AIS transceiver antenna spreader or stern?

[Water Dog]

My mast is out in the boatyard. I seek help determining whether to run a new coaxial cable to a spreader about 26' above the water or to a tower on the stern ~10 feet above the water.

I've read here and elsewhere that, generally speaking, the higher the antenna is above the water, the better, but that when transmitting, the antenna is sensitive to nearby objects such as the mast and shrouds. If the AIS antenna is mounted on a spreader, I would hang it upside down to avoid tangles with running rigging.

Given that the antenna will receive and transmit, and, if installed on the spreader, be between the aluminum mast and the surrounding cap and lower shrouds, how should I decide between the two locations (spreader vs. stern tower)?

More context

I have a never installed AIS transceiver (Vesper Watchmate XB9010) that I want to install.

I cruise the Santa Barbara Channel Islands and may venture further. I want redundancy.

I have two Vesper VA159 antennas designed to be shared between VHF radio and AIS transceiver. I also have the Vesper AIS/VHF splitter SP160, but would like to have that ready as a back up. So I intend to dedicate the masthead antenna to the VHF radio and the second antenna (that this post focuses on) to the AIS transceiver. If one fails, I have the splitter ready to be used.

Thanks!

Joe

[Been lurking here for a few years. Thanks all for the info you share!]

[pcmm]

My implementation has my AIS antenna on an arch at the stern. The Arch also holds the GPS antenna, a spare VHF antenna, Solar and is the davits for my dinghy. I like to have redundancy and as I've been through the Erie canal. using a handheld for the day is really annoying as the battery never last long enough. With my mast down I can connect my main VHF to the spare on the arch and use my full power main VHF with its ram mic.

[smac999]

on the stern will work just fine. you might get 30mile range instead of 40 mile. but if a boat is 40 miles away it's not about to hit you.

the stern also can serve as a backup antenna moved to the vhf if you ever lose the mast.

[wholybee]

If you aren't going to mount it on the masthead, the stearn is fine. Probably better to have it in the clear instead of against the mast.

AIS has much lower power than voice VHF. As such, for voice the range is limited by the horizon, and every meter higher matters. AIS range is limited by power, and won't reach the radio horizon, so height, while important, isn't as critical as voice. A short coax run with the best coax you can get will probably make a bigger difference.

[Water Dog]

pcmm, smac999, & wholybee: Thank you so much for your thoughts. I'm smiling because you all seem content with the stern mount. I know that this CF forum is notorious for divergent views. So, it's nice that your repliles to my first post are consistent, although that may change quickly.

[Pete7]

Joe, if its on the stern and the mast falls down, then at least you are in a fighting chance of using it for a VHF. The cable on our stern antenna is long enough to reach the AIS and in emergency the VHF which has the same coax plug.

[Jammer]

Good advice upthread.


From a radio standpoint either location is good enough.


The real questions are practical and involve where the antenna will be in the way least. Can you get a halyard or sail caught on a spreader-mounted antenna? I'm sure it's been done. Is it a serious problem on your boat? Beats me.


On the stern side, do you have solar panels? Will the antenna shade them? etc.


I do not have an AIS installation at this point but when I add one it will likely have a stern-mounted antenna.

[Jim Cate]

FWIW: We have some experience with this issue. We have both a masthead VHF antenna and one on the stern arch, about 12 feet above sea level. With the masthead antenna via a Vesper splitter we routinely see class A AIS from ships at 30+ miles, occasionally much more due to exceptional conditions. From the arch, with no splitter involved, the range is reduced to 10-12 miles for class A ships.

This is not theoretical projection, this is real world data. Interpret it however you wish.

Jim

[Jammer]

Jim,


Did you find the longer range to be useful in a practical sense, or just a novelty?

[Jim Cate]

Originally Posted by Jammer

Jim,


Did you find the longer range to be useful in a practical sense, or just a novelty?

,

Good question! In m view the longer range for picking up ships isn't so important, but the implied better sensitivity means that picking up weaker signals from closer range is more likely, and that is indeed useful. It also implies that our signal is better transmitted, and that ships can then pick it up at greater distances, and that, of course, is beneficial!

Jim

[Alden]

Hello Joe.


I had great performance with a B&G radio and AIS while using a splitter, through a single mast-top antenna. There is value in having the splitter, to prioritise signals and avoid interference.


I say the performance was great. It was **** to start with. The builder's electrician was dyslexic and put the two BNC terminated cables from the antenna and the radio into the wrong ports of the splitter. This damaged both devices. Once I discovered this and replaced both (at their cost), the range of the whole setup was good enough to blow socks off. I was dubious about splitters, until I saw how good they can be.

[deblen]

It is best to not mount any radio antennae "inside" the cone of protection provided by a vessel's rigging. At radio frequencies,wire rigging,wet "rope",wet sails,etc. act the same as if you put a solid metal cone over the mast to deck.


Also,mounting a vertical antenna within 2ft of a vertical metal mast,stay,wet object-will make the antenna directional ie: performs extra well in one direction and poorly 180deg opposite direction.


On most sailboats,you only have 2 choices to avoid these situations:
1. Very top (truck) of mast-longer distance & best omnidirectional performance.

2. Rear arch/pushpit rail-outside of rigging-somewhat less distance,but adequate for sailboat speeds,somewhat directional-easier for maintenance.

Cheers/Len

[Water Dog]

Thanks, all! This is giving me so much clarity!

Jim Cate: I'm wondering, if you were using an AIS transponder, were you aware of any differences in broadcasting your boat's AIS info between using the top of mast antenna/splitter vs. stern antenna?

[Jim Cate]

Originally Posted by Water Dog

Thanks, all! This is giving me so much clarity!

Jim Cate: I'm wondering, if you were using an AIS transponder, were you aware of any differences in broadcasting your boat's AIS info between using the top of mast antenna/splitter vs. stern antenna?

No, I did not do the necessary experiment to determine broadcast range differential. I suspect that there is a difference but can not verify that suspicion. Takes a willing partner in another boat and a series of contacts as the boats move apart... over a period of several hours... a big ask!

Jim

[Water Dog]

Thanks, Jim