17.5.4 Vhf and uhf communications

Antennas for VHF and UHF communication 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 determined by these aspects.

A comprehensive survey of VHF and UHF antennas can be found in (Rudge, 1986; Johnson, 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 the application than by the basic electromagnetic performance. The bandwidth of these simple elements is limited by the impedance characteristics, although most communication applications only require 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 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 possible. This is achieved with Yagi-Uda array, Figure 17.19, which consists of one driven elements, one reflector element and a number of director elements. 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 ensuring 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 polarised. Circular polarised 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 the mobiles. Base station antennas are mounted on towers and usually require to have 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 panelled dipoles. The influence of the tower on the antenna must be taken into account in the design.

Mobile antennas on vehicles, ships, aircraft or near humans pres­ent challenging problems to the antenna designer. In most cases the physical, mechanical and environmental aspects take precedence over the electromagnetic design. In consequence the ingenuity of the antenna designer is required to produce an antenna which works well in adverse conditions. For instance antennas on aircraft must not disturb the aerodynamic profile so cannot protrude from the body of the aircraft. The effects of corrosion, temperature, pressure, vibration and weather are other factors to be taken into account. Antennas for personal radios are constrained by the role of the operator and by the need for very compact designs commensurate with satisfying radiation safety levels. The human body acts partly as a director and partly as a reflector depending on the frequency of use and the relative position of the antenna to the body. The portable radio equipment has to be considered a part of the antenna system including the radio circuits, batteries and case. In general, improved performance will result when the antenna is held as far from the body as possible and as high as possible.