What is VSWR ?

This page describes - what is VSWR and mentions what is return loss and mentions difference between VSWR and return loss.

VSWR is the short form of Voltage Standing Wave Ratio. When a transmission line (cable) is terminated by an impedance that does not match the characteristic impedance of the transmission line, not all of the power is absorbed by the termination. Part of the power is reflected back down the transmission line. The forward (or incident) signal mixes with the reverse (or reflected) signal to cause a voltage standing wave pattern on the transmission line. The ratio of the maximum to minimum voltage is known as VSWR, or Voltage Standing Wave Ratio.

A VSWR of 1:1 means that there is no power being reflected back to the source. This is an ideal situation that rarely, if ever, is seen. In the real world, a VSWR of 1.2:1 (or simply 1.2) is considered excellent in most cases. In an EMC lab where many of the tests are very broadband in nature, a VSWR of 2.0 or higher is not uncommon. At a VSWR of 2.0, approximately 10% of the power is reflected back to the source. Not only does a high VSWR mean that power is being wasted, the reflected power can cause problems such as heating cables or causing amplifiers to fold-back.

There are ways to improve the VSWR of a system. One way is to use impedance matching devices where a change in impedance occurs. Baluns are used extensively in antennas to not only convert from balanced to unbalanced signals but also to match the impedance of the source to the antenna. It is common practice in EMC testing to include attenuators at any point where there is an impedance mismatch. One emissions standard, for instance, specifies using an attenuator at the connector of a biconical antenna since it has a high VSWR at some frequencies. One of the conducted immunity standards suggests using a 6dB pad at the input of the coupling device, which is commonly 150 ohms. Attenuators obviously cause power loss, but they reduce VSWR by providing an apparently better termination to a signal.

There are various ways of measuring and/or calculating VSWR. In the old days of open transmission lines, the voltage could be measured along the length of the line until the maximum and minimum values were found (which were 1/4 wavelength apart) hence the reference to Voltage Standing Wave Ratio. Thus, VSWR would be calculated by the following formula:

VSWR = Emax/Emin = Ei + Er / Ei - Er

Emax = Maximum measured voltage
Emin = Minimum measured voltage
Ei = incident wave amplitude in volts
Er = reflected wave amplitude in volts

What is relation between VSWR and return loss

Following equations are used to convert return loss to VSWR and VSWR to return loss.

VSWR = ( 1 + 10RL/20 ) / (10RL/20 - 1)

Return Loss = 20 * Log10 ( VSWR+1/VSWR-1)

VSWR and Return loss related links

Reflection Coefficient vs Return Loss vs VSWR
Return Loss to VSWR converter calculator

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