d the mechanical size, mass and environmental charac- teristics. Figure 17.2 Toroidal radiation pattern characteristics and secondly to design the reflectors and horns to produce the aperture fields. Aperture antennas can be designed to meet very stringent specifications. Omni-directional antennas con- Figure 17.1 Pencil beam radiation pattern sist of elements which are small in wavelengths, such as dipoles and monopoles. The radiation characteristics are influenced by the Antennas for communication systems fall into two broad ca- presence of surrounding objects. Non-electromagnetic factors such tegories depending on the degree to which the radiation is confined. as the size are often as important in the design as the radiation Microwave radio relay and satellite communications use pencil performance. For this reason the design of omni- directional an- beam antennas, where the radiation is confined to one narrow beam tennas is partly an empirical process in which expertise and previous of energy, Figure 17.1. Mobile communications are more likely to experience play an equal part with theoretical knowledge. require antennas with omni-directional patterns in the horizontal In between the large aperture antennas and the small element plane and toroidal patterns in the vertical plane, Figure 17.2. antennas lies array antennas which consist of two or more elements. Pencil beam antennas usually consist of one or more large to The radiation from an array antenna is determined principally by the medium reflectors which collimate the signals from a feed horn at physical spacing and electrical signals driving the elements rather the focus of the reflector. Both reflector and feed horn fall within the than the radiation characteristics of the elements themselves. generic class of aperture antennas because they consist of an aper- The detailed theory of antennas can be found in Stutzman (1981); ture which radiates into space. The design problem is to first deter- Elliot (1981); Balanis (1982); Silver (1984); Colin (1969). De