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Old 19-04-2013, 21:50   #1
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AIS Transponder and VHF Antennas on the Masthead?

I will have my mast down shortly. I am thinking of replacing all lights with LED's but I am also thinking of adding an AIS transponder antenna to the masthead.
Seeing that the two antennas will be in somewhat close proximity, is this going to cause more problems than it's worth or will they be able to live happily together ?
I am thinking of mounting a stainless plate that extends out past the two sides of the mast maybe a foot on each side and stick one antenna on each side so they would be separated by maybe 2.5 ft. One thing I don't really want is for my VHF squelch to open up every 30 seconds or so when the transponder transmits. As it is, I have issues with my VHF squelch opening up quite often while there is no signal on the air (squelch at max, Icom radio) - I will check the antenna connection to the VHF radio while the mast is down to see if there is anything I can see that might cause this noise issue.
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Old 19-04-2013, 22:13   #2
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Most manuals say the AIS should be directly below the VHF. If the same height a minimum of 3 meters separation. I tried a .number of ideas on the top of my mast, could not get an arrangement I liked, so put AIS on the mizzen.
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Old 19-04-2013, 22:37   #3
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Re: AIS transponder and VHF antennas on the masthead ?

I had an AIS sales guy tell me that you could put both antennas next to one another, no problem. Different freq.
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Old 20-04-2013, 00:51   #4
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Wink Re: AIS transponder and VHF antennas on the masthead ?

AIS and VHF ariels are the SAME BUT receive on seperate frequency's.

So have them at a minimum os 3 meters apart to get optimum reception.

I was given this information when my AIS was proffesionaly installed.
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Old 20-04-2013, 00:59   #5
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Re: AIS transponder and VHF antennas on the masthead ?

What about if one antenna is mounted on the spreaders? Will being next to the mast be a problem? It could not be mounted at the ends in case of fouling with the genoa, so would have to be relatively close to the mast.

The same question applies if you want to maintain bridge clearance, so mount it close to the mast, with the top of the antenna at masthead height.
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Old 20-04-2013, 01:07   #6
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Re: AIS transponder and VHF antennas on the masthead ?

I've got my AIS antenna on the upper spreader, positioned as far outboard as it can go before the antenna touches the shroud. The VHF antenna is on the masthead. This arrangement works great. Putting the AIS antenna on a lower spreader should also work well. Even putting the AIS antenna on the stern pushpit gives reasonable performance.

There are two reasons for keeping the two antennas separated; mutual interference and overload, and antenna detuning. I suspect that in most cases, with even a couple of feet separation, it's going to be OK, but that was something I didn't want to have to troubleshoot should it have been a problem.
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Old 20-04-2013, 02:48   #7
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Re: AIS transponder and VHF antennas on the masthead ?

We have one ariel and a signal splitter. We get AIS targets at over 15nm range and excellent VHF reception and transmission. We start to negotiate interactions with big ships at 10 miles without interruption of either the AIS or VHF signals. At 10 miles the big ships are receiving full AIS data from our transceiver.

Apparently there is a signal loss due to the splitter and it may be that our 76 ft mast is overcoming any signal loss problems.

AIS is a game changer and has completely changed how we deal with big ships.
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Old 14-07-2013, 08:48   #8
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Re: AIS transponder and VHF antennas on the masthead ?

Just reviving this thread. I am replacing my masthead mounted VHF antenna and coax, and installing a separate AIS antenna at the same time. I had planned to mount the AIS at the spreader but the installer has proposed to mount it at the masthead as well for simplicity. They would be one each side of the mast, separated by less than a metre.

In this thread there is a wide variety of opinion. The OP proposed 2.5 feet separation while a couple of comments said 3 metres separation is needed (impractical at masthead), and another said next to each other is OK. Searching the net I found various comments about an antenna height being a rule of thumb for a minimum. So that's 4 different viewpoints.

Has anyone actually mounted them close to each and found it was a problem or vice versa? The installer is due on Tuesday so I need to make a quick decision.
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Old 14-07-2013, 08:52   #9
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Re: AIS transponder and VHF antennas on the masthead ?

We have had them mounted next to each other with no noticeable problems at all. Meaning the VHF had good range and signal in all directions and the AIS received signals at long range from all directions.

The AIS was receive-only, not a transponder.

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Old 14-07-2013, 17:15   #10
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I have the AIS on the Bimini and the VHF on the mast head, that way if you loose the stick you could still have a working VHF antenna. By the way have the two antennas close to each other will probably screw up the omni-directional polar of both the antennas resulting in a blind spot on both.
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Old 14-07-2013, 17:29   #11
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Re: AIS transponder and VHF antennas on the masthead ?

Quote:
Originally Posted by bazzer View Post
By the way have the two antennas close to each other will probably screw up the omni-directional polar of both the antennas resulting in a blind spot on both.
In theory this is true. In practice for us, it was not. Again, our AIS was receive-only, so I have no data on the transmission for it, but do not expect it to differ from normal vhf.

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Old 14-07-2013, 17:45   #12
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Re: AIS transponder and VHF antennas on the masthead ?

AIS transmissions use the upper part of the VHF marine band, i.e., 161.975 and 162.025 MHz (channels 87B and 88B).

The problem with locating the antennas close together is that transmissions from one may cause not only interference with the other, but could cause damage to the receiver's front end.

IMO, it is not a good idea to use one antenna with a splitter, not only because of the insertion loss but because of possible failures which, again, could be damaging.

The marine VHF is arguably the most important piece of electronic gear aboard in terms of safety. It does not make sense to do ANYTHING which could compromise its use.

Bottom line: put the VHF antenna at the truck of the mast. Locate an AIS transponder antenna elsewhere....on the spreaders, on a mizzen mast, on a radar arch, or even on the pushpit.

Bill
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Old 14-07-2013, 17:58   #13
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I have argued elsewhere that using the masthead VHF with a splitter is a good idea for AIS. I would argue that transceiver AIS is arguably a far greater safety tool then VHF. Therefore it should demand the best resource. VHF at sea is rarely used, and outside English speaking waters , often a poor choice . Also the vast majority of time your VHF is not transmitting , so it makes sense to ultilise that high up antenna

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Old 14-07-2013, 18:03   #14
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Re: AIS transponder and VHF antennas on the masthead ?

Thanks for the quick responses, and Mark for your experience.

However, this morning I decided to call Vesper Marine for their advice as I am installing their system. They do not recommend sharing the masthead space due to proximity. He said over a metre may be acceptable but still not recommended.

For the spreader location he said it would be too close to either the mast or the rigging and again not recommended.

His suggestion was to use a splitter in preference, and mount only the AIS antenna at the masthead. However I already have all the kit ready to go, so will find somewhere around the stern area for the AIS. The bimini does not work for me because it is still under the boom.
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Old 14-07-2013, 18:28   #15
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Re: AIS transponder and VHF antennas on the masthead ?

Cheaper than a splitter is putting up a second VHF antenna.

My second is on the aft rail.

Usually my VHF is on the aft rail and the AIS is at mast top. But when I am in port, like now, I put the VHF up top to try and maximise that. But at sea, as Dave says, the AIS is the one you want with the best range.
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