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Old 05-06-2009, 10:07   #1
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AIS VHF Antenna Info

Does anyone have any experience and recommendations for the selection and placement of an AIS VHF antenna on a sailboat? I am planning on installing a Northstar NAIS 300L connected to a Northstar M121 Chartplotter with their BR-24 Broadband Radar. Thanks!
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Old 05-06-2009, 12:20   #2
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Use an antenna splitter so you can use your masthead antenna. It will shut off the AIS transmitter when you key the mike. The Raymarine has the splitter built into their tranceiver, The Raymarine unit also has a multiplexer so that you don't need a separate Gps antenna. Shine micro has a unit that has all the bits and pieces in one antenna - not FCC approved yet.
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Old 05-06-2009, 23:43   #3
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I don't understand these splitters: so when you key the mike, the AIS has no antenna. I know they use an end-stage that won't be easily damaged when transmitting without antenna but why would you do that on purpose?

Or are you talking about AIS receivers only?

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Old 07-06-2009, 10:42   #4
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why not install an 2nd vhf antenna and cable it so it can be used as backup antenna if you demast your vessel
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Old 11-06-2009, 06:34   #5
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We have a splitter that will take the AIS, VHF, and FM all using the masthead antenna.

The reason for using the antenna on the masthead is that you get maximum distance for reception and transmit purposes. If you put the antenna lower, you'll reduce it's "horizon".

You shouldn't put multiple antennas in close proximity when one or more are transmitters or you could damage the circuitry and/or degrade the preformance.

You need a splitter designed for the purpose.
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Old 11-06-2009, 06:49   #6
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I have a 8' x 1" ss tube/pole lashed to the pushpit where the boarding ladder is located. The pole is a great hand hold when boarding as well. It's lashed with about 12" of hitches of small stuff so no welding or custom ss work was requred and it takes standard antenna mounts. Atop this pole I have a GPS antenna and a VHF/AIS antenna which is about 14' or so above the waterline. I have no trouble receiving AIS from ranges up to 10-12 miles which is more than enough for the purpose which interests me - collision avoidance and calling vessels approaching my "guard zone".

The second antenna could be used as back up VHF is the masthead one goes south on me. The AIS antenna was a spare which has now been put to good use.

I am for reduncancy and the splinter idea flies in the face of that. Antenna or splitter fails and you lose AIS and VHF capability.
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Old 11-06-2009, 07:29   #7
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We also use a splitter for our AIS reciever. We purchased one designed for AIS use (smartradio 161) from Milltech marine: Products from Milltech Marine , and it appears that they have several models for AIS use.

I agree with the issue of the splitter failure and loss of the VHF. In three years, we have had no issues with the splitter. We've had a VHF radio fail, but not an antenna splitter. The ideal solution is to have a dedicated antenna for each use, but often our boats aren't really set up for that.

We carry two additional VHF antennas for redundancy. The primary antenna on the masthead, and a secondary on the arch, which is currently used for the DSC function on the SSB. Granted, I could use that antenna for the AIS (and might try it), but I really like the idea of long range on the AIS. We'll do some experiments to see if there is a significant change in the range of the unit. Keeping in mind, that the long range reception/display only works (assuming that the data is sent to a plotter/radar or computer navigation program) in a long range mode if the display is also set on that mode, and if we're in open waters. In other words, having a 12+ mile AIS reception range doesn't do a lot of good if the display is only set on 3 miles. Our third VHF antenna is the portable 'emergency antenna'.

The important thing is that you have AIS. Lots of options for antennas. Find what works best for your needs.

Steve
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Old 11-06-2009, 08:47   #8
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Install a second antenna. Don't put all your transmitting eggs in one basket.
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Old 11-06-2009, 14:32   #9
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I still don't know is these posters use a splitter for VHF + AIS transponder or just for VHF + AIS receiver. I mean, what does the switch do when both AIS and VHF want to transmit at the same time?

I will take this position: if you have an AIS transponder, you MUST have a separate antenna for it. If you have an AIS receiver you SHOULD have a separate antenna for it which can be used as VHF antenna in an emergency.

We have the 2nd antenna on our mizzen mast but for a sloop I would put it on a spreader.

ciao!
Nick.
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Old 11-06-2009, 15:11   #10
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Can I mount an antenna for AIS upside down under the radar anenna half the mast? Interfer with the radar?
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Old 11-06-2009, 15:44   #11
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I'm sure you can mount it that way but a standard VHF antenna is a ground plane (GP) antenna and it wouldn't work well. The standard way of dealing with this is to put it halfway on top of a spreader. Use a standard 1" threaded mount and select an antenna that fits on that, instead of the side-mount brackets.

You can spend a lot of money on a good antenna. But that doesn't mean you waste money. I would advise a Shakespear XT 5400 antenna for this setup. It looks like a 4' fibreglass tube. Inside it is a dipole. This means a cable comes out the side of the tube near the bottom. Inside the tube it runs up halfway where it connects to the dipole. The cable has some length and a connector. It was too short for me so I cut it near the antenna and soldered a plug back on; pulled a RG213 cable; soldered connectors to that one too and used a simple I-connector to connect the plugs at the antenna.
A dipole antenna outperforms the standard GP whips and when you put this on your spreader you might prefer it over the masthead antenna for your vhf. No problem, you can use either for VHF or AIS.

Also, mounted on the spreader it is likely to survive a lightning strike.

ciao!
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Old 11-06-2009, 16:20   #12
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My ais antenna is mounted on a 10' fiberglass pole on the port stern, so the tip is about 17' above the water. I am seeing AIS targets 17+ miles away, and in the Chesapeake, thats two pages of targets on my Garmin Plotters. I do not want to see (or bother) anyone any further away. ACR AIS B, using the antenna that came with it. I mounted the GPS antenna inside, under the cabin roof, and it works fine too.

My worst case scenario gives me 30 minutes to establish coms with any ship intercepting me, and two hours to figure out a way to miss anything standing still. That should do it.
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Old 11-06-2009, 21:13   #13
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If you have a closing speed of 25 knots (say, your 8 and their 17) then with 8 nautical miles you have a little over 20 minutes.

In shipping lanes, you have multiple targets (say in and out of NYC, for example).

Then in some places there are high speed passenger and other vessels which move much faster.

The other issue is that some vessels that you're concerned with include fishing vessels that may not have transmitter/recievers located 90' off the water...

I sail in the Chesapeake too, Sandy. For the Bay, your set-up is fine. You can probalby see Ladyhawk on your AIS occasionally ;^)
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Old 11-06-2009, 21:42   #14
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The problem with receiving long range AIS targets is that they (or you) are likely to change course or speed before they come close enough to become a factor in navigating. When you watch everything you quickly get an information overload, so it's important to filter it. Here in Panama I always "see" between 100 and 200 targets and need the software to select targets of interest.

If we keep continuous watch, we set the AIS alarms to 10 minutes TCPA. This is more than enough time to establish the safe way out. Way offshore we change it to 30 minutes but that's more for the wish to sight someone than for navigation needs.

I think the 17 mile range that Sandy has is enough; more than enough.

ciao!
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Old 12-06-2009, 16:40   #15
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I installed the dedicated AIS specific Shakespeare antenna on my Bimini,tip maybe 15ft. above W.L.,I pick up targets over 40 miles(really!) from my vessel.
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