ugh most communication applications only pattern. This constraint is removed with a dual reflector design. require relatively narrow bandwidths. With small elements, some form of impedance matching network is required. One problem with 17.5.4 VHF and UHF communications balanced dipole type antennas is that they are required to be fed by an unbalanced coaxial cable. A balun is needed to match the bal- Antennas for VHF and UHF communication systems take on a wide anced to unbalanced system and this is inevitably frequency sensi- variety of specific forms, but the vast majority are derivatives of the tive. generic dipole type antenna. The physical, mechanical and environ- Antennas for point-to-point links need to be directional and have mental aspects are generally more significant than for microwave as high a gain as possible. This is achieved with Yagi-Uda array, antennas because the smaller size of the antenna means that the Figure 17.19, which consists of one driven elements, one reflector radiation and impedance characteristics are partly determined by element and a number of director elements. Only the driven element these aspects. is connected to the feed line; the other elements are passive and A comprehensive survey of VHF and UHF antennas can be found currents are induced in them by mutual coupling, the spacing in (Rudge, 1986; Johnson, 1984). Antennas that give near uniform ensuring that this is in the correct amplitude and phase to give a coverage in one plane can be obtained from half wave dipoles or directional radiation pattern. Gains of up to about 17 dBi are monopoles. Complementary antennas such as loops and slots will possible from one Yagi-Uda array. Higher gains can be obtained by work equally well and the actual shape will be determined more by multiple arrays. The Yagi-Uda array is inherently linearly polarised. the application than by the basic electromagnetic performance. The Circular polarised arrays can be made either from crossed dipoles bandwidth of these simple elements is limited by th