et rectangular or circular horns are preferred. The maximum number reflector constrains the possible shapes because it is not possible to of beams depends on the tolerable aberrations since array elements which are off-axis will have degraded performance. Hemi/zone transmit reflector 17.5.3.4 Shaped beams Hemi/zone receive It is desirable to optimise the shape of the satellite beam on the Hemi/zone reflector earth's surface SO as to conserve power and not waste energy by transmit Hemi/zone receive feed illuminating portions of the oceans. An example is shown in Figure 17.16. Shaped beams can be produced in two ways. Multiple, West spot antenna overlapping beams produced by an offset parabolic reflector and an array of feeds can be used. This approach is an extension of the multiple beams and has the advantage that it is possible to design for reconfiguration by incorporating switching systems into the Earth coverage beam forming network. The alternative approach is to use a single, high performance feed and to physically shape the surface of the Earth spot antenna antenna reflector SO that power is distributed uniformly over a shaped beam region. Both approaches have received considerable attention in recent years. The multiple beam approach is well illustrated by the INTELSAT VI communication satellite, Figure 17.17, which produces multiple shaped beams to cover the main population regions of the earth. (Figure 17.18.) In order to be able to use the same satellites over the Atlantic, Indian or Pacific Oceans, the array feed consists of 146 elements which can be switched to produce the appropriate shaped beams (Bennett, 1984). Figure 17.17 INTELSAT IV antenna system The shaped reflector approach has the advantage of mechanical simplicity and lower weight at the penalty of fixed beams. The 17/16 Antennas Zones Zones Coverage Coverage ATLANTIC/PACIFIC OCEAN INDIAN OCEAN Figure 17.18 Shaped beam generated by INTELSAT VI antenna arbitrarily specify the amplitude and the phase of the synthesised characteristics, altho