If you’ve ever sung in the shower and hit a note where the sound seemed to amplify, you’ve experienced resonance. Every antenna has a particular frequency on which it is “resonant”. When we transmit on a radio frequency that is resonant with the antenna, very little power is lost and you are efficiently sending the most energy over the airwaves. Resonance is typically measured as the Standing Wave Ratio or SWR.
In the world of RF, any time you connect a transmitter to a circuit that’s not resonant, power is reflected back from the antenna into the transmitter. Worst case, this situation can damage your equipment, so you want to avoid a bad match at all costs.
Enter the antenna tuner. This is a device that uses capacitors and an inductor to match an antenna (or load) to a transmitter, even if the antenna is not resonant at the transmitted frequency.
Here’s a video that shows how to use an antenna tuner to create a good match with a minimal SWR.