17.4.2 Dual symmetric reflector antennas

The performance of a large reflector antenna can be improved and the design made more flexible by inserting a sub-reflector into the system, Figure 17.9. There are two versions, the Ca.ssegrain, where the subreflector is a convex hyperboloid of revolution placed on the inside of the parabola focus, and a Gregorian where a concave elliptical subreflector is placed on the outside of the parabola focus. In symmetric reflectors the Cassegrain is more common because it is more compact, but the electrical performance is similar for both systems.

The advantages of the dual reflectors are:

1. The feed is in a more convenient location.

2. Higher performance feeds can be used because the subtended angle is such that wide aperture diameter feeds are needed.

3. Spillover past the subreflector is directed at the sky which reduces the noise temperature.

4. The depth of focus and field of view are larger.

The study of the radiation characteristics and the efficiency of dual reflectors is similar to that for the prime focus reflector. Analysis of the radiation patterns depends partly on the size of the subreflector. If it is small then physical optics or GTD must be used on the subreflector. The main reflector is usually large so geometric optics is adequate.

The limiting factor to obtaining high efficiency in a standard parabola is the amplitude taper across the aperture due to the feed pattern and the space loss in the parabola (i.e. the illumination efficiency). By shaping the surfaces of a dual reflector antenna it is possible to increase the efficiency and produce a more uniform illumination across the aperture. A well known method to produce a high efficiency Cassegrain symmetric reflector antenna is due to Galindo and Williams (Galindo, 1964; Williams, 1965). It is a geometric optics technique in which the shape of the subreflector is altered to redistribute the energy more uniformly over the aperture. Then the shape of the main reflector is modified to refocus the energy and create a uniform phase across the aperture. After this process the reflector surfaces are no longer parabolic and hyper­bolic. The method works well for large reflectors. For small or medium size reflectors geometric optics is not adequate and physi­cal optics including diffraction must be used at least on the subre­flector.

17.4.3 Offset reflectors

In recent years the growth in communication systems has led to a tightening in the radiation pattern specifications and the consequent need to produce reflectors with low far-out sidelobes. Symmetric reflectors cannot be made to have low sidelobes because of the inherent limitations caused by scattering from the feed and feed supports. This blockage loss can be entirely eliminated with the offset reflector, Figure 17.10, which consists of a portion of a parabola chosen so that the feed is outside the area subtended by the aperture of the reflector. The projected aperture is circular, though the edge of the reflector will be elliptical. The removal of the blockage loss also means that smaller reflector antennas can be made efficient which has led to their widespread use as DBS receiving antennas.

In addition to the unblocked aperture, the offset reflector has other advantages, (Rudge, 1986, Page 185; Rahmat-Samii, 1986). The reaction of the reflector upon the primary feed can be reduced to a very low order so that the feed VSWR is essentially independent of the reflector. Compared to a symmetric paraboloid, the offset con­figuration makes a larger F/D ratio possible which in turn enables a higher performance feed to be used. The removal of the feed from the aperture gives greater flexibility to use an array of feeds to produce multiple beams or shaped beams.

The offset reflector antenna also has some disadvantages. It is much more difficult to analyse and design due to the offset geometry and it is only with the advent of powerful computers that this has become feasible. The lack of symmetry in the reflector means that when a linearly polarised feed is used, a cross-polarised component is generated by the reflector surface. When circular polarisation is used, a cross-polarised component does not occur but the offset surface causes the beam to be 'squinted' from the electrical bore-sight. Lastly the construction of the offset reflector is more difficult. However if the reflectors are made by fibreglass moulding this is not really significant. Also the structural shape can be put to good use because it is convenient for deployable configurations on satel­lites or transportable earth stations.