SECTION IV.C.(l) IV.C. MICROWAVE POWER TRANSMISSION SYSTEM IV-C-1 ANTENNA ARRAY a. RATIONALE A SPS (Solar Power Satellite) in geosynchronous orbit collects and converts solar energy to DC electricity by either the photovoltaic process or by solar thermal converters. This electrical energy is transmitted back to the earth using a high power microwave transmission system. The microwave power transmission system consists of microwave generators, waveguides, a large planar phased array, a closed-loop phasing system, and a tracking system. b. DEFINITION The phased array antenna has a diameter of one kilometer, with a 10-dB Gaussian taper and a maximum power density at the center of 21 kW/m2. The array consists of 7854 subarrays, each subarray being about 10 meters X 10 meters in area, and each subarray connected to its adjacent subarray by a flexible ground plane. Behind the waveguides of each subarray are mounted microwave generators which convert the high-voltage DC into microwave energy at S-band. The energy is then radiated into the slotted waveguides which transmit the RF electromagnetic energy in the form of a main lobe to an earth rectenna. The microwave generator may be a klystron or a crossed field amplifier (such as an amplitron) with the klystron generating 50 kW of RF power and the crossed field amplifier generating 5 kW of RF power. The waveguides are made of aluminum and, with the aid of phase control, will minimize the size of the sidelobes so that the efficiency of the antenna array will be as high as 90%. c. OPERATING CHARACTERISTICS The antenna array will perform its transmission function based upon a number of simultaneous actions. Each subarray will radiate through its slot apertures to create its own antenna pattern. The total number of subarrays will collectively transmit their beams to form a main lobe whose energy will be almost totally captured by the ground receiving antenna. The subarrays contain phasing circuits which provide phase front control for the propagated electromagnetic waves to accurately point the beam and focus the high power microwave beam in the presence of a non-homogeneous, time varying atmosphere and'ionosphere, thermal deformation of the array waveguide and structure, and phase variation of transmission lines, converters, and phase shifters. There are two basic approaches to phase control: command and adaptive. The command makes field measurements at the rectenna and transmits this information by a telecommunications link to the transmitting array. The adaptive utilizes a reference beam sent from the receiving antenna location to the space transmitting antenna to enable phase measurements and corrections to be made at each subarray. Numerous antenna types have been analyzed to accomplish the goal of high efficiency, low losses, reliable antenna pattern and low sidelobes. Conclusions are summarized below: (See Figure TV-C-1-1) L. Leopold Tracking and Communications Development Division
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