distribution across the array surface which maximizes the amount of RF power intercepted by the ground rectenna. The previous work by Raytheon and JPL (ref. 1,5) has shown that a truncated Gaussian taper is a good approximation for an optimum aperture distribution. This illumination function has the form The mainbeam pattern and sidelobe characteristics of the antenna will vary with amount of edge taper as shown in Figure IV.A.2-7. A uniform illumination, that is OdB taper, has a narrow mainbeam and the maximum density at boresight (center of rectenna). Increasing the amount of taper produces a lower boresight density, a wider mainlobe, and lower sidelobes. These power density curves are for a 1 km transmit array with no phase or amplitude errors and no failures. The rectenna collection efficiency, that is, the amount of flux density intercepted by the rectenna, is shown in Figure IV.A.2"8 for the same taper configurations. As would be expected, the collection efficiency for a given rectenna radius increases with the amount of taper. The model configuration has a 10 dB taper which means that the power density at the center of the array is 10 times that at the edge. For the no error/no failure conditions the 10 dB taper system gives a 90% collection efficiency at a radius of 4300 meters. IV.A.2(f) STEP-TAPER ANALYSES The 10 dB Gaussian taper will not be a continuous function across the array surface; rather it will be a physically-realizable quantized approximation. Three configurations, 5 step, 8 step, and 10 step approximations were investigated. Minimizing the number of steps
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