An antenna is a device that can convert a guided electromagnetic wave on a transmission line into a plane wave propagating in free space. It is typically used as part of a radio transmitter or receiver.

The antenna acts as a transducer, transforming the guided wave propagating on the transmission line into an electromagnetic wave that can propagate through space. In two-way communication, the same antenna can be used for transmitting and receiving signals.

The antenna also functions as a transformer, converting the impedance of the transmission line (usually 50/75 Ohm) into the free space impedance (120pi or 377 Ohm). Transverse electromagnetic (TEM) is a mode of propagation where the electric and magnetic field lines are all restricted to directions normal (transverse) to the direction of propagation.

A receiving antenna will convert the received electromagnetic waves from the beam into electrical signals.