17.4.4 Horn feeds for reflector antennas

A reflector antenna consists of the reflector plus the horn feed at the geometric focus of the reflector. Thus the correct choice and design of the feed is an important part of the design of the total reflector antenna. High performance feeds are necessary to achieve high performance antennas. The diameter of the feed in wave­lengths will be determined by the angle subtended by the reflector at the feed. A prime focus reflector with an F/D between 0.25 and 0.5 will have a subtended half angle of between 90 degrees and 53 degrees. Application of the general rule that beamwidth is approxi­mately equal to the inverse of the normalised aperture diameter shows that this means a feed with an aperture diameter of between about one and three wavelengths. Dual reflectors (Cassegrain or Gregorian) and offset reflectors have subtended angles between 30 degrees and 7 degrees, leading to feed diameters of between three and ten wavelengths.

Of particular interest in horn feed design is the polarisation per­formance and the quality of a feed is usually expressed by the level of the peak cross-polarisation. The radiation characteristics of horns are predicted by a two part process. Firstly the fields in the aperture are computed from a knowledge of the guided wave behaviour inside the horn. Secondly the aperture fields are used to compute the radiated fields. The Fourier transform method has been found to work very well for the case of horns. The main types will now be briefly described. For more details see (Love, 1976; Love, 1986).

17.4.4.1 Rectangular or square horns

These are the simplest type of horn, Figure 17.1 l(a) but they are rarely used as feeds for reflectors because they have very high cross-polarisation unless the aperture size is large.

17.4.4.2 Small conical horns

These can have reasonably good cross polarisation performance, Figure 17.1 l(b). They are widely used as prime focus feeds in small symmetric and offset reflectors. The basic design will have an aperture diameter of about one wavelength and is essentially an open-ended circular waveguide propagating a TE_n mode. The radiation pattern can be improved by adding one or more rings or chokes around the aperture, Figure 17.1 l(c). These have the effect of changing the distribution of current on the flange and creating a more symmetric radiation pattern. The theoretical design of the open-ended waveguide is straightforward, but the analysis of the choked version is much more complicated. As a consequence most small feeds are designed empirically with measured data.

17.4.4.3 Multi-mode conical horns

These improve the performance of conical horns by generating a second mode inside the horn in such a manner that the aperture fields are linearised. The second, TM11 mode, is generated by a step change in the conical horn diameter and the length of the horn is determined by the need to have the modes in the correct phase relationship at the aperture. The dual mode horn gives low crosspolarisation over a narrow band of frequencies. Although narrow band it is simple to make and of low weight.

The concept of adding higher order modes in a horn can be extended for other purposes. In tracking feeds a higher order mode is used to provide tracking information. The inherent crosspolarisation which occurs in offset reflectors can be cancelled by the appropriate addition of higher order modes (Love, 1986). Finally the main beam can be shaped to provide higher efficiency in prime focus reflectors although only over a narrow frequency band.