How does an antenna tuner compensate for an impedance mismatch in an antenna system?

An impedance mismatch means the antenna system exhibits capacitive or inductive reactance, or a resistance other than 50 ohms, at the operating frequency. An antenna tuner consists of a network of adjustable capacitors and inductors. By introducing the exact opposite capacitive or inductive reactance to the circuit, the tuner cancels out the reactance of the antenna system. This presents a perfectly matched, purely resistive 50-ohm load to the transmitter. It is critical to understand that a tuner does not physically change the antenna's actual resonant frequency or length. It merely acts as an electrical matching network, masking the mismatch from the transmitter so the radio can deliver full power without activating its protective fold-back circuitry.