https://en.wikipedia.org/wiki/Standing_wave_ratio

In __radio engineering__ and __telecommunications__, **standing wave ratio** (**SWR**) is a measure of __impedance matching__ of __loads__ to the __characteristic impedance__ of a __transmission line__ or __waveguide__. Impedance mismatches result in __standing waves__ along the transmission line, and SWR is defined as the ratio of the partial __standing wave__‘s amplitude at an antinode (maximum) to the amplitude at a __node__ (minimum) along the line.

The SWR is usually thought of in terms of the maximum and minimum AC __voltages__ along the transmission line, thus called the **voltage standing wave ratio** or **VSWR** (sometimes pronounced “vizwar”^{[1]}^{[2]}). For example, the VSWR value 1.2:1 denotes an AC voltage due to standing waves along the transmission line reaching a peak value 1.2 times that of the minimum AC voltage along that line. The SWR can as well be defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line’s __currents__, __electric field strength__, or the magnetic field strength. Neglecting transmission line loss, these ratios are identical.

The **power standing wave ratio** (**PSWR**) is defined as the square of the VSWR,^{[3]} however this terminology has no physical relation to actual powers involved in transmission.

SWR is usually measured using a dedicated instrument called an __SWR meter__. Since SWR is a measure of the load impedance relative to the characteristic impedance of the transmission line in use (which together determine the __reflection coefficient__ as described __below__), a given SWR meter can only interpret the impedance it sees in terms of SWR if it has been designed for that particular characteristic impedance. In practice most transmission lines used in these applications are __coaxial cables__ with an impedance of either 50 or 75 __ohms__, so most SWR meters correspond to one of these.

Checking the SWR is a standard procedure in a radio station. Although the same information could be obtained by measuring the load’s impedance with an __impedance analyzer__ (or “impedance bridge”), the SWR meter is simpler and more robust for this purpose. By measuring the magnitude of the impedance mismatch at the transmitter output it reveals problems due to either the antenna or the transmission line.