What is the purpose of an antenna tuner?

An antenna tuner (more accurately an impedance matching network) transforms the complex impedance of the antenna and feed line system to the standard 50-ohm output impedance expected by the transmitter. By providing this match, it maximizes the power transfer from the transmitter into the feed line. It is crucial to understand what a transmitter-side antenna tuner does not do: it does not change the actual Standing Wave Ratio (SWR) on the feed line between the tuner and the antenna, nor does it reduce power dissipation or loss in that section of the wire. The tuner only creates a 1:1 SWR match between the transmitter and the tuner itself. This prevents modern solid-state transmitters from automatically reducing power to protect their final amplifier stages.