An antenna tuner, or antenna tuning unit, is used
to match the impedance of an antenna to the characteristic
impedance of the antenna feeder and the output impedance of
a transmitter or input impedance of a receiver. Without the
use of an antenna tuner, large standing waves can
develop, which leads to
very inefficient power transfer, and
the possibility of
the output stage of a transmitter being destroyed by the
reflected power.
An antenna tuner is commonly built into modern
radios.