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VSWR Explained

AIS Transponders are generally very reliable pieces of kit that can work for many years, transmitting your position and receiving the positions of AIS equipped vessels within VHF range of your boat.

There are basically three things that an AIS Transponder needs in order to work correctly and an easy way to remember them is the abbreviation VGA, where….

V  =  Voltage, the transponder must have a good 12v DC supply voltage

G  =  GPS, the transponder’s GPS must be getting a good position fix

A  =  Antenna, the AIS/VHF antenna must be in good condition and well matched to the transponder

The Voltage can be checked either by connecting a Multimeter (set to measure DC Volts) across the Red and Black power wires of the transponder or even more easily by running the proAIS2 software and checking the value shown in the middle of the Diagnostics page (see image above).

The GPS reception can also be checked using proAIS2, by going to the Global Navigation Satellite System (GNSS) status tab and there you will see a good “bar graph” display of the satellite signals (see image below).

Checking the VHF/AIS Antenna, is the most difficult test and the one that we get the most technical support calls about, so we thought a blog post on the subject would help.

It is surprisingly common for yacht owners to install an AIS transponder with an antenna splitter and then find that their “old faithful” VHF antenna at the top of the mast is actually not as good as they thought it was. The reason for this, is that the top of the mast is a pretty hostile place and over time, the antenna will suffer high levels of salt water and UV exposure, coupled with large temperature swings, vibration, G forces, etc. all of which start to degrade the materials and cabling. Add this to the occasional stepping of the mast, with possibly a through-deck connection or cable join inside the hull and you can see that there are lots of possibilities for the antenna to not be in tip-top condition.

All AIS transponders perform a continuous series of self tests, including a VHF antenna test and this is often the first time, in many years, that the VHF antenna at the top of the mast has been tested, so it is not unusual for some to fail the test. The reality is, that a VHF radio or AIS will happily get some level of reception even with just a wire connected to it and so you can appear to be receiving AIS data but your transmission is poor, due to an old/degraded VHF antenna that has a high Voltage Standing Wave Ratio (VSWR).

The VSWR of an AIS transponder, is a measure of the level of standing waves present in the antenna cable (feeder). Standing waves are the signals/power that are not radiated in to the air, by the antenna (load), but reflected back down the cable to the transponder (source). In an ideal world, all of the power sent to the antenna would be transmitted through the air to other vessels, but this only happens if the impedance (AC resistance) of the source, feeder and load are identical (50 ohms).

This perfect matching of the impedances would result in a VSWR of 1:1 but in “real life” pleasure boat installations, it is more likely to be between 1.1:1 and 2.5:1. In the diagnostic page of our proAIS2 software, the measured VSWR is displayed and updated every time the transponder transmits. By keeping an eye on the Transmit Counter you can see when a transmission takes place and you will probably see a slight change (in the decimals) each time the VSWR is measured, which is normal. If you see a bigger fluctuation, this is indicative of a poor connection or water ingress in the antenna cable/connections.

If the measured VSWR goes above 5:1, the RED Status LED will illuminate and a VSWR alarm displayed in proAIS2 (see top image).

Understanding the importance of having a good VHF antenna, with a low VSWR to get the best possible AIS transponder performance (transmit range) is key and we hope this article helps our customers have a better appreciation of this technology.


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