F igure 7.25 Two Tone Pilot-Signal Retrodirective Antenna [Kaya, 1991b] Atmospheric Effects Power transmission by means of microwave technology is presently used for many ground applications and it is considered suitable for space power beaming. The electromagnetic radiation can be used for information transmission as well as for power transportation to or from space. Three different applications can be considered in this area: microwave transmission from earth-to-space, from space-to-space, from space-to-earth. Each one of these problems requires a different approach and choice of the optimal criterion for the parameters to be set. One of the most important parameters to be selected is the frequency of the electromagnetic radiation used for the power transmission. The choice of the frequency depends on many factors, such as the beam interaction with the atmosphere, the size and the mass of the instruments, the environmental and safety issue, the efficiency achievable and the technology availability and reliability. [Rybakov et al.,1991] The frequency employed since the early utilization of microwave transmission is 2.45 GHz. The reason was mainly the poor influence of the earth's atmosphere on the processes of electromagnetic propagation in S-band shown in Figure 7.26. The ionosphere of the earth is usually opaque to frequencies below 10 MHz and has strong effect on signals with frequencies up to 1 GHz. The frequency 2.45 GHz is still considered one of the most attractive also because of the good technical support available for both power generation and focusing of the radiation. At 2.45 GHz the diameter of an orbiting transmitting antenna is 1 km and the one of the ground receiving antenna is 10 km. Using higher frequencies the antenna and rectenna size can be considerably reduced (the antenna size is 10 times smaller at 35 GHz) and therefore also the costs decreases. But the efficiency of the power transmission, the lack of technology available and the effects of the beam interaction with the atmosphere at higher frequencies make the problem of the optimum criteria for the frequency selection particularly critical. The interaction of micro wave radiation with the earth's atmosphere is a very complex problem to be analyzed. When a microwave beam propagates through the earth's ionosphere many phenomena occur. The most important are summarized in the following. • The electromagnetic radiation is influenced by absorption of the oxygen molecules and water vapor. At higher frequencies, molecular absorption in the earth's atmosphere produces remarkable losses as the waves propagate through the atmosphere. Figure 7.27 shows the atmospheric attenuation over the spectral region up to 300. • Rain, mist and cloudiness produce an attenuation of the radiation. • The presence of the earth's magnetic field leads to Faraday rotation of the electric vector affecting any polarization measurement.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==