17.5.3.2 Spot beams

Spot beam antennas are required to produce a beam covering a small region of the earth's surface. The angular width of the beam is inversely proportional to the diameter of the antenna. Size consider­ations virtually dictate that some form of deployable mechanism is needed on the satellite and this leads to the use of offset reflectors with a dual-mode or corrugated feed horn. The constraints of the launcher mean that the maximum size for a solid reflector is about 3.5 metres. Larger reflectors can only be launched by using some form of unfurlable mesh or panel reflector. The trend towards smaller footprints on the earth can be met either by using a larger reflector or by using a higher frequency, both of which involve higher costs. To date most spot beam communication satellite have used two prime focus offset reflectors, one for transmit and one for receive, producing footprints on the earth's surface which are ellip­tical because of the curvature of the earth.

17.5.3.3 Multiple beams

It was early recognised that by using a single reflector and an array of feeds it was possible to produce multiple beams on the earth, Figure 17.15. This has the advantage that most of the antenna sub-system is re-used with the penalty of having to design and make the array of feed horns and the beam forming network behind the array. The array feed elements must be compact so that they occupy the minimum space in the focal plane of the offset reflector. At the same time the crosspolarisation must be low. This tends to mean that corrugated horns cannot be used and small diameter dual-mode rectangular or circular horns are preferred. The maximum number of beams depends on the tolerable aberrations since array elements which are off-axis will have degraded performance.

17.5.3.4 Shaped beams

It is desirable to optimise the shape of the satellite beam on the earth's surface so as to conserve power and not waste energy by illuminating portions of the oceans. An example is shown in Figure 17.16. Shaped beams can be produced in two ways. Multiple, overlapping beams produced by an offset parabolic reflector and an array of feeds can be used. This approach is an extension of the multiple beams and has the advantage that it is possible to design for reconfiguration by incorporating switching systems into the beam forming network. The alternative approach is to use a single, high performance feed and to physically shape the surface of the reflector so that power is distributed uniformly over a shaped beam region. Both approaches have received considerable attention in recent years.

The multiple beam approach is well illustrated by the INTELSAT VI communication satellite, Figure 17.17, which produces multiple shaped beams to cover the main population regions of the earth. (Figure 17.18.) In order to be able to use the same satellites over the Atlantic, Indian or Pacific Oceans, the array feed consists of 146 elements which can be switched to produce the appropriate shaped beams (Bennett, 1984).

The shaped reflector approach has the advantage of mechanical simplicity and lower weight at the penalty of fixed beams. The theoretical design process is quite extensive and involves a syn­thesis process with the input of the required beam shape and the output of the contours of the reflector surface. A single offset reflector constrains the possible shapes because it is not possible to arbitrarily specify the amplitude and the phase of the synthesised pattern. This constraint is removed with a dual reflector design.