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Old 22-07-2015, 19:22   #1
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AIS Antenna on Spreaders

Has anyone any experience with such an installation? Decent reception? Any interference with halyards or other lines? Maybe mount Wi-Fi antenna on opposite spreader?

How did you mount it and how was the cable routed?

Top of the mast is already too busy and I am too old to climb all the way up if the spreaders will do.

And will RG-8X be adequate for the antenna cable?

TIA
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Old 22-07-2015, 19:35   #2
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Re: AIS Antenna on Spreaders

This maybe isn't what you're looking for, but I installed a vesper splitter about 6 months ago and it works great. Solves the placement dilemma and hassle you mention. Failure would not inhibit VHF comms and, similarly, when turned off the VHF is still operable (thus can be fused and wired directly to the AIS breaker). Consumes negligible electrons.

Simple, works well, easy peasy. I carry a backup antenna, but mine is mounted above a cockpit radar tower so relatively easy access.
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Old 22-07-2015, 19:55   #3
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Re: AIS Antenna on Spreaders

Not to be too funny, but are you kidding? Unless you have invented an ais antenna that is about 3" high, not 3' that would not work.
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Old 22-07-2015, 20:20   #4
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Re: AIS Antenna on Spreaders

I have a dedicated AIS antenna on my top spreader connected to a Garmin AIS600 transponder, as well as a Garmin VHF300AIS radio, which has AIS receive capability built in, connected to the mast top VHF antenna. Both AIS receivers can feed my plotter via NMEA2000 bus. The transponder has a dedicated GPS. The AIS output from the radio is normally disabled to avoid confusing the plotter with two signals.

The AIS600 contains a splitter which I use to connect my backup VHF radio to the AIS antenna. Both radios and both AIS systems work well when operating.
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Old 22-07-2015, 21:09   #5
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Re: AIS Antenna on Spreaders

Don't listen to Guy. I too have my AIS antenna mounted on the spreader and it works fine.

It's the Shakespeare 5215AIS which is a bit under a metre tall. I made a bracket from stainless plate and had my rigger rivet that to the spreader half way between mast and spreader tip. The bracket positions the antenna almost level with the front of the mast and the forward diamond protects it from the genoa.

I know you didn't ask about splitters or separate antennas, but no doubt you will get more suggestions to go the splitter route. I put a lot of thought into whether it was better to have a splitter or a separate antenna. As with everything on a boat, there are compromises. The separate antenna worked out a little dearer after buying cable and the time for the rigger to fit it, but the difference was not significant. So for me it was a close decision, but came down to usage in practice.

The splitter argument
With a splitter, you would be using the best located antenna (masthead for me), so more range. But long range in AIS is not important for me so I discounted this benefit. As it turns out, I have tried swapping antennas and the difference in reception is marginal. Most splitters reduce the signal a little so it would be worthwhile to find one that minimises this (e.g. Vesper).

What I saw as a downside for the splitter solution was the fact that AIS communication is disconnected while you are on the radio. So if you happen to be on the radio when your AIS transmits, the signal will not go out, and you will also not receive incoming signals. You will not know this has happened.

There could be rare occasions when the AIS is effectively dumb and blind. Maybe you are on the VHF trying to raise the very vessel you want to see or be seen by on AIS.

The separate antenna argument
In this case you have redundcancy and abiliity to easily swap cables. The downside is you will not know if the mast shadow is interfering with transmission or reception. I haven't seen any evidence of this happening but it must affect the signal if the antenna is hidden by the mast. There is a small angle abeam where signal could be affected. As you get closer, with stronger signal strength this effect should reduce. Mounting on the starbord spreader could be a slight advantage.

So each solution has benefits and disadvantages.

Back to your question ..... yes a spreader mounted AIS antenna works fine.
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Old 22-07-2015, 21:33   #6
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Re: AIS Antenna on Spreaders

Quote:
Originally Posted by Wanderlust View Post
Most splitters reduce the signal a little so it would be worthwhile to find one that minimises this (e.g. Vesper).

What I saw as a downside for the splitter solution was the fact that AIS communication is disconnected while you are on the radio. So if you happen to be on the radio when your AIS transmits, the signal will not go out, and you will also not receive incoming signals.
The Vesper splitter actually amplifies the received signal, so signal reception is better when passed through the splitter compared to not.

Class A transmission rates are every 2 or 6 seconds (speed and ROT dependent). Unless one is sitting on the mic key keeping the VHF side of a splitter open, it is unlikely that missing 2 seconds of data update every once in a while will be meaningful. If it becomes meaningful, you have made a navigational error beyond any fault of an AIS splitter.

Class B transmissions are every 30 seconds, so having an open VHF mic will have a greater probability of longer time intervals between updates. However, most class B transponders are used in vessels that are moving at relatively slow speeds and are more maneuverable, so even missing a minute or two of updates here is insignificant in practice.

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Old 22-07-2015, 21:49   #7
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Re: AIS Antenna on Spreaders

I guess with the kind of sailing or motoring ole wanderlust does a loose halyard or sail never get near the spreader. Sure.
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Old 22-07-2015, 22:12   #8
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Re: AIS Antenna on Spreaders

Utilizing the existing masthead VHF antenna, I have a 3' antenna tuned to 156 mhz, not the 162 mhz for AIS transmissions [I may have those numbers slightly, or grossly, off!!] How much difference that makes I don't know. Plus some loss though the splitter.... Except maybe not with the Vesper?

On the spreader, I am at 30-35 ft. or so, not the 60+ ft. that the masthead antenna has. Plus, if located say 12-15" from mast, will that block enough of the transmissions to be an issue? Does the higher elevation make any substantive difference?

The splitter sounds the easiest install, but not sure it's the best performing option. I want to locate somewhere other than an antenna at the pushpit is why I am asking.

I do have concerns about potential damage to a spreader mount, but that might be just theoretical and not true in reality.
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Old 22-07-2015, 22:32   #9
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Re: AIS Antenna on Spreaders

In order to muddy up the question even further -

I have two fixed VHF radios, The nav station has the better Icom with a GPS interface for the DSC, LMR-400 cabling, masthead antenna and serves as the long distance/"this is important" radio of choice. Or when it is just too crappy outside, and this is my excuse to go below for a break

In the cockpit I have a Standard Horizon with a 5' fiberglass antenna on the stern rail. This I use mostly for line of sight communications as the range seems to top out at 7 or 8 miles most of the time.

For vessel crossing situations I am just as likely to be in the cockpit as at the nav station below. Maybe stepping on the AIS transmission/reception wouldn't be an issue (again, most of the time) if I utilize the cockpit radio whenever possible.

Haven't figured out where I would mount the AIS display either - down below or in the cockpit. Clearly, I haven't thought this through very well.
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Old 22-07-2015, 22:33   #10
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Re: AIS Antenna on Spreaders

Quote:
Originally Posted by Guy View Post
I guess with the kind of sailing or motoring ole wanderlust does a loose halyard or sail never get near the spreader. Sure.
Just because you've been proven wrong there is no need to get snarky. I'm not the only one who has success with it.

In my last trip away of several months I somehow managed to avoid that happening (Sydney to Whitsundays and return). The radar mount is far more exposed than the AIS antenna. Perhaps you would suggest others not mount their radar domes on the front of the mast? What about deck lights?
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Old 22-07-2015, 22:36   #11
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Re: AIS Antenna on Spreaders

Mounting the antenna on a spreader will not cause any problem with the AIS functioning. If you are worried about a halyard catching on the antenna consider mounting the antenna to the underside of the spreader. Radio signals can't tell if the antenna is upside down.
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Old 22-07-2015, 23:17   #12
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Re: AIS Antenna on Spreaders

Re Colemj's comment on splitters ..

Quote:
Originally Posted by colemj View Post
..... so even missing a minute or two of updates here is insignificant in practice.

Mark
Yep I mostly agree. I just saw a slight possibility that several Class B transmissions in/out could be lost. Maybe you or the other vessel (or both) changed course due to close proximity at night and you are trying to verify what you see or warn them by repeatedly calling them on VHF, thus preventing the AIS transmissions in either direction on those times. The chances of missing several in a row are remote, but people do win lotteries.

Anyway, I don't have a strong argument for or against either solution because they are each a compromise. For me it was a close decision and you won't find me arguing against splitters. They are an equally good solution.
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Old 22-07-2015, 23:51   #13
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Re: AIS Antenna on Spreaders

Quote:
Originally Posted by Wanderlust View Post
Just because you've been proven wrong there is no need to get snarky. I'm not the only one who has success with it.

In my last trip away of several months I somehow managed to avoid that happening (Sydney to Whitsundays and return). The radar mount is far more exposed than the AIS antenna. Perhaps you would suggest others not mount their radar domes on the front of the mast? What about deck lights?
If you think a 3' whip antenna mounted on the spreaders can't get caught on something when things go ape **** at 2 am, you are in La La land.
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Old 23-07-2015, 00:16   #14
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Re: AIS Antenna on Spreaders

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Originally Posted by redsky49 View Post
Haven't figured out where I would mount the AIS display either - down below or in the cockpit. Clearly, I haven't thought this through very well.
My solution was a swing mount similar to this..Click image for larger version

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Granted this is on a 27' with a small cockpit.

Or a second display?

On my boat, it really wants to be in the cockpit for whoever's on watch. Especially around 3AM after a couple cups of good black paranoia juice

With the vesper I have, the swing has the added benefit of articulating inside the cabin such that it's widely visible down below when performing it's anchor watch duties.
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Old 23-07-2015, 00:37   #15
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Re: AIS Antenna on Spreaders

My AIS antenna is on the top spreader, and the coax is routed down through the mast. I've had zero problems with halyards, or spinnaker rigging, and since the antenna is flexible I don't see how a halyard could really get seriously hung up.

But, when flying the spinnaker in extremely light wind the sailcloth has brushed against the antenna tip. This could rip the spinnaker. I'm considering running a light nylon line from the antenna tip up to the shroud above, which would keep this from happening.

The spreader-mount antenna definitely gets more range than my previous stern-rail mounted antenna. I don't think there are any significant mast "shading" issues, but I haven't done a careful pattern check.
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