AIS fault VSWR

Last night while sailing down to Martinique, an alarm came from the AIS: ”External fault on AIS unit: Antenna VSWR fault
It’s night and I’m alone on my watch and only have the possibility to check that the antennas are still there and that the cables are connected. There has been no alarm about missing GPS position from AIS, so I guess it is the VHF antenna that is the problem. If that is the case, it would reduce the reach of our AIS signal so we may not be visible on other AIS receivers and we couldn’t receive AIS signals from other boats.

After arriving at Martinique, I set about trying to fix the error myself. First I googled what VSWR means, Voltage Standing Wave Ratio.

Hmm, the alarm is given when the VSWR value is too high. There is nothing about what can cause high VSWR. Googles further and learns that the value gets high from a poorly tuned antenna.

Hmm, why is the antenna all of a sudden poorly tuned? Google even more, the wrong antenna cable may be a cause, but not on Sally. Finally, I find that VSWR can be high if the cable is off. Finally something I understand how it looks and how to fix it. It does also match my initial analysis that there is some problem with the antenna cable.

VSWR values and proAIS2

To see what the VSWR value is, I use the ProAIS2 program that I downloaded from the internet and installed on a PC. The PC is connected to the AIS with a USB cable. With that program, I can see the VSWR for the VHF antenna connected to the AIS. The program shows that VSWR is 10.2: 1. To find out more about VSWR, check out the Antenna Theory – VSWR page, which has a detailed description of VSWR.

When connecting the USB cable between the PC and the AIS, it is best to have the AIS turned off. The AIS will get power through the USB cable and can receive AIS signals. When you have connected and received MMSI, switch on the power to AIS so that the transmitter starts. To see what VSWR value you have, you need to wait until your AIS has sent out a position report, this is when the VSWR is measured.

VSWR reading 10.2:1

With ProAIS2 you can also set some other parameters in AIS, such as where the GPS antenna is located on the boat or what GPS satellites are used.

I check the cable joints on the antenna cable and find the problem in the joint of the mast. It’s a joint that has broken apart and it’s just to put it back together. I see that the joint of the VHF antenna is loose too.

The problematic joint

When I connect with proAIS2 after the repair, VSWR is 1.4: 1, and the alarm is gone. Nice feeling 👍😃

OK VSWR-value after fix
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