VSWR vs Return Loss

What is the difference between VSWR and Return Loss?

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- everything RF

Feb 21, 2019

VSWR (Voltage Standing Wave Ratio) and Return Loss both measures of the same parameter i.e., the signal reflected back in a transmission line.

When a signal is transmitted through a transmission line, some signal power is always reflected or returned to the source due to discontinuities in the transmission line. The measure of this reflected power is called Return Loss. The Return Loss is expressed in dB.

In an ideal transmission line where the load and source are perfectly matched, all the power input from the source is delivered to the load. However, practically this is not possible as the load and source cannot be perfectly matched, there is always some level of mismatch. This mismatch causes part of the signal to get reflected back from the load towards the source. The voltages and currents of the forward and reflected waves in the transmission line add or subtract at different points according to the phases, thus creating the standing waves. Voltage Standing Wave Ratio (VSWR) is defined as the ratio of the maximum to the minimum voltage on a loss-less line. It tells the severity of standing waves in a transmission line. Expressed as 3.0:1, 2.0:1. Higher is the value of VSWR higher is the mismatch. 

Perfectly Matched or Ideal VSWR is 1:1.

Complete Impedance Mismatch VSWR is ∞:1.

The Return Loss can be calculated from the VSWR and the VSWR can be calculated from the Return Loss using the equations below.