Understanding VSWR & Return Loss

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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.   

Return Loss

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 discontinuity could be the connection to a system, another transmission line or connector. The measure of this reflected power is called as return loss.  Return loss (RL) is the ratio of the reflected power to the incident power, in decibels (dB). 

In an RF system, when power is sent from the source to load, there are three main parameters to be considered: (1) incident power; (2) reflected power; and (3) power absorbed by the load (transmitted to the load). Incident power is independent of the transmission line (or cable) and load, it depends only on the configuration of the source and cannot be changed once transmitted from the source. Power absorbed by the load is essentially reflected power subtracted from incident power.

High Return Loss: A higher return loss indicates less power is reflected from the load. This is usually a desirable outcome.

Low Return Loss: A lower return loss indicates more power is reflected back from the load. This usually means that there is an impedance mismatch at the load.

For instance, a return loss of 20 dB translates to a VSWR of ~ 1.2:1, which means approximately 99% of total incident power is being transferred to the load while the remaining 1% gets reflected. In contrast, a return loss of 5 dB translates to a VSWR of ~ 3.5:1, which means 68% of the total incident power is being transferred to the load while the remaining 32% is getting reflected.

Return Loss vs VSWR

Both VSWR (Voltage Standing Wave Ratio) and Return Loss are measures of the amount of signal reflected by a connector. Return Loss is like VSWR and is generally preferred in the cable industry to a VSWR specification. The main difference between return loss and VSWR is that return loss is a logarithmic measurement, which makes it useful when displaying very small reflections whereas VSWR is a linear measurement and is useful to represent larger reflections.