Measuring and Monitoring Voltage Standing Wave Ratio (VSWR / SWR)

Voltage Standing Wave Ratio (VSWR or SWR) is an important stat for transmitter sites that should be continuously measured and monitored to be able to see – and frequently preempt – a problem before it has a significant impact.

VSWR / SWR is worth paying attention to on a wireless transmitter site as it:

  • Is a good measure of the overall health of the antenna system
  • Can highlight degraded system performance – coverage, voice quality, data throughput, etc.
  • Helps avoid damage and failure to transmitters and repeaters from high VSWR
  • Indicates if the system is performing as designed and intended

In an ideal system, all the RF power is transmitted through based on an exact match of impedances from the transmitter to the cable and antenna. The signal’s AC voltage sees no interference and no power is reflected. Real life systems have unexpected impedances from simple items like connectors or bigger issues like a damaged antenna which cause power to be reflected. VSWR is a ratio calculated based on the forward and reflected power, with the ideal system having a VSWR of 1.0 (or 1:1).

VSWR can have a significant impact on through power and continuous monitoring allows trends to be seen. As shown on this graph for example, monitoring may allow you to identify when VSWR climbs to just 2.0 which means 89% of power is still getting through and users may not yet be significantly impacted. Without VSWR monitoring, the first time you hear of the problem may be from users when VSWR has reached 4.5 with only 59% of power getting through.

VSWR Chart

The most common way that VSWR is measured is when a technician is on-site with test equipment. This is typically at the time of the initial install, when troubleshooting for a problem, or during an annual preventative maintenance check. This on-site approach usually interrupts the normal operation of the transmitter and its users during the test. Truck rolls are expensive and don’t provide continuous trending.

VSWR is important and should be monitored on a continuous basis for both ease and trending. Fortunately newer technology has brought fresh affordable options to remotely measure and monitor VSWR. There are two primary ways:

  • RF Sensor – a sensor is permanently placed in-line between the transmitter and antenna. These sensors measure forward and reflected power and thus VSWR can be calculated.
  • Digital Transmitters – newer digital transmitters have the built-in ability to measure VSWR. For example, Mototrbo repeaters from Motorola Solutions have onboard Repeater Diagnostics and Control (RDAC) which measures a variety of key operational statistics including VSWR.

Our RimotRF remote monitoring service for wireless transmitters and sites can continuously measure and monitor VSWR in either of these two ways:

  • Our RF sensor can securely measure and report forward power and VSWR for transmitters from 20MHz to 1GHz, analog or digital, across most equipment manufacturers
  • As a solution partner with Motorola Solutions, our Mototrbo repeater digital interface allows us to capture and report VSWR as measured by the Motorola RDAC diagnostics on the repeater.

With RimotRF, VSWR and many other parameters about the site and transmitters can be viewed remotely on a desktop or smartphone, and alert notifications sent when performance thresholds are exceeded.

Contact us to discuss how RimotRF can securely measure and monitor VSWR / SWR for your systems.

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