AIS Antenna Position

Nodak7

Nodak7

.
Sep 28, 2008
1,256
Hunter 41DS Punta Gorda, FL
Our AIS system shares the VHF Antenna. I have been told that our signal seems to come and go and could be lost whenever we are using the VHF radio it would override the AIS transmission. I was wondering about installing a dedicated AIS antenna on my cabin top. I definitely do not want to install another antenna at the top of the mast. Question: Would having it low on the boat affect its range? Thoughts on whether this is really an issue?
 
All U Get

All U Get

.
Oct 2, 2008
3,810
Pearson/ 530 Strafford, NH
I have two separate antenna at the masthead and get other boats over twenty miles away. Previous to this I had the AIS on the bottom spreader and the distance was nearer fifteen miles.
 
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dlochner

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Jan 11, 2014
12,824
Sabre 362 113 Fair Haven, NY
If you have a proper splitter, there should be little difference. AIS does not send out a continuous signal. There is a complicated system where different classes of AIS, A, B, B+ have different transmission priorities. Class B transceivers have the lowest priority. Most of us have Class B transceivers. As the various AIS transceivers talk to each other they are assigned a time slot to broadcast. The first broadcast is simple, just basic info including MMSI and position. On subsequent broadcasts additional information is provided. Depending on how many boats in the area are using AIS the frequency of broadcasts will vary.

If your friend is monitoring your progress over a websites such as MarineTraffic.com, be aware that there are blind spots in their coverage. MT relies on volunteer stations along the coast to pick up the AIS signals and send them to the website, once out of range MT uses satellites. Satellite data requires a paid subscription.

Lowering the height of the antenna will reduce range. There are calculators online that will do the calculations for you.

Because both AIS and VHF are broadcasting on similar frequencies, it is important to provide proper spacing between the antennas to avoid overloading the receivers. I think they need to be 3' or more apart, check the manuals.

Another issue may be the quality of your antenna cable and connections. An AIS broadcast is at 2 watts while VHF can be as high as 25 watts, more power equals stronger signal less likely to be over run by other transmitters. Check your connections and use a low loss coax cable. This year I replaced the old RG-8 cable with Times Microwave LMR-240 Ultraflex and was amazed at how much better the VHF (and presumably the AIS) worked, much better reception.
 
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leo310

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Dec 15, 2006
646
Catalina 310 44 Campbell River BC
I have place the AIS antenna on the solar arch and get over 20 miles range.
 
Brian D

Brian D

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Feb 17, 2006
5,274
Lancer 27PS MCB Camp Pendleton KF6BL
It has always surprised me that people think height is the answer to everything. And I am not picking on the OP, but this is in general.

If you really need help right away, height is the answer. The higher the antenna the better the chance of being heard. Unless a fast mover (fixed wing/ rotor wing) is headed your way, it is going to take time. Think about that. A Coast Guard vessel hears your plea and their max speed is 20 Kn. If they are 20 Knm away that will be an hour before they get to you.

Now think about AIS. No difference. If you can detect traffic at 15 Kn away, and they are moving at 30 Kn then you still have 30 minutes to move out of the way. So is that ample time to take or make a course correction?

The point is, if you detect a moving target that is on a collision vector with your boat, and that will happen in a few minutes, it makes no difference how high your antenna is. So put the antenna as high as you can, but by all means, PAY ATTENTION! Don't let technology be a substitute for your eyes and ears, and most notably your gut feeling.

Again, not directed at the OP, but for those who have this need for height.

Now, technology says, and I agree, use separate antennas for separate devices. Yeah, it looks cluttered but there will be no issues. If you are getting interference then start using mix 31 toroid chokes on your cables. Too many times we are looking for an easier solution when the solution is right there in front of our collective noses.

So, some may disagree with me and that is fine. I can live with that.
 
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leo310

.
Dec 15, 2006
646
Catalina 310 44 Campbell River BC
One of the main reason that I put my ASI antenna on the solar arch is that if I ever get de-masted or something happens to the VHF antenna I can use the ASI one as a back up.
 
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jssailem

jssailem

SBO Weather and Forecasting Forum Jim & John
Oct 22, 2014
22,948
CAL 35 Cruiser #21 moored EVERETT WA
Brian D said:
start using mix 31 toroid chokes on your cables.
Click to expand
This is what it is. Ferrite_chokes.html

Refresher... VHF and AIS transmissions are Line of Sight signals. Yes you can see farther from the top of the mast than from the deck. As Brian suggests, If you are alert, and react correctly at the first sign of a boat crossing your path - you will pass by each other like two ships in the night. You may not even see the wake.
 
D

dlochner

.
Jan 11, 2014
12,824
Sabre 362 113 Fair Haven, NY
Brian D said:
It has always surprised me that people think height is the answer to everything. And I am not picking on the OP, but this is in general.

If you really need help right away, height is the answer. The higher the antenna the better the chance of being heard. Unless a fast mover (fixed wing/ rotor wing) is headed your way, it is going to take time. Think about that. A Coast Guard vessel hears your plea and their max speed is 20 Kn. If they are 20 Knm away that will be an hour before they get to you.

Now think about AIS. No difference. If you can detect traffic at 15 Kn away, and they are moving at 30 Kn then you still have 30 minutes to move out of the way. So is that ample time to take or make a course correction?

The point is, if you detect a moving target that is on a collision vector with your boat, and that will happen in a few minutes, it makes no difference how high your antenna is. So put the antenna as high as you can, but by all means, PAY ATTENTION! Don't let technology be a substitute for your eyes and ears, and most notably your gut feeling.

Again, not directed at the OP, but for those who have this need for height.

Now, technology says, and I agree, use separate antennas for separate devices. Yeah, it looks cluttered but there will be no issues. If you are getting interference then start using mix 31 toroid chokes on your cables. Too many times we are looking for an easier solution when the solution is right there in front of our collective noses.

So, some may disagree with me and that is fine. I can live with that.
Click to expand
Slight thread drift, when I first installed radar on Second Star and was learning to use it I often set at a very long distance, 10-20 miles. After awhile I realized all I really need is about 5 miles or less. The detail is greater as is the accuracy. Even with a boat approaching at 30 knots, I have 2-3 minutes to take action. And by then I can see it.
 
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quadrille38

quadrille38

.
Apr 11, 2010
969
Hunter 38 Whitehall MI
Nodak7 said:
Our AIS system shares the VHF Antenna. I have been told that our signal seems to come and go and could be lost whenever we are using the VHF radio it would override the AIS transmission. I was wondering about installing a dedicated AIS antenna on my cabin top. I definitely do not want to install another antenna at the top of the mast. Question: Would having it low on the boat affect its range? Thoughts on whether this is really an issue?
Click to expand
When I installed AIS the general recommendation was to not use a splitter because it interfered with VHS. Since then splitter technology has dramatically improved and there doesnt seem to be any noticeable interference.

I have the VHF on mast head and AIS is mounted on part of the Bimini frame. So in answer to your question yes the lower height absolutely affects range. At bimini height my AIS range is maybe 5 to 10 miles. The coastal dunes do tend to block the signal and I can’t see boats out in Lake Michigan as from the inland lake until I get through the channel and out into the lake. Friends we sail with bought a few years after us and use splitter approach (the technology had improved in those couple of years). His range is usually around 30 miles but on some days he’s gotten 60 miles.

I am not too worried though because at 5 miles or so I can still see far enough to get out of the way if I see something on a concerning heading.
 
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BarryL

BarryL

.
May 21, 2004
1,064
Jeanneau Sun Odyssey 409 Mt. Sinai, NY
Hello,

Note that if you have two radios (VHF and AIS Transponder), only 1 can BROADCAST at a time. That means if your AIS is broadcasting your position, you will not be able to transmit a message. I also think (but I'm not sure) that if your AIS is broadcasting then you won't be able to receive a VHF transmission either.

Barry.
 
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dlochner

.
Jan 11, 2014
12,824
Sabre 362 113 Fair Haven, NY
BarryL said:
Hello,

Note that if you have two radios (VHF and AIS Transponder), only 1 can BROADCAST at a time. That means if your AIS is broadcasting your position, you will not be able to transmit a message. I also think (but I'm not sure) that if your AIS is broadcasting then you won't be able to receive a VHF transmission either.

Barry.
Click to expand
Yes this is accurate. The advanced powered splitters will prioritize the VHF transmission. Also the AIS transmission is a very brief data burst, at only 26.66ms long and they only The odds of the a transmission colliding is very low.

For more than probably ever wanted to know about AIS, here's a link to the Wikipedia page:

 
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