Questions relating to the setup and use of antenna tuners, the virtues and shortcomings of various types of tuners, etc.
An antenna tuner, also known as a transmatch or antenna tuning unit (ATU) is a device connected between a radio and its antenna to improve power transfer between them by matching the impedance of the radio to that of the antenna.
Transmitters transfer maximum power when they are presented with a fully resistive load of a specified value (typically of 50 ohms). Antennas are resonant only at particular frequencies. Operating into an antenna at other frequencies is possible, but at a reduced efficiency.
An antenna tuner contains a matching network of capacitors and inductors designed to cancel out the reactance that occurs when operating an antenna at non-resonant frequencies leaving just the resistive load.
Therefore, an ATU allows the use of one antenna on a broader range of frequencies. An antenna and transmatch are not as efficient as a resonant antenna due to feedline losses and losses in the ATU itself. Note that an ATU does not change the resonant frequency of the antenna, merely changes the impedance.