Overview

An antenna is a component that can both radiate and receive radio waves. Because of its dual function, it is called a reciprocal device. An ideal antenna would radiate all of the power delivered to it by the transmitter (usually through a transmission line) in the desired direction or directions with the desired polarity. Practical antennas cannot achieve this ideal performance, so many different designs have evolved to approach this ideal at different frequencies for different applications.

Despite the antenna's reciprocity property, a convention has been established in the electronics industry that only the antenna's radiation function will be described. It is understood that the antenna can also receive the same kinds of signals that it transmits. That convention is followed in the descriptions of antennas in this section.

Antennas are classified by their application, operating frequency, or both. An antenna for low radio frequencies can be more than a mile (1.6 km) long, but one that has been optimized for use in the microwave band can have a length measured in inches or centimeters. In practice, however, antenna measurements are given in units of their principal transmission wavelengths rather than in standard measurement units. This is done because antennas that differ significantly in size can be described in similar terms. This convention is also followed in the descriptions of antennas in this section.

An antenna can be a single length of wire or an array of conductors that will both radiate and receive radio frequencies. The...