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, impedance mismatch will cause some of the power to be reflected toward the source (like an echo). 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 reflection occurs, the voltage changes and the VSWR increases -- for example: 1.2:1 or 2:1.