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Raymarine forum - [TG11] AIS 500 fault message while transmitting on VHF

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Full Version: [TG11] AIS 500 fault message while transmitting on VHF
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I have a new Axiom Pro 12 interfaced with an old Raymarine AIS 500 unit and a new ICOM M506 VHF. All work fine except when I transmit on the VHF I receive a warning on the Axiom "External AIS unit fault:Tx malfunction".

Sometimes I will get a VSWR fault message. I have checked the VSWR and it is good. The VHF and the AIS all work properly. The messages do not seem to correspond to anything not actually working, and are more of a nuisance than anything.

Is this something to do with using the old AIS 500 when I upgraded, or maybe I have something not configured properly? Thanks for any explanation.
Hello Berwick,

Do the AIS and VHF share an antenna via AIS500's internal splitter, or do they have separate antennas?
Is the antenna the same as the AIS has always been connected to, or has something changed there?
Was there a VHF connected to the AIS before the new ICOM?

A Tx Malfunction warning when transmitting on VHF suggests to me either that the AIS tried to transmit at the same time and was blocked because the VHF was controlling the internal splitter - if you're using it, with a single antenna - or a supply dip caused by the current being drawn by the VHF. Does the Tx Malfunction appear every time you transmit or only occasionally, and does it make a difference whether you transmit on low or high power from the VHF?

Re. the VSWR warning: did you check the VSWR at the AIS frequency range, ~162MHz? Antennas that are good at around Ch16 on ~156MHz can be very bad at 162MHz.

If in doubt, the ProAIS PC software can give more diagnostic info, give that a go.

Thanks for the response. Answers to some of your questions:

-The AIS does share an antenna via the splitter.
-It is the same antenna it has always been connected to- a 19' Shakespeare.
-When I bought the boat the AIS 500 was connected to an old Raymarine VHF which had the same problem.
-When I upgraded to the Axioms and ICOM, we changed out everything, but the AIS 500 unit.
-Still having the fault warnings, but only when transmitting.
-I checked the VSWR on all transmitting channels (01-88), and it is 1:1. I do not know how to check the AIS channel.
-The faults occur occasionally, and not on every transmission.
-The faults do occur when transmitting on low power as well as high power.

I will have to buy connectors to be able to hook the AIS 500 to the PC USB port on the boat. I assume that when I hook it up with the AIS software there is a diagnostics tab? Thanks for any guidance.
Attached are pictures of my computer screen while attached to the AIS. One shows the fault, the other is while not transmitting and the fault has been cleared. Hope this helps. Berwick
Hello Berwick,

If the AIS and VHF are both trying to transmit at once, the VHF wins (the internal splitter favours the VHF for safety reasons), which is I'm pretty sure why you're sometimes seeing the Tx errors when transmitting on the VHF. To avoid this you'd need to use separate antennas.

Re. the high VSWR warning: I am pretty sure your antenna VSWR is not really 1:1, especially not across the whole frequency band. That's unrealistically low (1:1 is the ideal, zero-loss situation), so I think that either your meter is leading you astray or transmission line loss in your cabling is masking a much higher VSWR (see continuouswave's excellent explanation of this here.) The screenshots of ProAIS you've posted show normal Tx Forward Power (transmitted by the AIS) but high Tx Reflected Power (what's reflected back from the antenna to the AIS.) The Reflected Power should be <10, whereas your images show 57 and 156(!). I think that your VSWR measurement is incorrect and that you actually have (probably) a poor and intermittent contact in your antenna cabling which is giving high to very high VSWR in fact.

Thanks. Admittedly my meter is ancient. I will buy a new meter and repeat the test, but it looks like a seperate smaller antenna may be the answer.
i used another meter and the vswr readings are all within the acceptable range while transmitting on the vhf. I don't know how to check on the AIS frequency. I will try a smaller separate AIS antenna and see if that cures the issue. Thanks
No problem, good luck with it.

I finally got back to the boat, and installed a separate AIS antenna. No more issues. The VHF works great on its antenna and the AIS does not fault at all anymore. It seems the problem is solved. Thanks for your help!
I'm glad to hear the problem's solved, thanks for the feedback.

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