Figure 72 shows that the transmitted beam hits the Earth's surface at an angle of about 45 deg. This results in an elliptical spot which has a major to minor axis ratio of about 1.4 to 1. To simplify the analysis, the spot was considered to be circular, as if it were projected on a plane at the Earth normal to the beam centerline, as shown in Figure 73. Receiving Antenna. — The receiving antenna consists of a large array of dipole-reflector elements placed within the 90% power radius (3.85 km). The gain of each element is about 10 and has an effective area of about 50 cm.2 Spacing these elements about 0.6X apart will overlap the effective area enough so that very little power will go through. This gives a density of about 300 elements per square meter or a total of 1.38 x 1010 elements. Since the RF energy is converted directly to de, there is no problem of phasing the elements. Each element at the center of the array will contribute about 1.9 watts and tapers off to approximately 0.10 watt at the 90% power radius. A summary of the properties of the antenna system for different distributions on the transmitting antenna [(1 — r2)n, n = 0, 1/2, 1 and 2] is presented in Table 17. (Note that the table data are for X= 10 cm or f =»3,000 MHz, not 3,300 MHz. Effects of frequency selection and antenna diameter are shown in Table 17.) Figure 76 shows the noise temperature that an antenna sees as a function of frequency. We observe that at 3,300 MHz this temperature is about 2 to 3°K for the antenna in the zenith position (antenna with a narrow beam). This indicates this area potentially to be about the worst part of the spectram as far as interference is concerned. If all other noise is eliminated, we are working against a noise background of only -158 dBw. Noise power = KTB = 1.71 x 10"16 watts = -158 dBw where K = Boltzmann's Constant = T = temperature in B = bandwidth in Hertz = In radio astronomy, the limit normally identified is approximately -200 dBw.
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