AIS VHF Antenna

[Adonnante]

My next project is to fit an AIS transponder. The VHF antenna needs to be low as I'm not interested in targets over 6 miles. Does a helical antenna need to be vertical for best reception and transmission and would foam composite decks reduce the signal? I've always had my GPS antennas inside the boat and wondered if I can keep the VHF antenna safe and dry inside the boat.

Your thoughts would be most appreciated.

Peter.

 

[st599]

My next project is to fit an AIS transponder. The VHF antenna needs to be low as I'm not interested in targets over 6 miles. Does a helical antenna need to be vertical for best reception and transmission and would foam composite decks reduce the signal? I've always had my GPS antennas inside the boat and wondered if I can keep the VHF antenna safe and dry inside the boat.

Your thoughts would be most appreciated.

Peter.

Whips are polarised. If you're transmitting vertically and receiving horizontally you'll lose maybe 30 dB.

 

[Watson47]

My next project is to fit an AIS transponder. The VHF antenna needs to be low as I'm not interested in targets over 6 miles. Does a helical antenna need to be vertical for best reception and transmission and would foam composite decks reduce the signal? I've always had my GPS antennas inside the boat and wondered if I can keep the VHF antenna safe and dry inside the boat.

Your thoughts would be most appreciated.

Peter.

This might give you another option as it's compact;

http://www.saltyjohn.co.uk/metzmantavhfantennaaerial.htm

I am considering one of these and your post remined me to get on with it .. ! Salty John is a member here I believe and will no doubt give good advice.

 

[NeilE]

I have fitted an AIS Transponder and mounted a whip antenna on my "aerial farm" (basically a T bar on a pole on the stern). What I fitted was a specially tuned antenna for the AIS channels. I didn't believe it would be any better than a cheap helical "rubber duck" which it replaced when I had an AIS receiver.

I was entirely wrong - it was a revelation giving targets minimum 10 miles away and routinely for large ships 30 miles. On one famous day sitting a port near the Vilaine river on the west coast of France I was watching the shipping going round Penzance - but that was "sporadic E" reception and quite unusual.

So my recommendation is a special antenna for AIS. You have to mount an additional GPS antenna too, but I managed to put that just under the headlining. It works fine

Kindest

Neil

 

[Salty John]

Your VHF antenna should be vertical, as others have said.
It must be at least 1 meter away from the radio set.
It should be located away from vertically oriented metal structures; locating it below is not a good idea because there will always be fixed obstructions as well as moving obstructions (people). Having said that, it will, at times, receive well and give you the opportunity to call me clueless!
A standard VHF antenna covers the AIS frequencies. An 'AIS' antenna or an 'AIS optimised' antenna is marketing speak. I'm talking on a directly comparative basis - some will have replaced one make of antenna and antenna location with another antenna and antenna location and will choose to credit any resultant improvement to the choice of an 'AIS' antenna. On a directly comparitive basis you will be unable to tell the difference between an antenna centered on ch 16 or one centered on one of the AIS frequencies.
VHF is line-of-sight, so try to give the antenna the best view of where it's signal is coming from or going to, with the minimum of obstructions.
I think the OP would be best served by a VHF antenna mounted on the pushpit. It will give 10 to 15 miles range and can serve as an emergency radio antenna in the event of loss of the masthead antenna.

 

[lw395]

Best to move the GPS aerial outdoors too, if you want the best assurance that it will keep working properly in adverse conditions.

 

[Adonnante]

Many thanks for the superb response, and John I could never call you 'clueless' after such a full and clear answer. The pullpit it will be; the next problem will be routing the antenna cable, why can't boat builders put conduits in?

Cheers

Peter.