Voltage standing wave ratio

Voltage standing wave ratio (VSWR): In a transmission line, the ratio of maximum to minimum voltage in a standing wave pattern.

Note: The VSWR is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that which corresponds to a perfect impedance match, is unity (exactly 1.0).

Source: from Federal Standard 1037C and from MIL-STD-188

The VSWR is also defined as the ratio of maximum voltage to minimum voltage in a transmission medium.

 VSWR =  Vmax/Vmin
 Where,  Vmax = Voltage level of the transmitted wave
         Vmin = Voltage level of the reflected wave

If the characteristic impedance of the medium is equal to load impedance then VSWR is equals to 1. If the characteristic impedance (Z₀) is less than the load impedance (Z_l) then VSWR is greater than 1.

See also: standing wave ratio (SWR)