VSWR (Voltage Standing Wave Ratio)

Voltage Standing Wave Ratio (VSWR) or sometimes just Standing Wave Ratio (SWR) is a measure of how well matched an antenna is (in terms of impedance) to the transmission line it connects to. If the reflection coefficient is given by , then the VSWR is defined as: The VSWR is always a real and positive number for passive (non-powered) loads, as in generally the case. The smaller the VSWR is, the better the antenna is matched to the transmission line and the more power is delivered to the antenna. The minimum VSWR=1.0, in which case none of the power is reflected, which is the ideal case. Often antennas must satisfy a bandwidth requirement that is given in terms of VSWR. For instance, an antenna might claim to operate from 100-200 MHz with VSWR<3. This implies that the VSWR is <3 over the specified frequency range; further it implies the power reflected is less than half (<0.5) over the quoted frequency range. Physically, the VSWR is determined from the voltage measured along a transmission line leading to an antenna. It is the ratio of the peak amplitude of a standing wave to the minimum amplitude of a standing wave, as seen in the following Figure: In industry, VSWR is sometimes pronounced "viz-wer".