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Old 01-12-2010, 08:37   #31
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Shipborne class A units also have the capability to interrogate other ships AIS including class B units which will automatically acknowledge messages addressed to specific ships. This is one of the meanings for transponder; "A receiver/transmitter that will generate a reply signal upon proper electronic interrogation".

Eric
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Old 01-12-2010, 09:14   #32
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We have been using AIS for about three years now. I have to wonder, with all of the discussion of 20 miles verses 50 miles, what is the benefit of seeing a vessel 50 miles away verses the vessel 20 miles away, and if the vessel that is 50 miles away is coming towards you, he will show up at about the 20 mile limit. At that time, if he is traveling 20 MPH, he is still an hour away and plenty of time to make contact or react. Chuck
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Old 01-12-2010, 10:14   #33
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We have been using AIS for about three years now. I have to wonder, with all of the discussion of 20 miles verses 50 miles, what is the benefit of seeing a vessel 50 miles away verses the vessel 20 miles away, and if the vessel that is 50 miles away is coming towards you, he will show up at about the 20 mile limit. At that time, if he is traveling 20 MPH, he is still an hour away and plenty of time to make contact or react. Chuck
Of course you're right about 20 vs 50 miles. When the range drops to 5 miles you still have time to react (5 mile range, 25 kt closing speed = 12 minutes).

But, since vessels only transmit their name and callsign at six-minute intervals, you might have trouble immediately calling them by name. And, since at the edge of your reception range the AIS data can be marginal, you might not receive the ship's name before they pass (or collide!) Even with my spreader-top antenna, I've had situations where I never receive a ship's name. I'm sure the situation with my Class-B Transponder is even worse in this respect.

I think it is a good idea to do what you can reasonably can to maximize your AIS range. If the best you can do is a stern-mount whip, don't worry about it -- it's still going to be very useful.
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Old 21-08-2011, 17:23   #34
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Re: AIS Antenna Location / Splitter

Has anybody reading this topic had a splitter fail or has any one heard of a splitter fail? I have not. I have used one for a couple years now with no glitches. All this talk about fearing VHF AIS antenna splitters reminds me of sextants V/S GPS. How many splitter naysayers still have a sextant and reduction tables and the working knowledge to use the darn thing?
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Old 21-08-2011, 17:34   #35
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Re: AIS Antenna Location / Splitter

saben, have a read here and you might find some info. We have had two or three splitters fail in the last few years. I do installs for a living so I can tell you that it is not a myth and it does happen. I won't do an AIS installation on a splitter any more. Chuck
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Old 21-08-2011, 19:28   #36
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Re: AIS Antenna Location / Splitter

Saben, I'm not a pro like Chuck but I am an EE (& CS) & a Ham & I do installations for friends. As I mention below, it's not that I've had splitters fail but I've had problems getting them to work correctly. But that was a few years ago - perhaps the art has improved, & I'm glad yours seems to be working well. And despite having 5 GPSs on board & 4 computers all setup for nav, I do actually have paper charts & a sextant on board & I do know how to use them!
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Old 21-08-2011, 19:47   #37
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Re: AIS Antenna Location / Splitter

Good point Jon, it is not so much that the splitter fails, rather that it creates problems either for the AIS or the radio just when you need it the most. Having said that Miltech makes a very good splitter if one is a must, that we have never seen any problems with. Chuck
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Old 21-08-2011, 22:55   #38
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Re: AIS Antenna Location / Splitter

Quote:
Originally Posted by Waterwayguy View Post
...Miltech makes a very good splitter if one is a must, that we have never seen any problems with. Chuck
You're right, Chuck - Doug at Milltech Marine is a wonderful source of help & information. And since he has almost no overhead his prices are very good as well. I buy all my AIS gear from Miltech.
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Old 23-08-2011, 13:21   #39
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Re: AIS Antenna Location / Splitter

Thank's Chuck and John for your feed back on the splitters. When we talk about antenna splitters are we talking about apples, or apples and oranges. I have one of the Milltech Marine variety which I understand is a straight thru conductor for the VHF radio with a normally energized relay switch for the AIS tap. Upon radio transmission, or loss of power to the antenna splitter, the relay opens protecting the ais reciever. Do you know if this is how other splitters are constructed? Any idea what the failure mode of those units where you mentioned? I too have a sextant, just have to learn how to use it. Steve S/V Saben
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Old 23-08-2011, 20:30   #40
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Re: AIS Antenna Location / Splitter

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... I have one of the Milltech Marine variety which I understand is a straight thru conductor for the VHF radio with a normally energized relay switch for the AIS tap. Upon radio transmission, or loss of power to the antenna splitter, the relay opens protecting the ais receiver. Do you know if this is how other splitters are constructed? Any idea what the failure mode of those units where you mentioned? ... Steve S/V Saben
Hi Steve - Good questions. I've never opened one up but theoretically a splitter also needs to protect the VHF when the AIS transmits. So it has to sense both input lines & have 2 cut-out circuits. There may be cheaper splitters for receive-only AIS units that behave as you describe.

Chuck or Doug will have better hands-on experience but most transistor circuits will fail to a short first, & then if the power continues they may fail open. If your splitter really has a relay (instead of a transistor switch) then a good designer would design it to fail open, which is certainly better for your equipment than short. I'd think a relay would be too slow for good protection but perhaps not.

A friend mistakenly connected his AIS by just T-ing into his antenna line. He damaged the fragile input to his SR161 AIS receiver but it actually continued to work, just with reduced range (~4nm).
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Old 23-08-2011, 20:36   #41
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Re: AIS Antenna Location / Splitter

Like Jon, I have never taken one apart but can only speak to the results of failures. In two cases the AIS was damaged and in another the VHF was damaged. Both required returns to the manufacturer and the re-installs were done with seperate antenna. In a couple of cases neither the VHF or the AIS was receiving or transmitting through the antenna with the faulty splitter.
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Old 27-08-2011, 04:53   #42
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Re: AIS Antenna Location / Splitter

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Vesper Marine has a new splitter out that has low insertion loss and even has a pre-amp for the AIS receive port that provides 12db of gain...
But presumably no gain to the VHF, where it will cause a loss.
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