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Old 10-06-2020, 18:07   #1
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VHF vs AIS antenna

I am adding an AIS transceiver to my boat.
Saw a post that suggested getting a specific AIS antenna as AIS is at the extreme end of the VHF spectrum and a regular antenna is not optimized for it. I have an extra standard masthead 3 foot stainless whip antenna that I was planning on mounting on the stern rail. I understand it will work. Is the difference between the two antennas only the length of the whip or is there something special in the little cylinder? If I can do an easy modification to improve what I've got, I will do it, but I am unlikely to buy a new dedicated antenna.
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Old 10-06-2020, 19:05   #2
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Re: VHF vs AIS antenna

You are talking about tuning your whip antenna to the specific AIS frequencies. By the way, the AIS frequencies (161.975MHz and 162.025MHz) fall within the Marine VHF allocations. So you could, in reality, take a regular Marine VHF antenna and tune the whip for lowest SWR in that frequency range. You will need a SWR meter or antenna analyzer to perform the tuning. So in essence, yes, you can tune (cut to length) the whip you have.

JMHO
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Old 10-06-2020, 19:36   #3
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Re: VHF vs AIS antenna

the short antennas are normally fine.

I've run into issues with the 8' vhf antennas on AIS. they often fail.

when you set up the ais, most will give an antenna signal number. so you'll know if it's ok or not.
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Old 10-06-2020, 20:28   #4
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Re: VHF vs AIS antenna

I use a VHF antenna sized for channel 16 and it works brilliantly on AIS.
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Old 10-06-2020, 22:16   #5
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Re: VHF vs AIS antenna

I'm the one that posted the issue. It was certainly an issue on our boat with our AIS. I purchased a "regular" VHF antenna and mounted it. It had intermittent problems. When I contacted Raymarine, they suggested the tuning issue. I went with a Morad unit, specifically for AIS. It's been perfect. The other antenna is sitting in a tube around here somewhere.

Again, I posted because WE had an issue. You might not. I just know how much of a pain it is to put in an antenna. That's why we went with a top of the line unit.
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Old 11-06-2020, 03:09   #6
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Re: VHF vs AIS antenna

I bought the Em-Trak B-100.
I do have what I think is an antenna tuner in a box somewhere, but no idea how to use it. I was hoping there might be a standard best length for the whip. On the stern rail, I should be much easier to modify than at masthead if the regular length is unsatisfactory.
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Old 11-06-2020, 04:10   #7
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Re: VHF vs AIS antenna

we have a splitter and the vhf and ais share the same antenna

works perfectly

cheers,
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Old 11-06-2020, 04:47   #8
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Re: VHF vs AIS antenna

Quote:
Originally Posted by sanibel sailor View Post
I bought the Em-Trak B-100.
I do have what I think is an antenna tuner in a box somewhere, but no idea how to use it. I was hoping there might be a standard best length for the whip. On the stern rail, I should be much easier to modify than at masthead if the regular length is unsatisfactory.
The Em Trak software will tell you what the SWR is, though I am not sure how accurate it is. Ours is reading 2.1, I think from memory, yet reception is very good with a range of 10 miles from a pole on the pushpit (3m height). That will do for us, not really interested in anything beyond that.

Why not fit an antenna on the stern and see how it performs before chopping up a perfectly good antenna.

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Old 11-06-2020, 04:57   #9
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Re: VHF vs AIS antenna

Quote:
Originally Posted by chrisr View Post
we have a splitter and the vhf and ais share the same antenna

works perfectly

cheers,
Splitters for receivers are cheap. Transceiver splitters cost same as my transceivers. That is a compelling reason for my separate sternrail choice.
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Old 11-06-2020, 05:17   #10
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Re: VHF vs AIS antenna

My AIS and VHF shared a masthead antenna via a splitter, both worked flawlessly at ranges of 20 nm for the AIS and 12+ nm, for the VHF for 10 years. Picking up big ships before they cross the visual horizon is very valuable especially in crowded waters and you will lose the AIS range with a transom antenna. However, my masthead was 76 ft above the water.
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Old 11-06-2020, 08:37   #11
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Re: VHF vs AIS antenna

We have been very happy with the performance of our Vesper splitter working with our radio and Vesper AIS.
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Old 11-06-2020, 08:49   #12
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Re: VHF vs AIS antenna

Hi,
Regular VHF antenna's are tuned tot the middle of the band. AIS frequencies are with 162 MHz at the edge of the band.
Some AIS transponders do not like high SWR values (> 2,0) This can occur as the SRW for a particular antenna at the edge of the band is not optimal.
Fiberglass antenna's have - because of their construction - a slightly "broader" frequency spectrum and are less likely to show this effect as stainless steel whip antennas. Thin whip antenna's have a slightly "narrower" optimum frequency range.
There is a simple solution: In case of a stainless steel whip antenna, you can cut of approx 1/2 to 1 inch from the top of the rod. This will move the centre frequency (the resonance frequency) slightly up to the desired 162 MHz.
The disadvantage of this operation is that you cannot reverse it, and the antenna is not optimized for some channels on the lower edge of the VHF band i.e. ch 01.
But since this shortened antenna is only used for AIS there is no problem.
One of the best VHF antenna's on the market is the Celmar 0-1 (made in Denmark). Reasonable priced too. Cutting the top is easy.
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Old 11-06-2020, 09:03   #13
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Re: VHF vs AIS antenna

Quote:
Originally Posted by sanibel sailor View Post
I am adding an AIS transceiver to my boat.
Saw a post that suggested getting a specific AIS antenna as AIS is at the extreme end of the VHF spectrum and a regular antenna is not optimized for it. I have an extra standard masthead 3 foot stainless whip antenna that I was planning on mounting on the stern rail. I understand it will work. Is the difference between the two antennas only the length of the whip or is there something special in the little cylinder? If I can do an easy modification to improve what I've got, I will do it, but I am unlikely to buy a new dedicated antenna.
AIS works in VHF range (line of sight) so height is important. Use a splitter and your present antenna.

I installed a Vesper Marine Watchmate 850 and Vesper SP160 splitter six years ago and have not had one problem. Works great. Keep it simple.

~ ~ _/) ~ ~ MJH
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Old 11-06-2020, 09:04   #14
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Re: VHF vs AIS antenna

More important than perfect tune is placement. Having your AIS antenna at the masthead will perform so much better than the stern rail as to make perfect tuning irrelevant.
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Old 11-06-2020, 09:21   #15
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Re: VHF vs AIS antenna

Try this article: https://cruisingclub.org/article/ais...considerations
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