VSWR warnings on AIS

Poey50

Poey50

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Over winter I changed the VHF aerial to a Metz Manta and also upgraded the cable to RJ8X. It's made a good difference to reception and transmission also seems fine. However I'm now getting VSWR error messages from my Raymarine 650 AIS. I can receive AIS messages with no obvious problem but the error comes when trying to transmit position. I don't have access to a VSWR meter but the Pro-AIS software shows a VSWR reading of 42.5:1. That would suggest a short except for the otherwise well functioning VHF. It's no different with the aerial plugged straight into the AIS so it's not the splitter. I recently updated the AIS software but that also doesn't seem to have helped. Any thoughts?

Edit: Salty John was helpful and suggested trying another aerial. Just tried the emergency aerial and the AIS is all working fine. So must be the new aerial. I've got conductivity between core and outer sheath measured at the radio connector end which I assume is a short. Puzzled that it seems fine on VHF.
 
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The AIS units usually report quite well when there is a problem with the antenna, as you have found.

Easiest quick change is to cut off and replace the plug that you have at the transmitter end of the feeder cable. Did you make any changes at the through deck area?

Also, once you have cut the plug off you'll be able to double check resistance across both the antenna feed and also of the plug you have just cut off. This might narrow it down.

I do have a power and swr meter and pass through Chichester most days but it's unlikely to show anything different to what the AIS transponder shows.
Bear in mind, plugging the laptop into the AIS transponder with usb cable to check/configure can change the swr ratio that the unit reports so take the actual value with a pinch of salt if this is the way you are connecting to read the result.

I assume the AIS is alerting to the fact and then switching off future transmissions.
 
The Metz antenna will show a short to a multimeter, it's internally grounded. You can only check your connector isn't shorted by disconnecting at the antenna. Alternatively, as someone else has suggested, cut off the connector and test it. This will cost you a PL259, but it's a lot cheaper than a VSWR meter. If the plug is shorted I'm surprised the radio is working, but could be just a single strand of the centre conductor bent back (very common).
To have such a massive VSWR reading and the radio working indicates something wrong at the AIS end.
 
When you say the VHF radio is working, what do you mean?
Most radio transmitters will self limit the output power if they detect a high SWR. And you'd possibly not notice much difference in receive. So it could be that your VHF is only putting out a watt (when set to 25W) due to self limiting. Can you get heard by fairly distant stations? If only local are hearing you then this might be a possibility.
Testing for short circuit with an ohm-meter on coax to an antenna can be misleading. Certainly finding an apparent short circuit at the radio end does not necessarily mean a problem, it depends on the antenna type. You're measuring the DC continuity doing that. But your transmitter doesn't output DC.
An SWR meter between the VHF and the antenna coax, with the antenna connected and excluding the AIS system and its splitter might tell more, but you need an SWR meter.
A crude approximation might be to monitor the radios current in the 12V line on transmit (not on Ch 16) at full power 25W. It might be 4-5 amps. If its small, say <1A, suspect high SWR.
Another way, is try to contact a distant station (as above).
Otherwise borrow a meter.
 
Geoff thanks .. on 25 watts it's reading +2 amps above standby.

John - I've been up the mast so both ends of cable could be disconnected and there is a short as you first predicted. I can only think that the better reception I definitely have with the new aerial and cable is relative to the terrible one it replaced.

I don't have a spare connector but I'm hoping its the lower one. The Law of Sod hasn't been kind recently so hopefully my luck will change.
 
I wasn't going to do this as I don't have a spare PL259 on board. But the suspense was killing me (and I'm close to base and have a handheld) so I cut off the lower one and it is the culprit. So that should be an easy fix.

Thanks all.
 
Poey50 said:
I wasn't going to do this as I don't have a spare PL259 on board. But the suspense was killing me (and I'm close to base and have a handheld) so I cut off the lower one and it is the culprit. So that should be an easy fix.

Thanks all.
Click to expand...

Nice find.... Can't you unsolder the plug you've cut off? Although as you say, easier to use a new plug.
 
Just came back on to say that I managed to unsolder the old plug .. great minds...

All working perfectly now with the Pro-AIS diagnostic software showing a VSWR reading of 1.1:1.

Total result!
 
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