Where to Mount AIS Antenna?

[Dockhead]

Thanks to everyone who helped me figure out where to install the antenna for HF DSC through my Icom M802.

I am also installing AIS for the first time and next in line is that system's antenna.

Since AIS works over VHF Channel 70, I presume that it is strictly line of sight, and so height here is important. Or is it? I know I can't put it at the masthead lest it interfere with the regular VHF antenna. So should I put it on a spreader? The third spreader nearest the masthead? Or is the first, lowest spreader ok? The lowest spreader is about 9 meters above the water, about 30 feet. The masthead is 75 feet. Obviously the lower it is, the shorter the cable run, less signal loss, and less negative effect of windage.

I would put it on the pushpit if it wouldn't unduly curtail the range. I guess that will be around 3 meters above the water. The advantage there is that it will stay up in case, God forbid, the mast ever comes down, so can double as an emergency VHF antenna, plus cheaper and easier to install, less windage, etc., etc.

[Rhapsody-NS27]

When it comes to VHF antennas, I tend to be in the group of "higher is better" but considering the main antenna is at the masthead, the highest spreader, I would think could work just as well. It sounds like it would be high enough to send/receive any information that would be of interest to you. If your concern is interference with the main VHF antenna at the mast head, you might be able to try clamping the antenna to the spreader first to do some testing before a permenant mount is installed may be helpful.

[Goudurix]

After using the masthead VHF antenna, via the splitter built-in my Raymarine AS250 AIS receiver, I found out I had some interference on channels adjacent to AIS frequency, coming through the splitter.

I then decided to install a dedicated AIS reception antenna on the puspit, with its base about 2 m above water level. It received well enough but I suspect the HF-SSB now to have damaged the AIS receiving unit due to the proximity of the SBB-sloping wire antenna to the AIS receiving antenna on the pushpit (they are about 2.5m apart, on both side of the pushpit).
Though this might be due to spurious harmonic frequencies sent by my Yaesu FT897 Ham transceiver, you might be better of with the Icom 802 marine SSB since it has better specs.

Jan

[Jim Cate]

G'Day Dockhead,

I won't try to make a recommendation, but here is an observation on our boat:

Our original AIS/VHF antenna was on the radar arch, about 3 metres above the WL. We were able to receive AIS signals from ships at around 25-30 miles distance on a regular basis. Recently had the mast out and installed a proper 3 db whip at the masthead with good coax feedline. We now routinely see ships at over 100 miles, and have seen class B yachts at 25-30.

So, yes, higher is better, but it is a personal decision as to whether that additional range is useful. Frankly, I'm not so sure it is. Especially in busy areas like you sail in the target lists will get to unmanageable sizes with the extra range, so you will likely filter them out anyway, so w hat's the point?

Good luck with the decision

Cheers,

Jim

[LJH]

Originally Posted by Jim Cate

G'Day Dockhead,

I won't try to make a recommendation, but here is an observation on our boat:

Our original AIS/VHF antenna was on the radar arch, about 3 metres above the WL. We were able to receive AIS signals from ships at around 25-30 miles distance on a regular basis. Recently had the mast out and installed a proper 3 db whip at the masthead with good coax feedline. We now routinely see ships at over 100 miles, and have seen class B yachts at 25-30.

So, yes, higher is better, but it is a personal decision as to whether that additional range is useful. Frankly, I'm not so sure it is. Especially in busy areas like you sail in the target lists will get to unmanageable sizes with the extra range, so you will likely filter them out anyway, so w hat's the point?

Good luck with the decision

Cheers,

Jim

Dockhead,

I will make a recommendation from my observations.

As Jim points out target range is greatly increased by selecting a good antenna location. I decided to use the Vesperamirne antenna splitter and use my masthead VHF antenna (70 feet up). Like Jim I pick up class A signals at great ranges that lets me plan a route through shipping channels early. Class B signals are weaker and many come from poorly placed antennas. While I receive some class B signals at ranges over 25 miles, there are many that do not show up until the range is around 10 miles or less!

Don`t forget the principle of see and be seen. With the increased range of a high antenna placement, you will have target lists that need filtering in the Channel, but you will also increase the range that others can see you. Would you prefer that cargo ship to see you 1 hr TCPA or 15 mins? If you would rather be seen early, I would recommend a higher installation.

Cheers,
Lyndon

[Dockhead]

Thanks -- very useful advice from everyone!

[neelie]

I'd leave it low (say first spreader?) and accesible and rely on the other traffic to have their antennae mounted higher up - roughly speaking, its the sum of the two antennae heights which affect the range. After all, the traffic that's going to run you over and give you a bad hair day is probably over 40 meters tall.

[DotDun]

I went low, basically rail level. I still receive class A at 10-15 miles and class B 5-8 miles minimum. It's been reported to me that I've been seen at 5-8 miles.

IMO, on a 7kt boat, this gives plenty of time for course adjustments.

[Goudurix]

I really don't understand an advice to install it low...?
If you have the opportunity to install it high, like masthead, or use the existing marine VHF antenna with a splitter, do so.

If installing masthead or other high location try to use a coax that is better than the usual RG-58 while you're at it....every dB won is won.....

Good luck.

Jan

[chrisjs]

At a combined closing speed of even 22kts (15 + 7) your 5 mile range equates to only 13 minutes before impact!! More (higher) is definitely better!!!!
You do need to avoid having your VHF and AIS antennas close (at the mast head) because they can interfere and the outgoing AIS signal may even damage your VHF. The idea of mounting at the spreader seems like the best compromise.

[s/v 'Faith']

Went with mounting on radar mast, lower then radome to minimize interference, maybe 2 meters off the water.

In the Virgin Islands right now, had this set up for about 2000 miles and 3 months now.

I recommend lower, rather then higher. Do you care about a boat 100 miles away? Keep in mind all that clutter will show up when you zoom out. Shorter runs, back up (non-mast mounted) antenna, reasonable range and self selection of targets you actually care about....

Lower is better, IMHO.

[Thonord]

Originally Posted by DotDun

I went low, basically rail level. I still receive class A at 10-15 miles and class B 5-8 miles minimum. It's been reported to me that I've been seen at 5-8 miles.
IMO, on a 7kt boat, this gives plenty of time for course adjustments.

Thats what I would do. Who wants unnecessary weight in the mast and distant AIS info anyway. You'll see the vessel a lot earlier on the AIS than you used to see it visually, even with the antenna on the rail or pushpit.

Tom

[LJH]

Originally Posted by DotDun

I went low, basically rail level. I still receive class A at 10-15 miles and class B 5-8 miles minimum. It's been reported to me that I've been seen at 5-8 miles.

IMO, on a 7kt boat, this gives plenty of time for course adjustments.

Originally Posted by Thonord

Thats what I would do. Who wants unnecessary weight in the mast and distant AIS info anyway. You'll see the vessel a lot earlier on the AIS than you used to see it visually, even with the antenna on the rail or pushpit.

Tom

Dockhead sails in an area with high volume of traffic and a large variety of traffic that can include high speed ferries. It would be possible for his traffic to be doing 45 Kts. Assuming he is mounting a class B AIS and his antenna is on the rail that would limit his transmission range to 5-8 miles, or about 5 mins TCPA for a 45 kt rate of closure. Personally, I would rather been seen with a TCPA of at least 15-20 mins.

[DotDun]

Originally Posted by LJH

Dockhead sails in an area with high volume of traffic and a large variety of traffic that can include high speed ferries. It would be possible for his traffic to be doing 45 Kts. Assuming he is mounting a class B AIS and his antenna is on the rail that would limit his transmission range to 5-8 miles, or about 5 mins TCPA for a 45 kt rate of closure. Personally, I would rather been seen with a TCPA of at least 15-20 mins.

So what does the high speed ferry do with non-AIS vessels run over 'em?

[LJH]

Originally Posted by DotDun

So what does the high speed ferry do with non-AIS vessels run over 'em?

I will answer this with he just might.

Aircraft have an Air Traffic Control System and many collision avoidance systems, yet there are still many incidents that occur every year. You don't ask your pilot to turn his TCAS off when you board your flight.

Why do so many people whose life is made safer by these systems resist putting similar systems to good or proper use in their boat?

Accidents usually occur as the result of a chain of events. Make yourself visible and identify your self early - increase your odds. It is about his decision making as much as it it yours. You quickly become one less contact for him to concern himself with and reduce his workload slightly. Safer for all!

If you are concerned about the number of contacts you have on your contact list, learn to use filters to reduce your target list to those that concern you.