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.

17.5.4 Vhf and uhf communications

Antennas for VHF and UHF communication systems take on a wide variety of specific forms, but the vast majority are derivatives of the generic dipole type antenna. The physical, mechanical and environ­mental aspects are generally more significant than for microwave antennas because the smaller size of the antenna means that the radiation and impedance characteristics are partly determined by these aspects.

A comprehensive survey of VHF and UHF antennas can be found in (Rudge, 1986; Johnson, 1984). Antennas that give near uniform coverage in one plane can be obtained from half wave dipoles or monopoles. Complementary antennas such as loops and slots will work equally well and the actual shape will be determined more by the application than by the basic electromagnetic performance. The bandwidth of these simple elements is limited by the impedance characteristics, although most communication applications only require relatively narrow bandwidths. With small elements, some form of impedance matching network is required. One problem with balanced dipole type antennas is that they are required to be fed by an unbalanced coaxial cable. A balun is needed to match the bal­anced to unbalanced system and this is inevitably frequency sensi­tive.

Antennas for point-to-point links need to be directional and have as high a gain as possible. This is achieved with Yagi-Uda array, Figure 17.19, which consists of one driven elements, one reflector element and a number of director elements. Only the driven element is connected to the feed line; the other elements are passive and currents are induced in them by mutual coupling, the spacing ensuring that this is in the correct amplitude and phase to give a directional radiation pattern. Gains of up to about 17 dBi are possible from one Yagi-Uda array. Higher gains can be obtained by multiple arrays. The Yagi-Uda array is inherently linearly polarised. Circular polarised arrays can be made either from crossed dipoles or from helixes.

Antennas for mobile communications can be divided into those for base stations and those for the mobiles. Base station antennas are mounted on towers and usually require to have nearly uniform patterns in the horizontal plane with shaping in the vertical plane to conserve power. This can be achieved with a vertical array of vertical dipoles or other panelled dipoles. The influence of the tower on the antenna must be taken into account in the design.

Mobile antennas on vehicles, ships, aircraft or near humans pres­ent challenging problems to the antenna designer. In most cases the physical, mechanical and environmental aspects take precedence over the electromagnetic design. In consequence the ingenuity of the antenna designer is required to produce an antenna which works well in adverse conditions. For instance antennas on aircraft must not disturb the aerodynamic profile so cannot protrude from the body of the aircraft. The effects of corrosion, temperature, pressure, vibration and weather are other factors to be taken into account. Antennas for personal radios are constrained by the role of the operator and by the need for very compact designs commensurate with satisfying radiation safety levels. The human body acts partly as a director and partly as a reflector depending on the frequency of use and the relative position of the antenna to the body. The portable radio equipment has to be considered a part of the antenna system including the radio circuits, batteries and case. In general, improved performance will result when the antenna is held as far from the body as possible and as high as possible.