Power conversion equipment converts the direct current (DC) power from the collectors into high density microwave (RF) power; the microwave antenna transmits it to the ground-based rectifying antenna, called the rectenna. The microwave antenna is a phased-array transmitter of 1-km diameter and contains 7,220 subarrays or power modules. The antenna transmits 6.85 GW of power at 2.45 GHz. The subarrays are arranged to provide a 10-dB Gaussian power distribution across the array surface. The power density varies from 22.14 kW/m^ at the center module to 2.45 kW/m^ at the outer edge. At the earth’s surface, the power is 23 mW/cm^ at the rectenna center and 0.08 mW/cm^ in the first s idelobe. Each subarray covers 108 square meters and contains from 4 to 50 klystrons, depending on the power output of the subarray. Each klystron converts DC to 70 kW of RF. An alternative concept described in the SPS CDEP Reference System Report would use a 50-kW klystron tube that would result in the use of 6 to 50 tubes per subarray (Ref. 1, p. 30). The subarray radiates power through slotted waveguides on the surface. Electronic phasing equipment in each subarray processes a beam-phasing signal from the ground and focuses the microwave beam. Waste heat is dissipated by radiation. The subarray also includes power distribution and conditioning equipment, waveguides, amplifiers, and frequency-control electronics. The rectenna consists of a dipole network and diode rectifiers, which receive and rectify the microwave power; a power distribution and conditioning system, which collects and delivers the rectified DC power to the utility interface; and the structure that provides support to the dipole rectenna panels and components of the distribution system. The support structure also provides a ground plane for the microwave power. At 35° latitude, the rectenna area of 10 km by 13 km contains 814 rows of rectenna panels tilted 40° from the horizontal, providing an active intercept area of 78.5 km^. A total of 436,805 panels will be assembled on site and erected. In order to minimize electrical wiring from the rectenna panel area, two electrical switchyards will be employed, each with its own converter and relay building. The rectenna site, including auxiliary buildings and the buffer zone, has been estimated to be an elliptical plot 12 km by 15.8 km, with a total area of 149 km^. The configuration of the ground-based rectenna, which receives and rectifies the downlink power beam, has half-wave dipoles feeding Schottky barrier diodes. Two-stage, low-pass filters between the dipoles and diodes suppress harmonic generation and provide impedance matching. The rectenna is a series of serrated panels perpendicular to the incident beam, rather than a continuous structure. Each panel has a steel-mesh ground plane with 75-80Z optical transparency. This mesh is mounted on a steel framing structure, supported by steel columns in concrete footings. Aluminum conductors are used for the electrical power collection system.
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