Figure 7.23 Slotted Waveguide Radiator [Schroeder, 1980] More recent developments in the area of microwave monolithic integrated circuits indicates that they might be used for high power applications. They offer a number of advantages over solid state hybrid circuits including small size, light weight, high reproducibility and low cost. At present MMIC GaAs FET (field effect transistor) amplifiers exhibit an efficiency of only 10- 15%. Figure 7.24 shows a solid state antenna design where the antenna elements are integrated with the solar cells. Figure 7.24 Integrated Solar Cell and Solid State Amplifiers. [Rockwell, 1980] Phasing of the individual elements can be achieved in a number of different ways. For large antennas small perturbations of the antenna surface will affect the beam pointing. Therefore, continuous phase monitoring of the antenna elements is required. For long distance power transmission various retrodirective control schemes have been proposed. A retrodirective system uses a pilot beam transmitted from the rectenna. Each antenna element receives the pilot signal which is used to generate the appropriate phasing. The power beaming system for the METS experiment is an example of a two tone pilot signal system. [Kaya, 1991a] The pilot signals are slightly shifted in frequency with respect to the power carrier signal. The correct phase for each radiating element is fed to digital phase shifters producing the correct element phase. A block diagram of the METS phase control system is shown in Figure 7.25.
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