ined by these aspects. A comprehensive survey of VHF and UHF antennas can be found in both Rudge (1986) and Johnson and Jasik (1984). Antennas that give near uniform coverage in one plane can be obtained from half-wave dipoles or monopoles. Complementary antennas, such as loops and slots, will work equally well, and the actual shape will be determined more by applica- tion than by basic electromagnetic performance. The bandwidth of these simple elements is limited by the impedance characteristics, although most communication applications require only relatively narrow bandwidths. With small elements, some form of impedance matching network is required. One problem with balanced dipole-type antennas is that they are required to be fed by an unbalanced coaxial cable. A balun is needed to match the balanced to the unbalanced system, and this is inevitably frequency sensitive. Antennas for point-to-point links need to be directional and have as high a gain as sible. This is achieved with a Yagi-Uda array (see Figure 9.21), which consists of one driven element, one reflector element, and a number of director elements. Chapter 9 Antennas Only the driven element is connected to the feed line; the other elements are passive, and currents are induced in them by mutual coupling; the spacing ensures that this is in the correct amplitude and phase to give a directional radiation pattern. Gains of up to about 17 dBi are possible from one Yagi-Uda array. Higher gains can be obtained by multiple arrays. The Yagi-Uda array is inherently linearly polarized. Circular polarized arrays can be made either from crossed dipoles or from helixes. Antennas for mobile communications can be divided into those for base stations and those for mobiles. Base station antennas are mounted on towers and usually require nearly uniform patterns in the horizontal plane, with shaping in the vertical plane to conserve power. This can be achieved with a vertical array of vertical dipoles or other paneled dipoles. The influence of the tower on the antenn