Antenna, in electronics, device used to propagate radio or electromagnetic waves or to capture radio waves. Antennas are necessary to transmit and receive radio, television, microwave, telephone, and radar signals. Most antennas for radio and television consist of metal wires or rods connected to the transmitter or receiver. When an antenna is used for transmission (propagation) of radio waves, electric currents are made to oscillate along the wires or rods by the transmitter. Energy from this oscillating charge is emitted into space as electromagnetic (radio) waves. When an antenna is used for reception, these waves induce a weak electric current in the antenna wire or rod. This current is amplified by the radio receiver. An antenna can generally be used for reception and transmission on the same wavelength if transmission power is not too great.

The dimensions of an antenna usually depend on the wavelength, or frequency, of the radio wave for which the antenna is designed. Frequency is the number of electromagnetic oscillations per second. The length of a radio wave is equal to the speed of light (300 million m/sec) divided by the frequency. Thus, low-frequency waves have long wavelengths (hundreds of metres); high frequency waves have short wavelengths (centimetres). The length of an antenna must be such that it resonates electrically at the desired wavelength, just as the length of an organ pipe determines its pitch. The basic antenna length must be at least half the wavelength of the radio waves it is designed to transmit or receive. It can also be an integral multiple of the one-half wavelength, that is, 1, 1.5, 2, 2.5, etc., wavelengths. Those with such dimensions are called resonant antennas. A resonant antenna is an efficient propagator and receptor of electromagnetic energy at its design wavelength. The half-wavelength dimensional rule applies to all antennas except wire loop antennas. Small loop antennas used in transistor radios are resonant at the long, 300-m wavelengths of the broadcast (AM) band because they contain a core of magnetic material called ferrite. Ferrite loop antennas are used in ultra-compact transistor radios.

Electrical energy is fed to an antenna by means of a transmission line, or coaxial cable, with two conductors. If one conductor is connected to a grounding cable and the other to the end of a horizontal wire antenna, the antenna is said to be an end-fed long wire. If the antenna is split in the middle, with each side connected to one conductor of the transmission line, the antenna is called a dipole, the simplest and most fundamental antenna. The familiar “rabbit-ear” television antenna is a dipole.

A dipole transmits or receives most of its energy at right angles to the wire; little energy is transferred along the length of the wire. Such directivity is one of the most important electric qualities of an antenna. It allows transmission or reception to be beamed in a particular direction, to the exclusion of signals in other directions.

A second fundamental quality of an antenna is its gain, or sensitivity. Gain and directivity can be controlled by carefully spacing numerous dipole elements to make a single antenna array.

A third characteristic of antennas is their bandwidth, or resonance to signals covering a band of wavelengths. Ordinary home radio antennas must have broad bandwidths in order to receive stations of different wavelengths. Helical antennas used for space communications have very narrow bandwidths and high gain.

Vertically positioned antennas include the Marconi, or ground plane, antenna. It is usually a quarter-wave vertical rod, or whip, representing one-half of a vertical dipole, with the ground acting as the other half. Such whip antennas are widely used in high frequency mobile communications. The metal body of the car acts as the other half of the dipole. Long wave vertical antenna types include the half-wave used by broadcast (AM) band transmitting stations. The antenna is the entire transmitting tower. Large horizontal rhombic antennas give good directivity at long wavelengths; they are used for international radio communications.

Radio telescopes and radar systems operate at wavelengths shorter than 30 cm, called microwaves. Microwaves behave much like light waves, and microwave antennas are similar to spotlights. The microwave source can be a small dipole or the opening of a special tubing called a wave guide. Microwave energy is reflected from a metallic paraboloid that shapes it into a narrow beam. Many kinds of sophisticated radar antennas have been developed; some of these can change their beam direction electrically, without motion.