One or two antennas for AIS and VHF?

[Airassmith]

If I fit a dedicated antenna for an AIS unit can it be right next to the VHF antenna at the top of the mast ? Or is it better to use a splitter and use the AIS receiver and VHF transceiver with the single VHF antenna I have now?
Dave

 

[Boomshanka]

From memory of the 'instruction manual' (sheet of A4) supplied with the NASA AIS engine, there needs to be at least 3 metres between the AIS aerial and VHF. I've gone for a stub aerial on the pushpit for AIS... this forum has covered it in depth and it seems to be okay.

 

[lw395]

Best not to put the two antennas too close together, particularly side by side, they may couple and put a lot of power into the receiver of the AIS. Also you could get a radiation pattern with nulls in some directions.
My own solution is to put a second antenna about 12 feet up the backstay for the AIS, this has worked ok, although it does cut off before the VHF when you're up a river for instance.
A stubby helical on a post or even pushpit would probably be nearly as good, I'm inclined to avoid encumbring the VHF with any splitters, filters etc unless I knew them to be pretty low loss and very reliable!
My experience is with the NASA AIS 'radar' btw.
Hope that helps,
Chris

 

[SkipperMarv]

depends on the unit, some like the Raymarine unit use only one aerial

 

[Airassmith]

So the VHF on the masthead and the AIS on the lower spreaders might be the best way to go perhaps?
Thanks for the help.
dave

 

[Anonymous]

[ QUOTE ]
So the VHF on the masthead and the AIS on the lower spreaders might be the best way to go perhaps?

[/ QUOTE ]How far away do you want to see the shipping? Anything more than 12nm is pretty pointless for collision avoidance so you can put the AIS onto the cabin roof, pushpit, or wherever you like - remember, ships are quite tall and their antennae are high.

 

[cnh]

A 'stub aerial' - can you point me in the direction of the sort of thing you're talking about?

Nicholas Hill

 

[Gunfleet]

Salty John who posts on here does an antenna suitable for a pushpit. He's also generous with advice. The advantage of having it on your rail is you can use it as a vhf antenna if your mast falls down or if your masthead one simply stops working (which happened to me once)

 

[William_H]

Yes definitely fit another antenna on the push pit. And make sure you can use it for the main VHF in case of mast loss.

I use a homemade VHF antenna on the push pit with great success.
It is made from a piece of coax RG58 5mm diameter stuff. With 17.3 inches of outer insulation and earth braid removed. I then got some outer screen braid from RG8 coax cable (10mm diameter) again 17.3 inches long. It is soldered to the end of the main coax braid and is fed over the outside of the coax. So you have an antenna of 17.3 inches of exposed inner conductor with below that 17.3 inches of extra braid below on the outside to make a ground plane. Heat shrink plastic over the extra braid would be good. The lot is fitted inside some plastic tubing and clamped to the push pit. As high as possible but at least with the bottom of the extra braid above the rail.
The cable needs to run to the radio with a suitable plug connected.
Give it a try olewill

 

[EDM]

Two antennas are best.

One on the mast the other on the pushpit or coach-house.
You could make your own antenna for AIS, but best bet is to buy a backup VHF antenna and use that - AIS after all is at the top end of the Marine VHF channel allocation.

Should you loose your mast, and need help - finding out your home made antenna just blew up your VHF radio due to a very poor SWR will make your bad day simply worse.

If you are playing with your AIS receiver at home, you can make a simple antenna from a meta coat hanger, (42cms long each arm) onto a short coax run. Tape it on a window facing the sea - that should amaze the wife, and kids... but afloat I would use something purpose built.

Regards

Tim

 

[William_H]

In actual fact this style of home made antenna is far more reliable because there is no connection at the antenna. That is the cable is the antenna. So long term reliability is good. The VSWR you will find is excellent at mid band. If you want it to tune more accurately at top of band make both dimensions 17 inches. olewillk