Voltage Standing Wave Ratio (VSWR)

Any impedance mismatches along a transmission line causes partial reflection of the propagating signals. The impedance difference determines the magnitude of the reflection. The length of a mismatched section determines the lowest signal frequencies that reflect from the section. VSWR is a measure of that signal reflection.

With an incident sine wave into the switch module, some of the signal reflects down the line. This reflected wave interferes with the incident wave. VSWR is the ratio of maximum to minimum amplitude in the resulting interference wave, as shown in the following formula:

where p is the reflection coefficient

Reflections can also be represented as a logarithmic ratio of the reflected signal to the input signal. This ratio is called return loss:

RL (dB)	= 10 log (PIN/PREFLECTED)
	= 20 log (VIN/VREFLECTED)
	= -20 log (VREFLECTED/VIN)
	= -20 log |p|