What is VSWR?

VSWR (voltage standing wave ratio, sometimes called vertical standing wave ratio) is a measure of the effective power of a wireless signal as it passes through a power source, a transmission line, and finally into a load (e.g., the output of a power amplifier, through a transmission line, and finally to an antenna). For an ideal system, the energy transmitted is 100%, requiring precise matching between the source impedance, the characteristic impedance of the transmission line and other connectors, and the load impedance. Since there is no interference in an ideal transmission process, the AC voltage of the signal remains the same at both ends. In a real system, impedance mismatch will cause some power to be reflected in the direction of the signal source (like an echo). Reflections cause destructive interference, producing voltage peaks and troughs at different times and distances along the transmission line.

VSWR is used to measure the change in voltage and is the ratio of the highest voltage to the lowest voltage on the transmission line. Since the voltage remains unchanged in an ideal system, the corresponding VSWR is 1:1. When reflection occurs, the voltage changes and the VSWR increases - for example: 1.2:1 or 2:1.

VSWR is the voltage ratio on the transmission line:

VSWR = |V(max)|/|V(min)|

Where V(max) is the maximum value of the signal voltage on the transmission line, and V(min) is the minimum value of the signal voltage on the transmission line.

It can also be calculated using impedance:

VSWR = (1 Γ)/(1-Γ)

Where Γ is the voltage reflection coefficient near the load end, determined by the load impedance (ZL) and the source impedance (Zo):

Γ = (ZL-Zo)/(ZL Zo)

If the load is perfectly matched to the transmission line, Γ = 0, VSWR = 1:1