- Klystrons have a relatively high specific weight, - Klystrons have larger dimensions than amplitrons, - Klystrons operate at a higher temperature (400°C), - The efficiency is slightly lower than with amplitrons (86 %), and - Klystrons have to be actively cooled (heat pipes). I .2.3.2.3 Other RF-Generators Since the amplitron and the klystron are the devices which have been developed much more than all other candidate RF- generators, these will be discussed here in one sub-category. The most promising type of alternative microwave generator is a solid-state-device. Weight might be greatly reduced (0.1 kg7kW~compared-to~O.7 kg/kW for klystrons) and reliability could be significantly improved (lifetime is expected to be 30 years compared to 10 years with klystrons or amplitrons). Estimated figures for a field effect transistor generator are: voltage 10 V output 1-100 W overall efficiency 80-85 % gain 10-20 dB spec, weight 0,1 kg/kW passive cooling Similar devices are proposed for the GSSPS. Here 1,744,500 transistors are directly implanted in the surface of the 2100 m long, 4 m diameter waveguide mounted on the back of each solar panel, delivering 159 MW of RF-power. Three other types of potential RF-generators were only mentioned in literature: - the travelling wave tube, - the magnetron, and - the gyrocon. 1.2.3.3 Transmitting Antenna and Beam Control In this sub-category two substantial different approaches were found in the literature. Most SPS-concepts use antenna types consisting of slotted waveguide elements with directly mounted RF-generators. These concepts differ only in the design of sub-modules, power density distribution, etc. Typical figures for this kind of antenna are:
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