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 ""viswar""). 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, however this terminology has no physical relation to actual powers involved in transmission.The SWR can be measured with an instrument called an SWR meter. Since SWR is defined relative to the transmission line's characteristic impedance, the SWR meter must be constructed for that impedance; in practice most transmission lines used in these applications are coaxial cables with an impedance of either 50 or 75 ohms. Checking the SWR is a standard procedure in a radio station, for instance, to verify impedance matching of the antenna to the transmission line (and transmitter). Unlike connecting an impedance analyzer (or ""impedance bridge"") directly to the antenna (or other load), the SWR does not measure the actual impedance of the load, but quantifies the magnitude of the impedance mismatch just performing a measurement on the transmitter side of the transmission line.