tio must be high, spacing and the amplitudes and phases of the signals sent to the and the cross-polar discrimination needs to be high for dual polari- individual elements. In addition to being able to form a wide range sation operation, typically better than -25dB within the main beam of multiple or shaped beams, the array has the advantage that the region over a bandwidth of up to 500MHz. The VSWR needs to be cross-polarisation of the total array is lower than that of an individ- low (typically 1.06 maximum) in a microwave radio relay system in ual element. However the closeness of the array elements gives rise order to reduce the magnitude of the round echo. The supporting to mutual coupling between the aperture fields which can distort the structure must be stable to ensure that the reflector does not move radiation patterns (Clarricoats, 1984b; Clarricoats, 1984c). A signi- significantly in high winds. The reflectors must operate under all ficant disadvantage of an array is that a beam forming network of weather conditions, which means that a radome is often required. waveguide components must be used behind the array elements to This poses extra design problems because inevitably it degrades the produce the correct amplitudes and phases to the array. For large electrical performance. A long waveguide or coaxial cable feeder arrays this can be heavy, expensive and a significant part of the must be provided from the transmitter to the antennas. Not only design of the complete antenna system. must this be low loss but it must also be well made SO that there is no possibility of loose joins introducing non-linear effects. Finally the cost must be relatively low because a large number of reflectors 17.5 Antennas used in communication are required in a microwave communication system. systems The majority of antennas in use are prime focus symmetric reflec- tors often with shields and radomes. The design of the prime focus 17.5.1 Microwave line of sight radio reflectors follows the procedure discussed in earlier s