y of patterns SO that it is possible to synthesise almost any specified radiation pattern. The array factor for an N section, but it is often convenient in initial design to take the feed element linear array of equal amplitude is given by Equation 17.14. pattern as being given by Equation 17.18. (17.14) (17.18) Experience has shown that good quality feeds approximate well This is similar to the pattern of a line source aperture, Equation to this function. 17.5, and it is possible to synthesise an aperture with a planar array. The radiation patterns can be predicted from the aperture fields by There is a significant benefit to this approach. The aperture fields using the Fourier transform relations described in Section 17.3.1. are determined by the waveguide horn fields which are constrained This works well for large reflectors but for detailed design of small by boundary conditions and are usually monotonic functions. This to medium reflectors it is necessary to take account of the precise constraint does not exist with the array so that a much larger range form of the currents on the reflector surface and the diffraction that of radiation patterns can be produced. Optimum patterns with most occurs at the edges of the reflector surface. The former can be Specific antenna types 17/9 accomplished with Physical Optics theory (Rusch, 1970; Rusch, 1986) and is good for predicting the main beam and near-in si- delobes. The diffracted fields influence the far-out sidelobes and can be predicted using the Geometrical Theory of Diffraction (GTD) (James, 1986). 17.4.1.3 Gain of reflector antennas The gain of a reflector antenna can be calculated from Equation 17.19, where n is the efficiency of the reflector. (17.19) The total efficiency is the product of six factors: 1. The illumination efficiency is the gain loss due to the non-uni- form aperture illumination. The spillover efficiency is the gain loss caused by energy from the feed which radiates outside the solid angle subtended by 01 called the spillover. It is the fra