Figure 12 MPTS 48 kW Klystron Parameters Figure 13 MPTS 48 kW Klystron Power Budget 3. TRANSMITTING ANTENNA AND PHASE FRONT CONTROL Goubau and Schwering [1961] showed theoretically that microwave power can be transferred at high efficiency when the transmitting antenna is illuminated with an amplitude distribution that is near Gaussian, as illustrated for the MPTS in Figure 14, and when the phase front of the beam is focused on the receiving antenna. For the extreme transmission distance from geosynchronous orbit, the curvature of the phase front is very slight, but nevertheless the front must be controlled with high precision to maintain high efficiency. Figure 15 shows the effect of the transmitting antenna amplitude taper (from antenna center to its edge) on receiving antenna dimension for several high beam interception efficiencies. We can expect that attractive combinations will be found in the 5 dB to 10 dB range to limit the size of the receiving antenna while achieving high efficiencies. It is interesting to note that other microwave applications generally use a uniform illumination, or 0 dB taper, which achieves maximum intensity in the center of the beam but also places a higher proportion of power in sidelobes. This trend is illustrated in Figure 16. The recommended approach to control of the phase front to the required precision requires that the antenna be sectored into numerous subarrays. A typical quadrant for an antenna on the order of 1 km is shown in Figure 17, which also gives an example of how the array could be organized to provide the necessary center to edge amplitude taper. Figure 18 illustrates the factors entering into the choice of subarray size. Large subarrays individually have narrow, high gain radiation patterns that will result in large power loss if the overall array is mechanically offset to a substantial degree from pointing to the ground, due for example to attitude control limit cycling. This power loss cannot be offset electronically, so smaller more numerous subarrays are selected, as shown. (Total radiated power remains the same.) Phase control electronics must be present in each subarray (definition of a subarray) so that
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