SWR meter

[a] The meter indirectly measures the degree of mismatch between a transmission line and its load (usually an antenna).

Referring to the above diagram, the transmitter (TX) and antenna (ANT) terminals connect via an internal transmission line.

The diodes convert the magnitudes of the forward and reverse waves to the terminals FWD and REV, respectively, as DC voltages, which are smoothed by the capacitors.

For decades [2][3] radio operators have built and used SWR meters as a simple tuning and diagnostic tool.

Though called an SWR Meter, a low measured ratio indicates not only good match, but also clean A3, F3, or G3 emission without excessive harmonics nor spurious (out-of-channel) power.

Properly designed, a bridge circuit can not only indicate a match, but the degree of mismatch – making it possible to calculate the SWR.

[1]: 27‑03 An SWR meter does not measure the actual impedance of a load (the resistance and reactance), but only the mismatch ratio.

For accurate readings, the SWR meter itself must also match the line's impedance (typically 50 or 75 Ohms).