In an HF station, what device might allow the use of an antenna on a band it was not designed for?

An antenna tuner (often called a transmatch) is a device that transforms the impedance of the antenna system to match the 50-ohm output expected by modern transceivers. When you try to use an antenna on a frequency it was not cut for, the impedance mismatch creates a high Standing Wave Ratio (SWR) that can damage the transmitter. By adjusting the inductors and capacitors inside the antenna tuner, you can "trick" the radio into seeing a perfect 50-ohm load. This allows the transmitter to deliver full power safely, effectively letting you use the antenna on bands outside its natural resonant frequency. Other options like filters are used to block unwanted frequencies, and an SWR meter only measures the mismatch without actually fixing it.