C. Microwave Power Transmission System The MPTS consists of microwave generators, subarray elements, a phase control system, power distribution, a pointing control system, rotary joints, and structure. 1. Antenna Array The antenna is a planar phased array with a diameter of 1 km, comprising about 7850 subarrays. Each subarray is individually phase-controlled by reference to a "pilot" beam transmitted from the rectenna so that a narrow, coherent beam is transmitted to the ground. Power density varies in a 10-step approximation of a 10-dB Gaussian taper to maximize the power in the main lobe of the beam. 2. Microwave Generators Microwave generators convert de electrical power to microwave power for transmission to ground. Several types were considered, but only two, the amplitron and the klystron, were considered likely candidates and examined in detail. The amplitron's chief projected advantages are significantly lower specific weight (roughly 0.4 kg/kW versus 0.7 kg/kW for the klystron) and somewhat higher efficiency (88 percent vs. 86 percent). It uses a cold cathode, which enhances reliability, and can be passively cooled by a pyrolytic graphite radiator. However, it is relatively noisy and requires an absorptive filter. Tuning is accomplished by a motor-driven pole piece that is potentially unreliable. The klystron is capable of ten times the power output per tube of the amplitron. Therefore, fewer tubes are required, greatly simplifying antenna assembly. The anode voltage is twice as high, permitting a reduction in distribution system weight (unless the amplitrons are connected in series). The gain is much higher; the resulting low-rf input drive power makes phase control easier. Arcing and breakdown are prevented automatically by internal discharge. Although the kylstron was used for weight estimates in this study, it is too early to make a selection, and much work is needed on both tubes before a final choice is possible. 3. Subarrays The transmitting antenna includes a large number of subarrays, each of which is controlled as a unit. To minimize losses, the subarray should be approximately square. Power density is varied over the antenna by adjusting the number of microwave generators in each subarray.
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