Major disadvantage of Comar AIS Multi

BlueChip

BlueChip

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We've recently fitted an Comar AIS multi, this is the model with a aerial pass-through allowing use of ships VHF aerial for AIS reception.

What I have found is that the AIS introduces a lot of noise onto the VHF signal and its necessary to turn the squelch almost to the maximum setting on the VHF to get rid of it. This means that only the very strongest local VHF transmissions get through.

I contacted Comar tech support and they told me there is a small pre-amplifier inside the Multi to amplify the signal after the loss introduced in splitting the signal.
This amplifier inevitably introduces a raising of the noise level, the squelch circuit control adjusts the point where the noise stops coming from your loudspeaker, this has now moved due to the increased noise level.

I was playing around today with the receiver and it's quite definite that using the AIS with the ships VHF aerial results in a loss of all but the strongest VHF transmissions, this has significant safety implications and I'm suprised that it wasn't mentioned in recent reviews.

Has anyone else also found this?

I do carry an emergency VHF aerial, but I'll be fitting a dedicated aerial for the AIS next week.
 
Doesnt sound right this. The small amplifier will amplify the noise and also the signal in the same ratio so you shouldnt be losing the signal.

Suggest you send the unit back as faulty. If it isnt faulty but does this for a reason I cant think of, then its not fit for purpose.
 
I agree, while a dedicated additional antenna is our preferred route for AIS, the splitter should not introduce a limit to a critical piece of equipment.

This is also the first report of such interference we have come across.

For completeness, can you let us know which VHF and antenna you are adding the Comar to?
 
Bosun Higgs said:
Doesnt sound right this. The small amplifier will amplify the noise and also the signal in the same ratio so you shouldnt be losing the signal.

Suggest you send the unit back as faulty. If it isnt faulty but does this for a reason I cant think of, then its not fit for purpose.
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I find myself in complete concurrence with the Bosun. Its either faulty or not fit for purpose.
 
I have this unit too, and I cant say I have noticed any change in the VHF. Are you sure your wiring is ok, and you havent got a stray strand, or something.Or maybe something is indeed wrong with the unit.
 
Comar AIS

Interesting this, I've recently installed one of these aerial splitters with equally mixed results:

Most FM radio reception has gone and only the very strongest VHF signals get thro', but the AIS reception is superb.

Am going to do so more in the way of diagnostics (random plugging in of different things in different holes) prior to skipping it and getting a dedicated AIS aerial, which'll have to be mounted on the pushpit.
 
Bosun Higgs said:
Doesnt sound right this. The small amplifier will amplify the noise and also the signal in the same ratio so you shouldnt be losing the signal.

Suggest you send the unit back as faulty. If it isnt faulty but does this for a reason I cant think of, then its not fit for purpose.
Click to expand...

Wrong. Any amplifier will amplify the signal and noise AND add noise. This increase in noise is given in data sheets as the noise figure of the amplifier. A good amp may add 0.5 dB, a bad one 5 dB, but, unless your AIS runs at absolute zero, noise will be added.

This could easily push the receiver from good performance to bad performance.
 
Malthouse said:
I agree, while a dedicated additional antenna is our preferred route for AIS, the splitter should not introduce a limit to a critical piece of equipment.

This is also the first report of such interference we have come across.

For completeness, can you let us know which VHF and antenna you are adding the Comar to?
Click to expand...

The aerial on the mast was fitted by Westerly from new it's a Vtronix whip. The radio is an ICOM 603.


This is the exact reply I got from Comar tech support in response to my email about this problem:

"There is a reason for this, we have a small pre-amplifier inside the Multi to amplify the signal after the loss introduced in splitting the signal.
This amplifier inevitably introduces a raising of the noise level, your squelch circuit control adjusts the point where the noise stops coming from your loudspeaker, this has now moved due to the increased noise level.
Hopefully it should never reach a level where you cannot turn it off."

Now this reply doesnt exactly seem like this is an unknown problem to them and the last paragraph doesnt exactly fill me with confidence.

I subsequently emailed them back on the 19th of March, and said I was concerned about loss of weaker signals, but I never received a reply back from them.

There is really nothing that can be wired incorrectly, the aerial pass through uses a regular plug and socket, so nothing has changed there. Other than that just power and NMEA feed.
With everything in place if I turn the power off to the Multi the noise goes away, if I turn it on - the noise comes back.
 
I have the comar unit fitted to an Icom 603, I have not had any problems and there is no discern able difference in performance however, we did replace the antenna when the unit was fitted.

If your antenna is "original as fitted by westerly", I would think it will be of some age, I would look at the integrity of the antenna leads and connections first.
 
Deliberately picking up on this old thread as I have been planning to buy one of these units.

I am wondering what happens to the VHF signal if the Comar unit fails or is depowered. Does it fallback to a direct connection?

Now I am also concerned about this S/N ratio issue.

Has anybody any further news?
 
Don't know about the multi - we have the normal dual channel comar engine on a dedicated antenna and very happy with it ... never been happy with splitters of any sort .. hopefully it's an unjustified prejudice!
 
MoodyNick said:
I am wondering what happens to the VHF signal if the Comar unit fails or is depowered. Does it fallback to a direct connection?
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In this case, as soon as you start transmitting with your VHF you'll fry the receivers in the AIS receiver. After that it's anyone's guess what happens next. Ideally nothing and the VHF will still work, worst case the RF-amplifier is shorted out and is fried as well. You must make sure the power to the splitter is always present when the VHF is powered on.

It's a attractive solution to use a splitter in the effort to install but one that should be avoided if possible. As usual with these solutions it'll work under normal circumstances, but when the sh*t hits the fan these systems will always find a reason to fail.
 
I posted the original message with the problem. Since then Comar replaced the unit and I had the same problems with the new one. I had the VHF antenna and radio checked out and all are ok, so I never got to the bottom of the problem. I now use the Comar with a dedicated pushpit antenna and it works fine but that rather defeats the point of buying that particular model with the antenna sharing.
 
aluijten said:
In this case, as soon as you start transmitting with your VHF you'll fry the receivers in the AIS receiver.
Click to expand...

That depends on the design of the unit, which is what I am trying to find out.

I have always been concerned about separate splitters and avoided them. Now I am trying to avoid the hassle of a second antenna installation, so want to find out more.
 
BlueChip said:
I posted the original message with the problem. Since then Comar replaced the unit and I had the same problems with the new one. I had the VHF antenna and radio checked out and all are ok, so I never got to the bottom of the problem. I now use the Comar with a dedicated pushpit antenna and it works fine but that rather defeats the point of buying that particular model with the antenna sharing.
Click to expand...

Thanks BlueChip, that's a great help !!
 
AIS noise?

The AIS specs call for a dedicated antenna. All so called splitter solutions are NOT to be used. Why spend top $ on an AIS, save a few bucks on a separate antenna and compromise both AIS AND VHF.
If cost were the issue I'd rather use a dedicated coat hanger for the AIS than sharing an antenna.
 
I have a COMAR too and it saw us safely from Turkey to India.

The one piece of advice I was told when researching this subject was NEVER SPLIT THE VHF AERIAL, no matter what gizmos you are told will do the job.

With this in mind I ran two independent aerials for each AIS and VHF down my rigging and through the deck. I used self-amalgamating tape around the cables to the rigging and whilst it wasn't the prettiest solution I have never had a problem (actually, you barely notice it). Since we were about to embark on a long journey through busy waters it was a case of opting for a practical solution, and this was it.

Whilst you already know that separate aerials is ideal but not practical my advice is run separate aerials no matter how difficult or ugly they are to install.

Sorry if this isn't solid technical advice, it's just my personal experience. :)
 
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