Power transmitter and microwave beam. The microwave transmitter antenna is a planar phased array system approximately 1 km in diameter composed of 7220 subarrays, each 10.4 m by 10.4 m across. Aluminum slotted waveguides form the radiating surface of each subarray, while the klystron tubes are mounted on the back. Near the center of the array, each subarray has 36 klystrons (70 kw each), while subarrays near the edge have only 4, with varying numbers of tubes per subarray between, providing a 10-step tapered profile in the emitted power density, decreas- 2 ing from 22.1 kW per square meter at the center to 2.46 kW/m at the edge. (An alternative configuration, using 50 kW klystrons, has 50 tubes per subarray at the center, decreasing to 6 per subarray at the edge.) Each subarray has a RF receiver and phasing electronics to process the pilot beam signal returned from the rectenna on the ground. The phasing signals provide the subarrays with the necessary information to form a single coherent beam focused at the center of the rectenna. The operating frequency of the microwave transmitter array is 2.45 GHz for the Reference Design. Rotary joint. Since the photovoltaic array must face the Sun at all times while the transmitter array must face the Earth at all times, relative motion amounting to one revolution per day must be provided between the two major segments of the power satellite. In order to minimize gravity gradient torques on the power satellite, its long axis is oriented normal to the orbital plane and parallel to the Earth’s axis of rotation. The transmitter array is then located at either the north or south end of the photovoltaic array and connected to it by the rotary joint. The rotary joint consists of a turntable made up of two concentric rings about 350 meters in diameter separated by roller bearings. Electrical power is carried through the rotary joint by brushes sliding along three slip rings (one for each of the main power buses) . The slip rings range from about 8 meters to 15 meters in diameter, and have a cross section of about 30 cm (radial thickness) by about 50 cm (height). Both the outer and inner faces of each ring are coated about 1 cm thick with coin silver (90% Ag. 10% Cu). Caliper-like structures support pairs of brush assemblies (each about 50 cm wide) distributed around each slip ring to transfer electrical power to or from both sides of each slip ring. Steel springs provide the necessary pressure on the brushes to minimize arcing problems without excessive
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