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In these relationships, the beamwidths are given in degrees. The paraboloidal antenna described so far is center-fed, in that the primary horn is pointed toward the center of the reflector. With this arrangement the primary horn and its supports present a partial blockage to the reflected wave. The energy scattered by the blockage is lost from the main lobe, and it can create additional sidelobes. One solution is to use an offset feed as described in Sec. 6.14. The wave from the primary radiator induces surface currents in the reflector. The curvature of the reflector causes the currents to follow curved paths so that both horizontal and vertical components are present, even where the incident wave is linearly polarized in one or other of these directions. The situation is sketched for the case of vertical polarization in Fig. 6.19. The resulting radiation consists of copolarized and cross-polarized fields. The symmetry of the arrangement means that the cross-polarized component is zero in the principal planes (the E and H planes). Cross-polarization peaks in the 45 planes, assuming a coordinate system as shown in Fig. 6.5a. Sketches of the copolar and cross-polar radiation patterns for the 45 planes are shown in Fig. 6.20.
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Figure 6.20 Copolar and cross-polar radiation patterns. (Courtesy of FCC Report FCC/OST R83-2, 1983.)
6.14 The Offset Feed Figure 6.21a shows a paraboloidal reflector with a horn feed at the focus. In this instance the radiation pattern of the horn is offset so that it illuminates only the upper portion of the reflector. The feed horn and its support can be placed well clear of the main beam so that no blockage occurs. With the center-fed arrangement described in the previous section, the blockage results typically in a 10 percent reduction in efficiency (Brain and Rudge, 1984) and increased radiation in the sidelobes. The offset arrangement avoids this. Figure 6.21b shows a development model of an offset antenna intended for use in the European Olympus satellite. The main disadvantages of the offset feed are that a stronger mechanical support is required to maintain the reflector shape, and because of the asymmetry, the cross-polarization with a linear polarized feed is worse compared with the center-fed antenna. Polarization compensation can be introduced into the primary feed to correct for the cross-polarization, or a polarization-purifying grid can be incorporated into the antenna structure (Brain and Rudge, 1984). The advantages of the offset feed are sufficiently attractive for it to be standard on many satellites (see, e.g., Figs. 7.6 and 7.22). It is also used with double-reflector earth station antennas, as shown in Fig. 6.24 later, and is being used increasingly with small receive-only earth station antennas.
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(b) Figure 6.21
(a) Ray paths for an offset reflector. (b) The offset feed for a paraboloidal reflector. (Courtesy of Brain and Rudge, 1984.)
Antennas
6.15 Double-Re ector Antennas With reflector-type antennas, the feeder connecting the feed horn to the transmit/receive equipment must be kept as short as possible to minimize losses. This is particularly important with large earth stations where the transmit power is large and where very low receiver noise is required. The single-reflector system described in Sec. 6.14 does not lend itself very well to achieving this, and more satisfactory, but more costly, arrangements are possible with a double-reflector system. The feed horn is mounted at the rear of the main reflector through an opening at the vertex, as illustrated in Fig. 6.22. The rear mount makes for a compact feed, which is an advantage where steerable antennas must be used, and access for servicing is easier. The subreflector, which is mounted at the front of the main reflector, is generally smaller than the feed horn and causes less blockage. Two main types are in use, the Cassegrain antenna and the Gregorian antenna, named after the astronomers who first developed them.
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