AIS antenna: masthead or rail mount

[J Clark H356]

I have Mast mounted antenna at 58 feet with a splitter for IAIS made by Digital Yacht. At Indian Key 1-1/2 years ago, I saw ships near the Dry Tortugas at 155 NM. On Kentucky lake 27 to 30 NM is average, but can be less due to terrain. Today, the max distance I saw was 27.7 NM.

Coming back up Tenn-Tom I had mast down and used a Shakespeare antenna made for AIS mounted on 10 feet of PVC attached to my stanchion and coax cable to my pick up point at the mast/deck joint. I would see 8 to 10 miles and using INAVX could see the mile marker of the tow and had plenty of time to work out where I wanted to pass. There is a big difference in 16 feet and 58 feet in terms of range for your receiver.


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[DotDun]

Originally Posted by 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.

I believe we agree.

The point I'm trying to make is that spending lots of $$ and performing herculean tasks (at the detriment of VHF comms) in order to receive AIS signals at 50, 40, 30 nm out really doesn't provide value to navigation. A rail mount AIS antenna will give you 20+ minutes to make navigation decisions, more than enough time necessary to avoid a collision.

[colemj]

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.

The difference between 10min and 30+min can easily be the difference between changing direction a degree or two or having to tack/jibe. In open water, I prefer to see as far as possible, and certainly further than 10min out.

I can avoid a collision with just seconds of warning (do it all the time with logs, etc), but that doesn't mean I prefer short warning times, nor does it make one dangerous to prefer them. Do you intentionally restrict your eyesight and hearing to only 10 minutes out? Your depth sounder to 15'? Keep your radar on 1.5nm setting?

I will consider keeping the docklines on and staying put if we ever get to a dock.

Reception is only half the equation. A class B should strive to make the most of the low power it is restricted to. Two class B's mounted on rails can be a very short communication distance - and one or both of those class B's can easily be moving 20kts+.

A single anecdote about a buddy's splitter should not be generalized. There can be poor splitters and poor installations out there, but anyone contemplating a splitter can be assured that there exists units that have zero insertion loss, give full priority to VHF voice, and 100% fail over to VHF voice should something go wrong.

IMO, the only valid reason to NOT use a quality splitter is redundancy.

Mark

[colemj]

Originally Posted by DotDun

The point I'm trying to make is that spending lots of $$ and performing herculean tasks (at the detriment of VHF comms)

Again, there is zero detriment to VHF comms if one chooses a quality splitter.

The money part is arguable. A good splitter costs $200-250. A good antenna/coax system can be similar money.

Mark

[Capt.Alex]

When I installed my AIS I decided to to take the opportunity to build in some redundancy to my comms. I added a second VHF antenna to the stern arch for the AIS and added a splitter and a second VHF Radio in the aft cabin. If my primary radio or masthead antenna fails, or God forbid, the mast comes down, I have a second full function VHF and the AIS still works. As others have said, the reduced range isn't that big of a deal.


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[DotDun]

Originally Posted by colemj

The difference between 10min and 30+min can easily be the difference between changing direction a degree or two or having to tack/jibe. In open water, I prefer to see as far as possible, and certainly further than 10min out.

I agree in the theory.

My experience has been slightly different.

Next time you're tempted to change course a degree or two at 20 minute TCPA, refrain and simply stand on to see what the CPA is at 10 minutes. Still plenty of time to change course and most likely without a tack/gybe, unless you're running on the edge. I find the CPA has changed a fair amount in that 10 minutes.

When you make a course change at 20 minutes, you're still watching at 10 to make sure you're good, hence my point, CPA has great variability over time. (If CPA calculations were 100%, you wouldn't need to watch a target after making a course change.)

Originally Posted by colemj

I can avoid a collision with just seconds of warning (do it all the time with logs, etc), but that doesn't mean I prefer short warning times, nor does it make one dangerous to prefer them. Do you intentionally restrict your eyesight and hearing to only 10 minutes out? Your depth sounder to 15'? Keep your radar on 1.5nm setting?

Really? The conversation isn't about restrictions, it's about the RoI of making additions.

FWIW, I set my radar on 6nm range to get the best resolution for night time collision avoidance. It allows me to see the little guys (FG 20 footers) at about 1.5nm.

Originally Posted by colemj

I will consider keeping the docklines on and staying put if we ever get to a dock.

Reception is only half the equation. A class B should strive to make the most of the low power it is restricted to. Two class B's mounted on rails can be a very short communication distance - and one or both of those class B's can easily be moving 20kts+.

My experience has been a Class B using a rail mount has a range of ~8nm. Two of them with questionable installation may see each other at only 4nm. Still further than radar can pick up a 30' FG sailboat with no reflector and plenty of time to avoid a collision.

No evidence, but I would guess a rail mount Class B xcvr can be seen by a 100+' high antenna on the big guys at 12-15nm minimum.

Originally Posted by colemj

A single anecdote about a buddy's splitter should not be generalized. There can be poor splitters and poor installations out there, but anyone contemplating a splitter can be assured that there exists units that have zero insertion loss, give full priority to VHF voice, and 100% fail over to VHF voice should something go wrong.

IMO, the only valid reason to NOT use a quality splitter is redundancy.

Mark

[DotDun]

Originally Posted by colemj

Again, there is zero detriment to VHF comms if one chooses a quality splitter.

Agree, as long as the box is the middle of the coax works as advertised and it never breaks in an unexpected manner.

Originally Posted by colemj

The money part is arguable. A good splitter costs $200-250. A good antenna/coax system can be similar money.

Mark

Even at WM prices, an antenna/coax is about 1/2 the cost. Add an X switch and you have the best of both worlds, you get the range desired and redundancy.

I've certainly spent boat bucks on more foolish things than an antenna splitter or X switch. But, I've yet to see the value in extending AIS range past what I consider useful.

[colemj]

Originally Posted by DotDun

Next time you're tempted to change course a degree or two at 20 minute TCPA, refrain and simply stand on to see what the CPA is at 10 minutes. Still plenty of time to change course and most likely without a tack/gybe, unless you're running on the edge. I find the CPA has changed a fair amount in that 10 minutes.

I have already spent considerable time watching this. On open water, large ships do not change their bearings in any practical sense regardless of how rough the water is. In rough water, our bearing can change considerably - but a 0nm CPA rarely changes away from +/- 0.25nm, which is very close in this situation - particularly at night.

In non-rough water, I can watch a CPA stay rock steady from far away. For two small recreational Class B's, I agree that CPA's change constantly.

Perhaps this has more to do with the specific gear on our boats? I don't think the large ships are a variable.

Mark

[DotDun]

Originally Posted by colemj

I have already spent considerable time watching this. On open water, large ships do not change their bearings in any practical sense regardless of how rough the water is. In rough water, our bearing can change considerably - but a 0nm CPA rarely changes away from +/- 0.25nm, which is very close in this situation - particularly at night.

At what time interval?

I agree at 10 minutes, but at 30 minutes or even 20 minutes, I've seen it change more than that.

Originally Posted by colemj

In non-rough water, I can watch a CPA stay rock steady. For two small recreational Class B's, I agree that CPA's change constantly.

Perhaps this has more to do with the specific gear on our boats? I don't think the large ships are a variable.

Mark

You have more open water experience than I do. Mine experience is mostly the FL Straits, GoM, Yucatan area, hence it's very possible the big boys are actually making course changes and calling me names for 'playing chicken' with them as lately I stand on until 10 minute TCPA.

I agree .25nm CPA is too close.

I use my Furuno NN3D for CPA calculations.

[jt_kilroy]

Higher the better, no doubt about it.

On Edit - I feel the real value isn't you seeing them, its them seeing you. You are carrying a class B right?

[Ndavies]

Wow - this post may be the next "what's the best anchor"

I gave location of the antenna much consideration when I installed AIS. I opted for rail mount near the stern. I agree that masthead would be better, but I didn't go this route for 2 reasons: (i) I was not sure if a second antenna on the masthead could interfere with my primary VHF, and also didn't want the hassle of threading a new cable down the mast; and (ii) I decided against a VHF splitter as its just one more thing to go wrong.

My rail mount antenna seems to do well and results are consistent with previous posts.

Neil

[hamburking]

In addition to reception, keep in mind that class B AIS will be transmitting a short burst every minute at 2watts. Your stern rail antenna is likely right next to where all your people are sitting. Although 2watts is not a lot of power, its a lot more than a cell phone, and people are picky about that...and this is transmitting (a few seconds per minute) all the time.

In short, masthead is better.

I think not everyone realizes you can buy a splitter and share your masthead vhf antenna with the AIS. This splitter is an electronic device, not just a Y shaped connector.

[DotDun]

Originally Posted by hamburking

In addition to reception, keep in mind that class B AIS will be transmitting a short burst every minute at 2watts. Your stern rail antenna is likely right next to where all your people are sitting. Although 2watts is not a lot of power, its a lot more than a cell phone, and people are picky about that...and this is transmitting (a few seconds per minute) all the time.

I wonder what they think of using a 5watt VHF handheld 3 inches from their head?

Originally Posted by hamburking

In short, masthead is better.

I think not everyone realizes you can buy a splitter and share your masthead vhf antenna with the AIS. This splitter is an electronic device, not just a Y shaped connector.

[colemj]

Originally Posted by hamburking

In addition to reception, keep in mind that class B AIS will be transmitting a short burst every minute at 2watts. Your stern rail antenna is likely right next to where all your people are sitting. Although 2watts is not a lot of power, its a lot more than a cell phone, and people are picky about that...and this is transmitting (a few seconds per minute) all the time.

This is of no safety concern at all. Most powerboats, by nature, have rail-mounted or similarly low antennas. Even many HF installations have part of their antenna transmitting straight through their boats inches from their heads.

Mark

[Chuck Hawley]

Two thoughts on AIS antennas:

1. One of the best uses of AIS is to find a person in the water with a small AIS transmitter/beacon like the Kannad or Ocean Signal. The higher antenna on the mast will do a dramatically better job of "hearing" the beacon. Putting the antenna low might limit the range of detection to 0.5NM or so.

2. If you're going to put a second antenna on the masthead, that's a bad idea. Consider using a low-loss splitter like those from Em-Trak for both your VHF and AIS. While there is a high-loss splitter on the market (AVOID!), the Em-Trak model has negligible loss and solves the problem.

Chuck