What is VSWR?

What is VSWR or Voltage Standing Wave Ratio?

1 Answer
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- everything RF

Jan 10, 2021

Voltage Standing Wave Ratio (VSWR) is defined as the ratio between the incident and reflected wave in an RF component or system. It determines the efficiency of power transmission from a source to the load through a transmission line (or cable). The minimum value of VSWR can be 1:1 (or simply 1), which is an ideal condition where 100% of power from source is absorbed by the load. However, in real-world applications, VSWR is rarely found to be 1:1 and systems are designed to keep the VSWR as close to unity as possible.

In an ideal RF transmission system, the impedance of source and transmission line (or cable) must be perfectly matched with the impedance of the connected load. VSWR can be defined as the numerical measure of how well the impedances of these integral components are matched.

VSWR Calculation

There is a mathematical equation to accurately calculate the VSWR of an RF system.

As it can be seen from the above equation, VSWR is always a real and positive number with a minimum value of 1.

In the ideal case where the source, transmission line and load all have an impedance of 50 Ω the VSWR will be 1:1 (perfectly matched).

A higher VSWR indicates more reflected power i.e., reduced efficiency in power delivered. In real-world RF components, a VSWR of lower than 1.5:1 is considered to be good, however, VSWR values can go up to 3:1 or higher depending on parameters such as operational frequency, bandwidth, power handling, etc. VSWR is a very important parameter in RF transmission systems where a high VSWR can reduce the power delivered to an antenna or system significantly. This can lead to reduced range, heating of cables, damaged amplifiers, etc.

Click here to see a Table of corresponding VSWR and Return Loss values.

Click here to use the online VSWR calculator.

Shawn  Jun 28, 2022

Thought this was a terrific short tutorial on VSWR!