Standing wave ratio

SWR of a vertical HB9XBG Antenna for the 40m-band as a function of frequency

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.

Voltage standing wave ratio (VSWR) (pronounced "vizwar"[1][2]) is the ratio of maximum to minimum voltage on a transmission line . For example, a VSWR of 1.2 means a peak voltage 1.2 times the minimum voltage along that line, if the line is at least one half wavelength long.

A SWR can be also 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 deprecated term has no direct physical relation to power actually 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 interpret the impedance it sees in terms of SWR only if it has been designed for the same particular characteristic impedance as the line. 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.

  1. ^ Knott, Eugene F.; Shaeffer, John F.; Tuley, Michael T. (2004). Radar cross section. SciTech Radar and Defense Series (2nd ed.). SciTech Publishing. p. 374. ISBN 978-1-891121-25-8.
  2. ^ Schaub, Keith B.; Kelly, Joe (2004). Production testing of RF and system-on-a-chip devices for wireless communications. Artech House microwave library. Artech House. p. 93. ISBN 978-1-58053-692-9.
  3. ^ Silver, Samuel (1984) [1949]. Microwave Antenna Theory and Design. IEE. p. 28. ISBN 0863410170.