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Old 05-26-2015, 10:27 PM   #1
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Need an AIS antenna for AIS

Is antenna specific for AIS required for an AIS transceiver or is a VHS type antenna sufficient?


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Old 05-26-2015, 11:06 PM   #2
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Gawd Jim you are in the wars about the AIS thing. You have convinced me to spend my boat bucks on fuel, not AIS yet.

What happened to the guy that installed It? Methinks he pooched it.
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Old 05-26-2015, 11:07 PM   #3
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A VHF antenna is sufficient.
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Old 05-26-2015, 11:13 PM   #4
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Need an AIS antenna for AIS

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A VHF antenna is sufficient.

That's what I understood. Talked to tech support today at length. They are sending me another unit. I receive fine but the unit doesn't transmit. I've had enough of it.


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Old 05-27-2015, 12:46 AM   #5
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AIS operated over a couple of the VHF channels, so a VHF antenna is fine. I think the fine print fro AIS says it's supposed to be no more than a 3db antenna, but I'm not sure why. And I supposed an AIS antenna could be tuned to the narrower band used by the AIS channels rather than the whole VHF band. But personally I think it's splitting hairs. I've just got a VHF antenna and it works great.
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Old 05-27-2015, 12:47 AM   #6
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Oh, and I think antenna height matters about ten thousand times more than the antenna selection.
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Old 05-27-2015, 01:10 AM   #7
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Oh, and I think antenna height matters about ten thousand times more than the antenna selection.

I'm guessing that the dedicated antenna is +20' above the waterline, probably more.


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Old 05-27-2015, 10:46 AM   #8
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I'm guessing that the dedicated antenna is +20' above the waterline, probably more.


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Yes, that's right. I never really appreciated the difference that antenna height makes until this boat where they are up really high. I just double checked my air draft yesterday and the antennas top out at 39'. I pick up AIS targets (Class A and B) from much greater distances than on my old boat. And on the old boat I had one of those made-for-AIS antennas.
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Old 05-27-2015, 11:57 AM   #9
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You can get antennas built and tuned to optimize AIS from Digital Antenna (my preferred antenna maker) or Shakespeare for the same price as VHF.
4 ft AIS Marine Antennas - 4.5dB Gain
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Old 05-27-2015, 02:05 PM   #10
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Yes, that's right. I never really appreciated the difference that antenna height makes until this boat where they are up really high. I just double checked my air draft yesterday and the antennas top out at 39'. ...
Gee, and I thought my air draft was high!
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Old 05-29-2015, 01:17 PM   #11
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You can get antennas built and tuned to optimize AIS from Digital Antenna (my preferred antenna maker) or Shakespeare for the same price as VHF.
4 ft AIS Marine Antennas - 4.5dB Gain
This is what I did. I have a dedicated AIS antenna. If you use a VHF antenna with both a radio and AIS on the same antenna you will need to get a switch box.
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Old 05-29-2015, 05:03 PM   #12
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If you have to buy an antenna for the AIS, why not get one specified for AIS?


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Old 05-29-2015, 07:25 PM   #13
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An antenna really doesn't care anything about what it is receiving, it just collects electromagnetic waves in the frequency range for which it was designed. VHF radios and AIS both operate in the same frequency range. They use simple dipole antennas which are non-directional in the horizontal plane. Dipoles are inherently broad band. The gain can be changed by adjusting the dipole length which simply narrows the vertical width of the beam. You cannot increase the gain by making it "narrow band" for a particular frequency. There is a lot of snake oil sold in the antenna world, just look at TV antennas. Don't spend extra money to buy a dipole antenna that that purports to work best for some particular purpose, like digital TV or AIS.

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Old 05-29-2015, 08:52 PM   #14
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Right, I think the only optimization for an AIS antenna would be to tune it to the couple of channel range used by AIS rather than the center of the whole VHF channel range. But I doubt it makes a huge difference, and probably none that any of us can detect.

As for "why not get an AIS antenna if such a thing is available", I think that makes sense if you are working from scratch. I used a VHF antenna because I wanted to match the lengths of my 4 antenna. That drove the decision. So far no regrets.
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