Figure 7.32 Antenna Paddle [Kaya, et al, 1991] The power amplifiers are of the F-class type GaAs-FET (Field Effect Transistor) semiconductor, which is already more than 50% efficient. The FET is part of the antenna paddle, a sandwich structure of 3 layers which consists of the microstrip antenna, ground and FET amplifiers. The entire transmitting antenna consists of these four antenna paddles. Each antenna paddle is mounted with 18 microstrip antennas. The benefits of the microstrip antennas are their light weight, ease of fabrication and ease of implementation into mass production because they are flat. This concept integrates the antenna component and FET amplifiers. Three economical and technical advantages that can be realized but not limited to with the use of FETs are discussed. Firstly, the FET power amplifiers can be directly connected to the DC input of the solar cells which supply the bias power for the FETs as is demonstrated in the METS project. This direct connection makes it possible to design a much simpler satellite structure. Secondly, although GaAs FETs are typically low power devices compared to klystrons and magnetrons and FETs are temperature limited to a few tens of watts each, the FETs are relatively inexpensive and can be used in large quantities. Lastly, solid state transmitters are continuously being improved by the demands of other electronic markets such as communications, power converters and integrated circuit technologies. Therefore, there are more economical gains and incentives for FET manufacturers to improve their FET devices with higher frequency capabilities and higher power efficiency operations. Better power efficiencies of the individual FETs can lead to less hardware for both power transmission and thermal management given a fixed power output for the satellite. Solar power satellite technologies are heading toward the integration of sub-systems into one unit. Then, the units can be used as building blocks that are connected into many satellites of several different power output needs. Another interesting proposal may be to combine the two examples discussed in the previous two studies, an antenna integrated with an amplifier and an antenna integrated with solar cell. This can be accomplished by including the solar cell material on the fully developed antenna and FET amplifier that is already developed for the METS experiment. Magnicons For Microwave Space Power Beaming A new microwave source which could have potential as a highly efficient, high power, and high frequency source for space power beaming is a magnicon [Manheimer, Gold, Seo, 91]. In their paper study, Manheimer, et al, discuss a conceptual point design of a continuous wave magnicon at X-band, which has an efficiency of approximately 85%, a power of 200 kW and a gain of 18 dB. Other comparable microwave sources include: klystrons, magnetrons, and gyrotrons. Medium power klystrons and high power klystrons can be used in continuous wave mode. A medium power klystron, operating at frequencies between 1.3 and 18 GHz, can deliver up to 1 kW output power. High power klystrons (for example, "superklystrons") are capable of delivering 1 MW output power at 350 MHz with a 65% - 70% efficiency. Continuous wave magnetrons can deliver up to 6 kW of power at 2.45 GHz with efficiencies up to 70%. The Soviets have developed a magnicon as a source to power accelerators at a frequency of about 1 GHz. Although, they have proposed designs at higher frequencies up to 7 GHz. The magnicon is an example of a "swept beam" device that operates more efficiently than a klystron or gyroklystron. With a magnicon, the electrons are phased to gain rather than lose energy.
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