What does an antenna tuner do?

An antenna tuner, more accurately called an antenna matching unit or transmatch, uses adjustable internal inductors and capacitors to transform the impedance of an antenna system. Its primary purpose is to ensure the antenna system presents a standard 50-ohm load to the transceiver. By matching the transceiver to a mismatched antenna system, the tuner prevents high SWR at the transmitter. This allows modern solid-state radios to operate at full power without triggering their built-in protection circuits, which would otherwise reduce power output. It is important to note that an antenna tuner does not actually alter the physical resonant frequency of the antenna itself or fix the mismatch on the feedline; it merely masks the mismatch at the transmitter end so the radio can deliver power efficiently.