LDR also promises a more compact storage and easier deployed in space when compared to the existing technology. LDRs are highly resistant to damage from micro meteorites and space debris since that only the droplet generator and the collector can be damaged. On the other hand special consideration has to be taken to avoid potential contamination of the spacecraft due to the backscattering of the working fluid evaporation. Droplets which impact the spacecraft surface at high velocity can release additional contaminants. The LDR technology is presently under development. Up to now no unresolvable technical issues have been identified. Lately, the droplet generator at startup and shutdown has been investigated. Additional work is needed on the collector. Demonstration of a prototype could be planned on the Space Station. This promising technology could be considered in many midterm applications. It could bring new hope to the solar dynamic systems. 7.2 Power Transmission Wireless power transmission can be presently performed through two possible technologies, microwave and laser. They are presented in this section discussing the generation of the transmission beam, beam propagation, reception and conversion to electricity. 7.2.1 Microwave Transmission In this section the general principles of microwave power beaming and reception are discussed. Different types of antennas suitable for microwave power transmission are examined. Special attention is paid to phased array antenna technology. The atmospheric effects on the transmitted beam are adressed. An overview on the microwave sources technology for power transmission is presented. The last part of this section concerns rectenna technology and siting. Microwave Antennas Figure 7.20 Mainlobe Radiation Pattern The basic radiation characteristics of any antenna is determined by its dimensions and the transmitting wavelength. If the transmitter is emitting an equal amplitude and phase wave front, the far-field radiation intensity will be a simple diffraction pattern. Approximately 90% of the transmitted energy will be within the mainlobe of the antenna, as shown in Figure 7.20 while the rest is spread out into the grating sidelobes. The angle defined by the mainlobe is roughly X/Dt (radians), where Dt is the diameter or width of the antenna. In the case of a perfectly circular aperture the beam width is given by 2.44X/Dt. For an antenna located at a distance H, the transmitting antenna will produce a footprint with a diameter Dr.
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