AIS antenna: masthead or rail mount

[Calypso52]

Is there a material difference in reception if the antenna is mounted on the masthead versus on the rail. Clearly it is easier to mount it on the stern pulpit if only until the next time I pull the mizzen mast. Thanks.

[monte]

Yes, big difference, try mount it as high as possible ASAP.

[rgesner]

AIS and VHF isn't purely line-of-sight because the atmospheric conditions and sea state will affect if, but this distance to horizon calculator will give you an idea of the relative distances for different heights.
Distance to the Horizon Calculator

There is a lot more in this recent thread http://www.cruisersforum.com/forums/...?highlight=ais

[Zanshin]

Mounting the AIS antenna higher will certainly make a big difference.


But does it matter if a ship sees you at 40nm or only at 15nm or vice-versa? I have my antenna mounted on the aft rail and have seen cargo ships on the open sea change direction to avoid me (I transmit as well) when about 15 miles away; I don't know if they saw me earlier, but that distance is more than enough for me.

[Oliver L.]

Calypso,

I had mine on the aft railing and I was pretty much blind forward. Finally I installed a splitter on the VHF antenna and now I can see (and be seen) roughly 30 nm.

Oliver

[JPA Cate]

Calypso 52,

Friends have a stern mounted AIS. Ours is masthead ~66 ft., sharing the VHF antenna via a splitter. We consistently receive the signals from both class A and class B transmitters at significantly greater distances.

Ann

[MarkJ]

I agree with Zanshin.
I have both mounts and can swap my antennas to either VHF or AIS.
At sea I always have the AIS on the masthead antenna, and in port if I have friends about I have the VHF on the masthead.
The effective range difference is about what Zanshin said. 40:15 miles for the AIS

[Bill_Giles]

We have ours on the pushpit rail, we get a good 12 to 15nm range (we receive only). We have arranged some additional coaxial in the line so that if the mast comes down we have an immediately ready VHF aerial.

[monte]

I think it's hard to argue that seeing a target 40 miles out with a CPA of over an hour isn't safer than seeing it at 15 miles with a CPA around 30 minutes. We routinely see ships at 20 miles plus with a masthead antenna on a splitter, yachts at around 10 miles and occasionally yachts at around 1.5 miles. I assume the yachts at 1.5 miles have a low antenna or dodgy connection. Double the range = double the safety factor, so why wouldn't you, unless you sail in a bay 15 miles wide.

[Bill_Giles]

Where I sail in the English Channel ships are frequently changing direction, although predictably as they follow the TSS. I have never been troubled by only having 'sight' of a ship at twice normal visual range. I guess that we only have a small screen and having ships shown out to 40nm would be bad cluttering.

[DotDun]

IMO, it's a waste of time making navigation decisions more than 10 minutes from TCPA. There are simple too many variables to use a AIS borne TCPA more than 10 minutes and then using that information to make changes to direction/speed.

If you are not confident to avoid a collision within 10 minutes, please keep the docklines on and don't venture out.

Point: rail mount AIS is all you need for collision avoidance. But, if you get off on seeing ships far away, certainly the top of mast will give you a much better range. But, choosing longer AIS at the determent of VHF comms* is completely foolish, again IMO.

*A buddy boat recently chided me for my rail mount AIS, he could only see me 8 miles out. Same friend then mentioned he could only hear my side of VHF comms when we were right next to each other. I had no problem carrying out VHF comms @ 20nm. He could not hear the other half but refuses to believe his antenna splitter has anything to do with it.

Using an X switch as MarkJ does is the best of all worlds, it allows you to manually switch between the 2 antennas.

[El Pinguino]

Mine is taffrail mounted.... 'sees' ships at 20miles... as stated above by quite a few anything over 5 miles away is of interest but of no consequence.
Not worth the fuss and bother of sending another cable up the inside of the mast.
Some say they have seen ships alter for them when over 15 miles away on the basis of AIS reception... I think that is wishful thinking...

[monte]

Originally Posted by DotDun

IMO, it's a waste of time making navigation decisions more than 10 minutes from TCPA. There are simple too many variables to use a AIS borne TCPA more than 10 minutes and then using that information to make changes to direction/speed.



If you are not confident to avoid a collision within 10 minutes, please keep the docklines on and don't venture out.



Point: rail mount AIS is all you need for collision avoidance. But, if you get off on seeing ships far away, certainly the top of mast will give you a much better range. But, choosing longer AIS at the determent of VHF comms* is completely foolish, again IMO.



*A buddy boat recently chided me for my rail mount AIS, he could only see me 8 miles out. Same friend then mentioned he could only hear my side of VHF comms when we were right next to each other. I had no problem carrying out VHF comms @ 20nm. He could not hear the other half but refuses to believe his antenna splitter has anything to do with it.



Using an X switch as MarkJ does is the best of all worlds, it allows you to manually switch between the 2 antennas.

I guess you may be referring to vessels altering course with regard to the too many variables comment? And maybe the 10 minutes being close to shore or in high traffic areas? I certainly wouldn't want a 10 minute warning mid ocean or even close to shore. The longer the warning the better, but in built up areas shorter CPAs might be acceptable. 10 minutes to make collision avoidance decisions? Really, we start making those decisions as soon as possible and 10 minutes wouldn't be anywhere near enough time.

[DotDun]

Originally Posted by monte

I guess you may be referring to vessels altering course with regard to the too many variables comment? And maybe the 10 minutes being close to shore or in high traffic areas? I certainly wouldn't want a 10 minute warning mid ocean or even close to shore. The longer the warning the better, but in built up areas shorter CPAs might be acceptable. 10 minutes to make collision avoidance decisions? Really, we start making those decisions as soon as possible and 10 minutes wouldn't be anywhere near enough time.

I've sat and watch too many CPA of <.1nm at 15/20 minutes TCPA change considerable, ending up at >.5nm CPA without either vessel making course/speed changes. Hence, TCPA more than 10 minutes does not warrant navigation adjustments for me. It may very well be my chart plotter being unable to calculate good CPA at large distances.

YMMV

[monte]

Fair enough DD. We also see CPA changing over time and we (and most ships) generally aim for a minimum CPA of around 1 mile in open water. Obviously 1 mile isn't always possible in dense traffic. The sooner we can determine if course changes might be required, or contact with the other vessel might be required, the better as far as we are concerned. Here's a snap from mid Atlantic. Obviously most of the ships are of little concern but its still good to know whats out there and possible crossing situations ASAP, especially when horizon scans might be 15 minutes between. The range ring is 20 miles.