5. MICROWAVE SUBSYSTEMS The microwave power transmission system (MPTS) converts electrical power to microwave radiation and transmits this radiation as a coherent beam which must be kept focused on the receiving antenna (rectenna). Electrical power is converted to microwaves by RF amplifiers. Solid state amplifiers were rejected because their low efficiencies would greatly increase the size of the SPS power conversion and distribution systems and because considerable non-lunar material would be required to cool the amplifiers. The gyrocon, klystron, and magnetron amplifier concepts were selected for study. They are discussed in sections 5.1, 5.2, and 5.3. The gyrocon was eventually dropped from consideration due to its low state of development. The masses and projected efficiencies of the magnetron and klystron are listed in Table 5-1. The magnetron design uses more non-lunar material but is more efficient, reducing the size of the power conversion system. Thus the magnetron is preferred if the power conversion system has high non-lunar mass, and the klystron is preferred if the power conversion system contains little non-lunar mass. The critical non-lunar specific mass for the power conversion system is 0.074 kg/kW; below this value, the klystron gives lower total non-lunar mass than the magnetron. This critical value was estimated using the projected efficiencies shown below. TABLE 5-1 COMPARISON OF KLYSTRON AND MAGNETRON DESIGNS Configuration of microwave antennae was outside the scope of this study, so the configuration used in the Boeing report(6) was adopted. In that design, microwave power is radiated from slotted waveguides arranged in a planar array. Two waveguide designs were considered. Both are compatible with the Boeing design, and both can be constructed entirely of lunar materials. One design, selected by General Dynamics, uses a conductive coating of aluminum in a foamed glass waveguide. The other design uses a pure aluminum waveguide. Because the waveguides do not bear a structural load, the foamed glass may be acceptable for this application. The aluminum waveguide will require stable temperatures in order to stay within the length tolerance. Providing thermal stability under varying solar illumination would greatly complicate the antenna design, so the General Dynamics waveguide is used.
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