Shaped reflectors Apertures Offset reflector with array feed Apertures and Arrays VHF/UHF Communications Yagis Dipole arrays Slots Elements and Arrays Mobile Communications (Base stations) Dipole arrays Elements and Arrays Mobile Communications (Mobile) Monopoles Microstrip Elements HF Communications Dipoles Monopoles Elements 17/4 Antennas -3dB Beamwidth Main Beam 1st Sidelobe Level Sidelobes Degrees Figure 17.3 Typical rectangular radiation plot number of handbooks, such as, Rudge (1986); Johnson (1984); Lo fied by the beamwidth between the -3dB points on the main beam (1988); Milligan (1985). and the sidelobe level. Communication antennas radiate in either linear polarisation or 17.1.1 Antennas used in communications circular polarisation. In modern communications cross-polarisation is important. This is the difference between the two principal plane Table 17.1 lists the principal types of antennas which are used in patterns and is specified relative to a reference polarisation, called communications. Under each category of communication system, the co-polar pattern. There are three definitions of the cross-polari- the specific type of antenna which is usually used is given. The last sation. The one in normal use with reflector antennas and feed column gives the generic type of either an aperture antenna, an array systems is Ludwig's third definition (Ludwig, 1973) which assumes antenna or a small element antenna. The generic type describes the that the reference polarisation is that due to a Huygens source. It general radiation characteristics which can be obtained from the most closely corresponds to what is measured with a conventional antenna and is useful because it makes the explanation of the antenna test range. performance easier. The power gain in a specified direction is defined by the ratio of The following sections describe first the generic antenna charac- the power radiated per unit solid angle in direction 0,4 to the total teristics, then the specific antenna types and then brief discussion of p