 normally expressed in decibels. In some instances the front to back ratio is important, especially when interference in the opposite direction to the main direction is an important feature. When designing or adjusting a beam antenna it is found that the maximum gain and optimum front to back ratio do not normally coincide exactly and a compromise has to be made. There are many different types of directive antenna and there are a number of different techniques that can be used to make an antenna directive. A simple half wavelength straight wire antenna will exhibit the maximum radiation (and sensitivity) at right angles to the axis of the antenna. By increasing the length, lobes form and the angles of main radiation tend to align more with the axis of the antenna. 82 Newnes Guide to Radio and Communications Technology Figure 4.5 Polar diagrams for wire antennas of differing lengths Directive antennas can be made by using two or more antennas. By altering the phase relationship between the signals applied to the two antennas the signal can be reinforced in one direction or another. Similarly signals can be cancelled out in some directions. It is not necessary to have to feed all the elements in a directive array. Elements placed close to the driven element will have an effect. The non-driven elements known as parasitic elements pick up the signal and re-radiate it with a different phase. Although it is not possible to have complete control over the phase and magnitude of the re-radiated signal, it is nevertheless still possible to develop highly directive antennas using this approach and the construction of the antenna is simplified by the fact that complicated phasing systems are not required. The popular Yagi antenna to be described later uses this approach. Bandwidth An antenna has a certain frequency bandwidth over which it can operate satisfactorily. There are two main factors that limit the performance away Antenna systems 83 from centre frequency. One is the variation in impedance and the other is the chang