VSWR (Voltage Standing Wave Ratio) is a measure of how well an antenna is matched to the cable and radio that feed it. VSWR is written as a ratio, such as 1.5:1, where 1:1 is a perfect match and every higher figure means more of the signal reflects back towards the radio instead of being radiated.

VSWR takes its name from what a mismatch does inside the cable. When part of the signal reflects back, the outgoing and reflected waves combine into a standing wave, with points of higher and lower voltage along the cable, and VSWR is the ratio of the highest voltage to the lowest. Cellular antennas and cables are designed around an impedance of 50 ohms, and a mismatch occurs when the antenna does not present 50 ohms at the frequency in use.

VSWR converts directly into the share of power reflected. A VSWR of 1.5:1 reflects 4% of the power, 2:1 reflects about 11% and 3:1 reflects 25%. The signal lost to reflection at 2:1 is about 0.5 dB, which is small next to the loss in a long cable run, while at 3:1 it rises to about 1.25 dB.

VSWR varies with frequency, so antenna datasheets quote a maximum VSWR for each band, and a wideband cellular antenna is often better matched on some bands than others. A well-matched cellular antenna typically achieves 2:1 or better across its main bands. A figure above 3:1 on a band points to a poor match on that band.

A poor VSWR on an installed system usually points to a fault rather than the antenna design: a damaged or water-filled cable, a loose or corroded connector, an antenna used outside its frequency range, or a ground-plane-dependent antenna mounted without a ground plane. VSWR describes the same match as return loss, which expresses it as a figure in dB.