17.3 Generic antenna types

17.3.1 Radiation from apertures

The radiation from apertures illustrates most of the significant properties of pencil beam antennas. The radiation characteristics can be determined by simple mathematical relationships. If the electric fields across an aperture, Figure 17.4, is E_a(x,y) then the radiated fields is given by Equation 17.3, whereis given by Equation 17.4. (Oliver, 1986; Milligan, 1985).

For high or medium gain antennas the pencil beam radiation is largely focused to a small range of angles around 0 = 0. In this case it can be seen from Equation 17.3 that the distant radiated fields, and the aperture fields are the Fourier transformation of each other. Fourier transforms have been widely studied and their properties can be used to understand the radiation characteristics of aperture antennas. Simple aperture distributions have analytic Fourier trans­forms, whilst more complex distributions can be solved numerically on a computer.

The simplest aperture is a one dimensional line source distribution of length This serves to illustrate many of the features of aperture antennas. If the field in the aperture is constant, the radiated field is given from Equation 17.3 as in Equations 17.5 and 17.6.

width and is .The first sidelobe level is at -13.2dB which is a disadvantage of a uniform aperture distribution. The level can be reduced considerably by a tapered aperture distribution where the field is greatest at the centre of the aperture and tapers to a lower level at the edge of the aperture. For example if Equation 17.7 holds, then the first sidelobe level is at -23dB.

The energy which was in the sidelobes moves to the main beam with the result that the beamwidth broadens to . In practicealmost all antennas have natural tapers across the aperture which result from boundary conditions and waveguide modes. Rectangu­lar apertures are formed from two line source distributions in ortho­gonal planes.

Circular apertures form the largest single class of aperture an­tennas. The parabolic reflector is widely used in communications and is often fed by a conical horn. Both the reflector and the horn are circular apertures. For an aperture distribution which is inde­pendent of azimuthal angle the simplest case is uniform illumina­tion which gives a radiated field as in Equation 17.8, where J₁ (x) is a Bessel function of zero order.

This can be compared to and is also plotted i n Figure 17.5.

The first sidelobe level is at-17.6dB. Table 17.2 lists a number of circular aperture distributions and corresponding radiation pattern properties. The pedestal distribution is representative of many re­flector antennas which have an edge tapers of about -10 dB corresponding to .The Gaussian distribution is also important because high performance teed horns ideally have Gaus­sian aperture distributions. The Fourier transform of a Gaussian taper which decreases to zero at the edge of the aperture gives a Gaussian radiation pattern which has no sidelobes.