transmission therefore, will emphasize new microwave systems rather than identify new methods of transmission and reception. The two examples discussed in this section integrate sub-system level components into one unit, such as a transmitter and solar cell into one power beaming unit. The advantages of integrating these systems for a space system application include: reductions in mass, size and thermal loss, as well as increases in power conversion and transmission efficiencies. These advantages are gained by the elimination of distribution wires and, interconnects between electronic components, and the use of new and lighter materials for the solar cells, antennae, amplifiers, etc. Integrated Microwave Antenna and Solar Cell The development of new and more efficient solar power satellite designs is dependent upon new technologies of photovoltaic receivers (AlGaAs, InP, and CIS) along with improvements in RF and solid state electronics. One new concept has been proposed by Landis and Cull [Landis, 1991]. They propose to integrate the solar cell and microwave antenna into a monolithic building block as shown in Figure 7.31, that can be replicated and combined into a light collection area which is also used as the transmitting aperture. At this time, this design is only a concept which has not yet been implemented into hardware. Two technical advantages are gained. The total satellite mass can be reduced by eliminating the need for a separate light collection area and transmitter which also eliminates some electronics and wire for power conditioning and distribution. This reduction in wire and electronic hardware also reduces the waste thermal management subsystem to small self-contained radiating surfaces. Secondly, because of the larger aperture that could be realized with the monolithic building block units, this integration could yield a narrower microwave beam at the receivers, hence smaller receivers can be implemented. Figure 7.31 Solar Cell with Microwave Antenna, (conceptual design) [Landis, 1991] Tubes are well developed and have been used as a DC to RF converter choice for most high power applications. However, receiving micro wave power and converting it to DC power has been demonstrated with solid state electronic devices and microwave rectifying antennas (rectennas) using thin-film techniques on a thin plastic substrate. It is now reasonable to use solid-state electronics instead of tubes as the microwave source for solar power satellites. The use of an FET transmitter instead of a tube will be demonstrated in the next section. In conclusion, the integration of antenna with solar cell concept consists of three technologically feasible elements which have been previously analyzed, but require further development. Microwave antennae are directly integrated at the solar cell level. Secondly, both the photovoltaic and microwave devices are constructed with thin-film technology. And thirdly, the antenna can be steered by controlling each element's phase with the addition of computational electronics. New FET Microwave Transmitter In the near future, solid state transmitters could provide solar power satellites with long life, high reliability, low maintenance and high DC to RF conversion efficiencies at high frequencies. Solid state transmitters are continually being improved for efficiency and higher frequency operations by using new materials and manufacturing process techniques. A research group (Chief: Prof. H. Matsumoto) was organized to promote a METS (Microwave Energy Transmission in Space) experiment. [Kaya, Matsumoto, Akiba, 1991] In the METS experiment, the antenna was integrated with the amplifier in a three layer structure as shown in Figure 7.32. The new transmitter system will be used to radiate a monochromatic microwave of 2.45 GHz.
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