power transmitter in space was addressed by several companies. But it was the contribution of R. M. Dickinson of JPL that pointed the design in the direction that it has since taken. The concept, as shown in Fig. 5, was an electronically steerable array composed of modules comprised of two magnetrons acting in combination with a passive combiner to excite a section of slotted waveguide array. Dickinson's concept motivated an intensive evaluation of the magnetron directional amplifier as a generator for the SPS. The evaluations used the common microwave oven magnetron for experimental data. It was determined that this tube generated very little extraneous noise, was highly efficient, and had an internal feedback mechanism to regulate its cathode temperature to achieve the longest possible life [11]. A subsequent study from NASA designed a specific magnetron for the SPS application with a projected 50 year life, 85% efficiency, and an external control loop to eliminate the power conditioning between the photovoltaic array and the microwave generator. The magnetron in combination with the slotted waveguide array became a radiation module that was combined with other modules to form a subarray of the large, one- kilometre-diameter SPS transmitting array as shown in Fig. 6. The SPS magnetron application was recently updated with new technology [15, 16]. This updating represents to a large extent the status of the conceptual design of the transmitter in the Solar Power Satellite.
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