This circuit is for a QRP (low power) antenna tuner, a.k.a. a transmatch, for use in the short wave amateur radio bands from 3-30 Mhz. It allows a wide variety of antennas to be connected to a low power transmitter. When the circuit is properly tuned, the maximum transmitter power will be delivered to the antenna. It is used in conjunction with a standing wave ratio (SWR) meter. This is a fairly generic antenna tuner circuit.
The purpose of a transmatch is to match the impedance of a transmitter, typically 50 ohms, to an unknown antenna impedance. The circuit consists of a variable series inductor followed by a variable capacitor to ground. Most transmitter outputs consist of