Voltage standing wave ratio

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.

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