enna is to efficiently transform the currents in a circuit or waveguide into radiated radio or microwave energy. The purpose of a receiv- ing antenna is to efficiently accept the received radiated energy and convert it into guided form for detection and processing by a receiver. The design and construction of an antenna usually involves compromises between the desired electromagnetic performance and the mechanical size, mass, and environmental characteristics. Antennas for communications systems fall into two broad categories depending on the degree to which the radiation is confined. Microwave radio relay and satellite communica- tions use pencil beam antennas, where the radiation is confined to one narrow beam of energy (see Figure 9.1). Mobile communications are more likely to require antennas with omnidirectional patterns in the horizontal plane and toroidal patterns in the vertical plane, as shown in Figure 9.2. Pencil beam antennas usually consist of one or more large to medium reflectors that collimate the signals from a feed horn at the focus of the reflector. Both reflector and feed horn fall within the generic class of aperture antennas because they consist of an aperture that radiates into space. The design problem is first to determine the aperture fields that will yield the specified radiation characteristics, and second to design the reflectors and horns to produce the aperture fields. Aperture antennas can be designed to meet very stringent specifications. Omnidirectional antennas consist of elements that are small in wavelength, such as dipoles and monopoles. The radiation characteristics are influenced by the presence of surrounding objects. Nonelectromagnetic factors, such as size, are often as important in the design as radiation performance. For this reason the design of omnidirectional antennas is partly an empirical process in which expertise and previous experience play an equal part with theoretical knowledge. Chapter 9 Antennas Fig. 9.1 Pencil beam radiation pattern. Fig. 9.2 Toroidal rad