17 - Antennas

Abstract

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. In between the large aperture antennas and the small element antennas lies array antennas consisting of two or more elements. The radiation from an array antenna is determined principally by then physical spacing and electrical signals driving the elements rather than the radiation characteristics of the elements themselves. 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. The principle of reciprocity is one of the most important properties of an antenna. It means that the properties of an antenna when acting as a transmitter are identical to the properties of the same antenna when acting as a receiver. For this to apply, the medium in between the two antennas must be linear, passive, and isotropic, which is always the case for communication systems.