Types of antennas

Antennas form the link between the guided parts and the free-space parts of a communication system. The purpose of a transmitting antenna is to efficiently transform the currents in a circuit or wave­guide into radiated radio or microwave energy. The purpose of a receiving antenna is to efficiently accept the received radiated energy and convert it to 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 communication systems fall into two broad ca­tegories depending on the degree to which the radiation is confined. Microwave radio relay and satellite communications use pencil beam antennas, where the radiation is confined to one narrow beam of energy, Figure 17.1. Mobile communications are more likely to require antennas with omni-directional patterns in the horizontal plane and toroidal patterns in the vertical plane, Figure 17.2.

Pencil beam antennas usually consist of one or more large to medium reflectors which 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 which radiates into space. The design problem is to first deter­mine the aperture fields which will yield the specified radiation 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 con­sist of elements which are small in wavelengths, such as dipoles and monopoles. The radiation characteristics are influenced by the presence of surrounding objects. Non-electromagnetic factors such as the size are often as important in the design as the radiation performance. For this reason the design of omni-directional an­tennas is partly an empirical process in which expertise and previous experience play an equal part with theoretical knowledge.

In between the large aperture antennas and the small element antennas lies array antennas which consist of two or more elements. The radiation from an array antenna is determined principally by the physical spacing and electrical signals driving the elements rather than the radiation characteristics of the elements themselves.

The detailed theory of antennas can be found in Stutzman (1981); Elliot (1981); Balanis (1982); Silver (1984); Colin (1969). Design information and descriptions of particular types can be found in a number of handbooks, such as, Rudge (1986); Johnson (1984); Lo (1988); Milligan (1985).