rf - What is the standing wave ratio (SWR)?

In rf electronics, we often characterize a component or antenna by its standing wave ratio (SWR). What is the definition of this parameter and how does it affect a circuit?

The motivation for this question is that while answering another question, it would have been useful to be able to refer to a definition of SWR within the site in order to explain how the SWR is measured.

Answer

Any transmission line has a characteristic impedance, usually denoted Z₀. If a transmission line is terminated with impedances that match Z₀, a signal launched at one end will be completely absorbed at the other end, and no energy will be reflected back to the source. The voltage and/or current measured at any point along the line will be the same as any other point.

However, if a termination impedance is not matched to the transmission line, energy will be reflected back into the line, and this "reverse" signal will interfere with (add to or subtract from) the "forward" signal.

If the signal is a fixed-frequency sinewave, this interference will produce "standing waves" on the transmission line. This means that the measured voltage or measured current in the line will vary periodically with the distance from the impedance discontinuity. If the termination impedance is greater than Z₀, there will be a voltage maximum at that point; if it is less, there will be a current maximum there.

The definition of "standing wave ratio" (SWR) is the ratio between the maximum voltage (or current) found at any point along the line to the minimum value found at any other point along the line. Sometimes the term VSWR is used to explicitly denote the voltage ratio. The value of this ratio is directly related to the ratio of Z₀ to the termination impedance Z_T. Specifically,

SWR = Z_T/Z₀, if Z_T > Z₀

SWR = Z₀/Z_T, if Z_T < Z₀

When a component or antenna is characterized with an SWR measurement, this is always specified with respect to a particular nominal transmission line impedance (usually 50Ω or 75Ω, depending on the intended application). This is just another way of stating how close the impedance of the device is to the nominal value.