iation pattern. -3-dB Beamwidth Main Beam Sidelobes First Sidelobe Level Fig. 9.3 Typical rectangular radiation plot. Degrees Between the large aperture antennas and the small element antennas lie array antennas that consist of two or more elements. The radiation from an array antenna is determined principally by the physical spacing and electrical signals driving the elements rather than the radiation characteristics of the elements themselves, such as that shown in Figure 9.3. The detailed theory of antennas can be found in Stutzman and Thiele (1981), Elliot (1981), Balanis (1982), Silver (1984), and Collin and Zucher (1969). Design information and descriptions of particular types can be found in a number of handbooks, such as Rudge (1986), Johnson and Jasik (1984), Lo (1988), and Milligan (1985). Antennas Used in Communications Table 9.1 lists the principal types of antenna that are used in communications. Under each category of communication system, the specific type of antenna used is given. The last column gives the generic type of aperture antenna, array antenna, or small element antenna. The generic type describes the general radiation characteristics that can be obtained from the antenna and is useful because it makes the explanation of performance easier. Basic Properties Table 9.1 Antennas Used in Communications Systems Specific Type Generic Type Microwave line-of-sight radio Prime-focus reflector with small Aperture Earth Stations (large) Dual reflector with corrugated Aperture horn feed Earth Stations (medium) Offset reflector with corrugated Aperture or dual-mode horn feed Direct Broadcast Satellite Receiving Prime-focus symmetric or offset Aperture Antennas reflector Flat plate antennas Array Satellite Antennas (spot beams) Offset reflector with single feed Aperture Satellite Antennas (multiple beams) Offset reflector with array feed Aperture and Array Satellite Antennas (shaped beams) Shaped reflectors Aperture Offset reflector with array feed Aperture and Array VHF/UHF Communications Yagis, dipol