What does an SWR meter measure to determine the SWR?

A Standing Wave Ratio (SWR) meter contains a directional coupler that samples the RF energy flowing through the transmission line. It separates and measures the voltage of the signal traveling from the transmitter to the antenna (forward voltage) and the voltage bouncing back from the antenna (reflected voltage). By comparing these two voltage values, the meter computes the SWR, giving the operator a clear picture of the impedance match. It does not measure radiated RF energy through the air, conductor temperature, or common-mode currents on the outside of the coax shield.