VSWR (Voltage Standing Wave Ratio, sometimes referred to as the vertical standing wave ratio) is a measure of the effective transmission power of a wireless signal through a power source, a transmission line, and ultimately into a load (eg, a power amplifier output through a transmission line, ultimately to an antenna). For an ideal system, the transmission energy is 100%, which requires an exact match between the source impedance, the characteristic impedance of the transmission line and other connectors, and the load impedance. Since there is no interference in the ideal transmission process, the AC voltage of the signal remains the same at both ends. In practical systems, partial impedance will be reflected toward the source (as an echo) due to impedance mismatch. The reflection causes destructive interference, and voltage peaks and troughs are generated 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 in the ideal system remains the same, the corresponding VSWR is 1:1. When a 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 is also possible to use impedance calculations:

\(VSWR = (1 Γ)/(1-Γ)\)

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

\(Γ = (ZL-Zo)/(ZL Zo)\)

If the load exactly matches the transmission line, \(Γ = 0\), \(VSWR = 1:1\)

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