An antenna tuner, transmatch or antenna tuning unit (ATU) is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna. An antenna tuner matches a transceiver with a fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna) impedance which is unknown, complex or otherwise does not match. An ATU allows the use of one antenna for a broad range of frequencies. An antenna plus matcher is never as efficient as a naturally resonant antenna due to additional induced losses on the feed line due to the SWR (multiple reflections), and losses in the ATU itself, although issues of pattern and capture area may outweigh this in practice. An ATU is actually an antenna matching unit, as it is unable to change the resonant frequency of the aerial. Note that similar matching networks are used in other types of equipment, such as linear amplifiers to transform impedances. See impedance bridging for details.
Read more about Antenna Tuner: Basic Principle of Wide-band Designs, Basic Principle of Narrow-band Designs, Connecting An ATU, Sample Application: Multiband Shortwave Transmitter