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Old 19-01-2018, 10:21   #1
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Best practice for VHF & AIS antenna placement

The received wisdom seems to be that the best VHF antenna is a 1/2 wave, center-fed dipole placed at the top of the mast. These don't require a ground plane and have modest gain so that they are less affected by heeling than a higher-gain antenna might be.

What is the best type and placement for a separate AIS antenna on boats with but a single mast?
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Old 19-01-2018, 12:32   #2
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Re: Best practice for VHF & AIS antenna placement

If the VHF (regardless of type) antenna is at the masthead, the best place for a second VHF antenna (which is what the AIS really is) would be below the VHF antenna, so there is very little interaction between the two. In practice that probably means mounting it on the spreader, compromising the protected position with the slight interference from the proximity of the mast and rigging.
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Old 19-01-2018, 13:44   #3
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Best practice for VHF & AIS antenna placement

If your going to use a separate AIS antenna, consider Vesperís dual antenna that is supposed to work for both. That way you would have a back up VHF if needed, although I believe a regular AIS antenna would work well enough as a VHF antenna.
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Old 19-01-2018, 17:32   #4
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Re: Best practice for VHF & AIS antenna placement

Our AIS antenna is mounted on the Bimini top rail. It works decent at that height. It also doubles as a backup VHF antenna should we loose the mast.
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Old 20-01-2018, 10:32   #5
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Re: Best practice for VHF & AIS antenna placement

Is there a specific reason you want a separate antenna? We have a splitter under the nav station for VHF, AIS, and broadcast Radio. It works extremely well and we have very long range AIS and VHF operation in practice. There have been no interoperation issues in the 4 years we have had the boat with this setup.
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Old 20-01-2018, 11:05   #6
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Re: Best practice for VHF & AIS antenna placement

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Originally Posted by svmobert View Post
Is there a specific reason you want a separate antenna? We have a splitter under the nav station for VHF, AIS, and broadcast Radio. It works extremely well and we have very long range AIS and VHF operation in practice. There have been no interoperation issues in the 4 years we have had the boat with this setup.
I just had new instruments installed and they did the same thing. A splitter for one antenna to both AIS and the VHS radio. While I was dubious at first, I haven't had any issues. Thanks for the testimonial. I feel better about the config now!
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Old 20-01-2018, 11:32   #7
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Re: Best practice for VHF & AIS antenna placement

I would prefer a separate AIS antenna to a splitter as so VHF use does not interrupts AIS.
As the height of the AIS antenna is not very important you may intall that antenna on the Bimini if you have one ore even on the pushpit. My AIS antenna is about 12 feet above the waterline and I received signal from lighthouses 600 NM away.
Good luck
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Old 20-01-2018, 13:59   #8
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Re: Best practice for VHF & AIS antenna placement

Unless you talk constantly a splitter makes more sense than 2 antennas.
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Old 20-01-2018, 14:45   #9
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Best practice for VHF & AIS antenna placement

Supposedly a Vesper splitter and one antenna will outperform individual antennas.
https://www2.vespermarine.com/antenna-splitter-sp160
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Old 20-01-2018, 15:55   #10
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Re: Best practice for VHF & AIS antenna placement

You asked about placement of a separate AIS antenna, which is what we also considered- and we have 2 masts...

Despite the simplicity of putting one of each antenna on each mast, we opted for redundancy [we have 2 fixed VHF radios- one for back-up] and installed a Vesper dual tuned AIS/VHF antenna on each mast head [1 wired to each VHF radio] and a single Vesper splitter. [The splitter can be quickly rewired in the event one antenna and/or connection failed.]

Best wishes deciding what is ideal for your set-up.

Cheers! Bill

This is a very functional set-up for us.
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Old 20-01-2018, 16:22   #11
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Re: Best practice for VHF & AIS antenna placement

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Originally Posted by a64pilot View Post
Supposedly a Vesper splitter and one antenna will outperform individual antennas.
https://www2.vespermarine.com/antenna-splitter-sp160
And out-cost!

My AIS antenna was <$60 plus ~$30 in coax.
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Old 21-01-2018, 11:34   #12
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Re: Best practice for VHF &amp; AIS antenna placement

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And out-cost!

My AIS antenna was <$60 plus ~$30 in coax.
That is a significant savings, DotDun.

Can you provide the specifics for the rest of us? [Source, product specifications, any reviews or testing results, etc.]

Thanks! Bill
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Old 21-01-2018, 11:34   #13
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Re: Best practice for VHF &amp; AIS antenna placement

Quote:
Originally Posted by a64pilot View Post
Supposedly a Vesper splitter and one antenna will outperform individual antennas.
https://www2.vespermarine.com/antenna-splitter-sp160
Supposedly according to whom?
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Old 21-01-2018, 11:44   #14
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Re: Best practice for VHF &amp; AIS antenna placement

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That is a significant savings, DotDun.

Can you provide the specifics for the rest of us? [Source, product specifications, any reviews or testing results, etc.]

Thanks! Bill
https://www.defender.com/product.jsp...062&id=1143468

plus a piece of coax and splice
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Old 21-01-2018, 11:50   #15
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Re: Best practice for VHF & AIS antenna placement

Thank you all for the insightful replies.

I note that the Vesper antenna splitter has significant insertion loss on the VHF (voice) side of things -- 1.5 dB on receive and 1 dB on transmit, according to the manufacturer. Whether this is enough to matter is debatable and in practice is going to depend on the situation and other aspects of the installation.

VHF voice is going to be noise limited in a busy harbor and line-of-sight limited in most other cases, but the insertion loss might make a difference in fringe situations where there are intervening obstructions like trees.

Cost wise the splitter, at $249, is about the same, or perhaps a little more money, than a higher quality antenna, mount, and co-ax.

Food for thought.
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