Standing Wave Ratio (SWR)

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The Standing wave ratio is often measured in terms of the Voltage Standing Wave Ratio (VSWR). Put simply, this is the ratio of the maximum to the minimum voltage of a standing wave (which is the instantaneous sum of incident and reflected waves). Ideal is a figure of 1:1 which means that 100% of the incoming signal passed through the component without any reflection. In that case, there would be no standing wave. A 2:1 VSWR (or mismatch) means that 12% of the incoming signal was reflected.

When a transmission line is terminated with an impedance, Zx, that is not equal to the characteristic impedance of the source and the transmission line, Zo, not all of the incident power is absorbed by the termination. Part of the power is reflected back to the generator so that phase addition and subtraction of the incident and reflected waves create a standing wave pattern on the transmission line.