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Old 11-04-2012, 17:03   #1
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AIS antenna

Getting conflicting advice and would like some input. Currently our AIS receiver is attached to a VHF antenna on the deck - range not the best. Difficult to put one up the mast so we want to replace the old one with a better version if they exist. Does AIS require a specific different type of antenna or are they just VHF?
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Old 11-04-2012, 17:07   #2
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Re: AIS antenna

Just VHF. A "Special AIS Antenna" is just a marketing ploy.
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Old 11-04-2012, 17:11   #3
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Re: AIS antenna

While it is possible to make an antenna tuned exactly to the AIS frequency, the very very slight improvement in technical performance would not be noticed in any real way. Just getting a better coax / connectors would be more effective.

AIS frequency remains inside the marine VHF band.
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Old 11-04-2012, 17:18   #4
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Re: AIS antenna

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Originally Posted by Wotname View Post
Just VHF. A "Special AIS Antenna" is just a marketing ploy.
Actually it is not a ploy if you have a transceiver. For my Furuno FA-150 I tried a regular VHF antenna and it did not work. I called Furuno tech support and they said I need an AIS transceiver antenna, I changed out my regular antenna for this AIS antenna and it then worked...but this is just for a transceiver and not the OP's situation. A regular VHF antenna will work for a receiver.

Shakespeare Antenna Specifications: Shakespeare 396-1-AIS Broadband VHF
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Old 11-04-2012, 17:19   #5
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Re: AIS antenna

You are much better off trying to get the Antenna to the top of the mast. I have two VHF antennas on my boat and play with the AIS to see which one worked better (as it wasn't obvious).

What I found was that I received targets in San Fracnscio Bay at about 8-10 miles on the transome antenna (15 feet high) and at about 35-40 miles on the mast head antenna (65 feet off the water).

The problem I have found is when in heavy weather the swaying mast causes me to loose targets... I don't know the technical reason why, just that when the mast is moving back and forth I will loose targets over a couple miles away.
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Old 11-04-2012, 17:22   #6
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Re: AIS antenna

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Originally Posted by jeremiason View Post
You are much better off trying to get the Antenna to the top of the mast. I have two VHF antennas on my boat and play with the AIS to see which one worked better (as it wasn't obvious).

What I found was that I received targets in San Fracnscio Bay at about 8-10 miles on the transome antenna (15 feet high) and at about 35-40 miles on the mast head antenna (65 feet off the water).

The problem I have found is when in heavy weather the swaying mast causes me to loose targets... I don't know the technical reason why, just that when the mast is moving back and forth I will loose targets over a couple miles away.


Higher gain antennas have shorter lobes in the vertical axis. So when you heel with a high gain antenna the lobe is pointing skywards on one side of the lobe and pointing at the water on the other side of the lobe.
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Old 11-04-2012, 17:35   #7
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Re: AIS antenna

Quote:
Originally Posted by David M View Post
Actually it is not a ploy if you have a transceiver. For my Furuno FA-150 I tried a regular VHF antenna and it did not work. I called Furuno tech support and they said I need an AIS transceiver antenna, I changed out my regular antenna for this AIS antenna and it then worked...but this is just for a transceiver and not the OP's situation. A regular VHF antenna will work for a receiver.

Shakespeare Antenna Specifications: Shakespeare 396-1-AIS Broadband VHF
I think the Furuno Tech support guy was just taking the easy option to effect a fast fix but rather than derail this thread I will PM you later as I am very interested in finding out more about this.
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Old 11-04-2012, 18:08   #8
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Re: AIS antenna

It helps to read the specs. AIS is on 2 marine channels (87B and 88B) at 162MHz (+/-0.025MHz). This is at the extreme high end of the marine VHF range. Looking at only the Digital Antenna specs, their standard VHF 4.5db antenna has a 2.0:1 SWR at 162MHz, while their AIS 4.5db antenna has a 1.5:1 SWR at 162MHz. Yes, that would make a noticeable difference in performance.

My AIS antenna (the Digital Antenna 4.5dB AIS above) is about 12ft high, on the radar mount on the transom. I regularly see USCG movements at the mouth of the Columbia River, about 100 miles away. YMMV
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Old 11-04-2012, 18:57   #9
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Things are a lot clearer now. Many thanks. New antenna to be ordered and connections checked.
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Old 11-04-2012, 22:29   #10
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Re: AIS antenna

If you are receive-only AIS there is not a great deal of worry about the antenna resonance being retuned for AIS from the normal antenna. Receivers can tolerate mismatched antenna-feedline conditions without much loss of signal.

If you are installing an AIS-transponder, the transmitter in your transponder may not enjoy working into a regular VHF Marine Band antenna because the VSWR will be rather high at the AIS frequency. If you have a transponder you need a dedicated antenna, anyways, so you might as well get an antenna tuned for the AIS frequencies.

As far as I can tell, the only difference between a regular VHF Marine Band antenna and its AIS equivalent is the antenna tuning. The AIS antennas are probably tuned to have better VSWR around 162-MHz. A typical regular antenna is probably tuned for 156.8-MHz (Channel 16).

Here is a good article explaining the frequencies used:

continuousWave: Whaler: Reference: VHF Marine Band Channels

Here is an AIS antenna I had designed by G0KSC and fabricated by him. It is useful for a shore station, not a vessel:

continuousWave: Whaler: Reference: AIS Antenna
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Old 12-04-2012, 05:41   #11
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Re: AIS antenna

Quote:
Originally Posted by CarinaPDX View Post
It helps to read the specs. AIS is on 2 marine channels (87B and 88B) at 162MHz (+/-0.025MHz). This is at the extreme high end of the marine VHF range. Looking at only the Digital Antenna specs, their standard VHF 4.5db antenna has a 2.0:1 SWR at 162MHz, while their AIS 4.5db antenna has a 1.5:1 SWR at 162MHz. Yes, that would make a noticeable difference in performance.

My AIS antenna (the Digital Antenna 4.5dB AIS above) is about 12ft high, on the radar mount on the transom. I regularly see USCG movements at the mouth of the Columbia River, about 100 miles away. YMMV
I seriously doubt one would notice any difference on a AIS receiver with the VSWR numbers you have quoted for the different antennas. And the only time any possible difference could be noticed is when the receive signal is right at the lowest threshold of the receiver sensitivity (which almost be never).

However if that is an issue, then a far more effective way of getting more signal into the receiver would be any (or all) of the following:
Raise antenna height
Upgrade coax
Reduce local (onboard) RFI.

But get an AIS tuned antenna if it floats your boat .
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Old 12-04-2012, 05:54   #12
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Re: AIS antenna

I went to Ham Radio Outlet and got a universal mag mount and stuck it on my cabin roof. Stuck up the canal, and surrounded by trees, I see movements 18 miles up and down the river with my tranceiver.

When I get to it I will cut the antenna to the specified length for AIS, then it will be tuned.
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Old 12-04-2012, 06:00   #13
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Re: AIS antenna

Another way to receive AIS is to use a VHF with buildin AIS like the RO4800 which I recently bought to replace my old VHF.
There are other VHF with buildin AIS also like the Standard-Horizon but the RO4800 was the cheapest here.

So I only replaced my VHF with the new one using the old cables and old antenna on the mast, connected GPS to the new VHF and bingo: Received AIS signals from ships over 100 miles away in some good weather conditions. By the way I connected the output of the RO4800 also to the PC to see the ships in the maps. Works great.

Gerhard
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Old 12-04-2012, 06:31   #14
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Re: AIS antenna

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....... If you have a transponder you need a dedicated antenna, anyways, so you might as well get an antenna tuned for the AIS frequencies.........
While recommended, a dedicated antenna is not always needed.
Here is a splitter suitable for a transponder.
Antenna Splitter for Class B AIS Transponders
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Old 12-04-2012, 06:37   #15
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Re: AIS antenna

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Originally Posted by David M View Post
For my Furuno FA-150 I tried a regular VHF antenna and it did not work.
What, exactly, does that mean? Nobody could receive your signal? If that's the case, or your range was very limited, I'd say you had a problem with your antenna system.

Quote:
Looking at only the Digital Antenna specs, their standard VHF 4.5db antenna has a 2.0:1 SWR at 162Mhz, while their AIS 4.5db antenna has a 1.5:1 SWR at 162Mhz. Yes, that would make noticeable difference in performance.
.5db (the difference between the two figures given) would not be noticeable. For 2 watts class B, that would be an antenna output of 1.94w as opposed to 1.78w and for class A (12w), 11.64 as opposed to 10.68w.

If you are going to obtain a new antenna strictly for AIS, then the prudent thing to do would be to get one tuned specifically for the AIS frequencies but a standard VHF antenna would work equally (≤ .5db difference) well.

Eric
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