An antenna tuner, a matchbox, transmatch, antenna tuning unit (ATU), or antenna coupler is a device connected between a radio transmitter or receiver and its antenna to improve power transfer between them by matching the impedance of the radio to the antenna’s feedline. Similar matching networks are used in other equipment (such as linear amplifiers) to transform impedance.An antenna’s impedance is different at different frequencies. An antenna tuner matches a radio with a fixed impedance (typically 50 Ohms for modern transceivers) to the combination of the feedline and the antenna; useful when the antenna’s feedline impedance is unknown, complex, or otherwise different from the transceiver. Coupling through an ATU allows the use of one antenna on a broad range of frequencies. However, despite its name, an antenna ‘tuner’ actually only matches to the antenna feedline – an antenna ‘tuner’ does not and cannot change the resonant frequency of the aerial.Operating an antenna far from its design frequency and compensating with a transmatch is not as efficient as using a resonant antenna with a matched-impedance feedline. If there is still a high SWR (multiple reflections) in the feedline beyond the ATU, any loss in the feedline increases, heating the wire instead of sending out a signal. Additionally, losses in the ATU itself can also waste power.