d and to physi- cally shape the surface of the 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 (see Figure 9.19), which produces multiple shaped beams to cover the main popula- tion regions of the earth (see Figure 9.20). To be able to use the same satellites over the Atlantic, Indian, or Pacific oceans, the array feed consists of 146 elements that can be switched to produce the appropriate shaped beams (Bennett & Braverman, 1984). Chapter 9 Antennas Fig. 9.18 European contoured beam. Hemi/Zone Transmit Reflector Hemi/Zone Receive Reflector Hemi/Zone Transmit Hemi/Zone Receive Feed West Spot Antenna Earth Coverage Earth Spot Antenna Antenna Fig. 9.19 INTELSAT IV antenna system. The shaped-reflector approach has the advantage of mechanical simplicity and lower weight, at the penalty of fixed beams. The theoretical design process is quite extensive and involves a synthesis with the input of the required beam shape and the output of the con- tours of the reflector surface. A single offset reflector constrains the possible shapes because it is not possible to arbitrarily specify the amplitude and the phase of the synthesized pattern. This constraint is removed with a dual-reflector design. Satellite Antennas for Telemetry, Tracking, and Command Zones Zones Coverage Coverage Fig. 9.20 Shaped beam generated by INTELSAT VI antenna: (a) Atlantic/Pacific Ocean; (b) Indian Ocean. Fig. 9.21 Yagi-Uda array. Reflector Driven Directors VHF and UHF Communications Antennas for VHF and UHF communications systems take on a wide variety of specific forms, but the vast majority are derivatives of the generic dipole-type antenna. The physical, mechanical, and environmental aspects are generally more significant than for microwave antennas because the smaller size of the antenna means that the radiation and impedance characteristics are partly determ