Antennas

An antenna is a conductor, or arrangement of conductors, that converts electrical signals into radio waves and radio waves back into electrical signals. In a transmitter the oscillating current driven into the antenna sets up time-varying electric and magnetic fields that propagate outward as electromagnetic radiation; in a receiver the passing wave induces a small voltage that the radio amplifies and demodulates. Because an antenna works equally well in both directions, the same design can serve for transmission and reception, a property described by the reciprocity theorem.

The physical size of an antenna is tied to the wavelength of the signal, and wavelength is inversely proportional to frequency. A simple resonant element such as the half-wave dipole is roughly half a wavelength long, so low-frequency antennas are large while microwave antennas can be very small. Antennas are commonly grouped by form and behaviour: wire types such as the dipole and the loop; directional arrays such as the Yagi-Uda; ground-plane and monopole designs that work against a conducting surface; and aperture antennas such as the horn and the parabolic dish used at higher frequencies.

Several parameters describe how an antenna performs. Gain and directivity express how strongly the radiated energy is concentrated in a preferred direction. The radiation pattern maps signal strength against angle. Polarization — vertical, horizontal or circular — describes the orientation of the radiated electric field, and matching polarization between two antennas improves the link. Impedance matching, often aided by a balun between a balanced antenna and an unbalanced feed line, ensures power transfers efficiently rather than being reflected back to the source.

Frequently asked questions

Why does antenna size depend on frequency?

Resonant antenna dimensions are a fraction of the signal's wavelength, and wavelength shrinks as frequency rises, so higher-frequency antennas are physically smaller.

What does antenna gain mean?

Gain measures how much an antenna concentrates radiated power in a particular direction compared with a reference antenna, usually expressed in decibels.

What is a balun?

A balun is a transformer-like device that connects a balanced antenna, such as a dipole, to an unbalanced feed line like coaxial cable while preserving impedance matching.