Twin areials

[Tupaia]

I currently have a mast head VHF antenna and a dedicated AIS (transponder) stubby antenna just above my bimini, limited range.



I previously had a splitter and found unacceptable interference on certain VHF channels, hence the stubby.


I am thinking of installing a second mast head antenna, one for VHF and the other for AIS.



Should I :
1. Ignore all the instructions and install them side by side about 300mm between them and see what happens.


2. Install one up and one down, obviously the downward one would be 150mm away and parallel the the mast. Good or bad?


3. Install a stubby ahead of the radar dome, protected by the baby stay ahead of that.


4. Install on a spreader - really don't like this idea.



5. Suggested alternatives.

[GordMay]

Would an Antenna Switch [used in reverse] work?
"Choosing a Coax Switch" ➥ https://www.onallbands.com/choosing-...itch%EF%BB%BF/

[SailingHarmonie]

You are very likely to have MORE interference issues if you mount both antennas at the masthead. A great deal of work for what is likely to be a poor result.

My first suggestion would be the spreaders. It's a common solution, and works well.

Second choice would be to find a splitter that works well. Some do. The masthead height on AIS, while not critical, is nice to have.

I am not sure I understand how a splitter causes "interference" on the VHF, unless the coax connections to the device are bad. The vast majority of the time on a TypeB a splitter is doing nothing but sharing the incoming signal with the two receivers. I might be missing something, but I am sure one of the splitter haters will quickly enlighten me...

I don't have a splitter because I have the luxury of two masts and two antennas. But I sail on several boats that use splitters and the real world performance of the AIS and VHF seems fine.

[Tupaia]

Originally Posted by SailingHarmonie

You are very likely to have MORE interference issues if you mount both antennas at the masthead. A great deal of work for what is likely to be a poor result.

My first suggestion would be the spreaders. It's a common solution, and works well.

Second choice would be to find a splitter that works well. Some do. The masthead height on AIS, while not critical, is nice to have.

I am not sure I understand how a splitter causes "interference" on the VHF, unless the coax connections to the device are bad. The vast majority of the time on a TypeB a splitter is doing nothing but sharing the incoming signal with the two receivers. I might be missing something, but I am sure one of the splitter haters will quickly enlighten me...

I don't have a splitter because I have the luxury of two masts and two antennas. But I sail on several boats that use splitters and the real world performance of the AIS and VHF seems fine.

Really not keen on the spreader idea, I know a lot of boats do it. The means of attachment to the spreader would worry me.


My splitter went back to the manufacturer because of consistent and significant interference on CH17 - quick short range channel choice. It came back and it did not interfere on CH17 but other channels still seemed to suffer and that did not occur with the AIS switched off. Maybe I should source and alternative splitter. Things move on.


Maybe I replace the existing mast head and coax and then try a splitter.

[Hartleyg]

FWIW, we've had our AIS antenna (1/2 wave vertical whip) on the radar mount (~35') for 15 years, and it seems to work well. We also have a masthead VHF antenna and our "day-to-day" helm VHF antenna atop the solar panel mount back aft, about 15' above the water. Works fine for coastal/harbor work and you don't have to listen to the USCG blatting away 100+ miles up the coast . Never hear anything interfering from the AIS (and I know what the AIS sounds like).


Hartley
S/V Atsa

[sailingharry]

AIS antenna is on my mind these days. My current antenna is mounted on my arch, but due to an accident my arch is being replaced. Everything is up for re-evaluation.
In my current setup, I have about an 8 mile maximum range. I sail the crowded waters of Annapolis, so have a lot of targets to examine. I can physically SEE freighters before I get AIS. A real issue.
While I really don't care about 20 mile range (send or receive), 8 mile is a bit short. Moving to spreaders or radome (moving to mast to get shadow off the solar) is a thought, as is a splitter at the masthead.

[svmobert]

A good splitter (zero loss Digital Yacht for example) and a good combo antenna (GAM SS-2 VHF-AIS Split antenna) would be far better than two masthead antennas.

For what it's worth, we had a splitter and standard vhf antenna (not an ais/vhf combo) at the masthead and had really great performance for AIS and VHF, better than many other boats we sailed around/with

[wholybee]

Originally Posted by sailingharry

AIS antenna is on my mind these days. My current antenna is mounted on my arch, but due to an accident my arch is being replaced. Everything is up for re-evaluation.
In my current setup, I have about an 8 mile maximum range. I sail the crowded waters of Annapolis, so have a lot of targets to examine. I can physically SEE freighters before I get AIS. A real issue.
While I really don't care about 20 mile range (send or receive), 8 mile is a bit short. Moving to spreaders or radome (moving to mast to get shadow off the solar) is a thought, as is a splitter at the masthead.

If you are only getting 8 mile receive on AIS then something is really wrong/broken with your installation. Moving the antenna or combining it with the VHF antenna via a splitter might fix it by accident, but not knowing the problem might not fix it or might create that problem for your VHF.

You should take a careful look at your current installation.

[donradcliffe]

I agree that you must be having an installation problem. I can pick up most ships 30 miles away with a stainless whip antenna mounted at bimini height. The other consideration is you may have some big noise generators on your boat. Try turning off all other electronics and led lights an checking your receive range again.

[Jammer]

Originally Posted by Tupaia

1. Ignore all the instructions and install them side by side about 300mm between them and see what happens.

No need for that because lots of people have run that experiment. It will perform worse than your splitter did. What you'll get is receive desensitization, that is, when your AIS is transmitting you'll get dropouts in voice reception, and vice versa. When they're both transmitting, you'll get intermodulation distortion, which will interfere with various VHF users on non-marine frequencies.

2. Install one up and one down, obviously the downward one would be 150mm away and parallel the the mast. Good or bad?

Better than #1 but still too close. It would sort of work, with a dipole (longer, around 1.2m long) antenna. With most shorter antennas the mast ends up being, electrically, part of the antenna, leading to receive desense.

3. Install a stubby ahead of the radar dome, protected by the baby stay ahead of that.

Could work OK if the masthead antenna is longer (1.2m+) and thoroughly decoupled from the feedline and the mast through the use of a current balun at the top of the mast.

4. Install on a spreader - really don't like this idea.

That will work electrically but will get caught in the sails.

5. Suggested alternatives.

Fix what you have. Seriously. The VHF/AIS splitters ordinarily work quite well. If yours wasn't then there's probably something wrong with your feedline, antenna, or the splitter itself. Have someone with the proper equipment and expertise check the installation.


The other alternative is to get a better AIS antenna and put it on whatever length mast you think you need to get it high enough. Switching to a proper 1.3m+ long antenna will make a big difference, Galaxy 5225 XT AIS or similar. Use quality feedline. Consider mounting it to the taffrail or side deck with a good ratcheting mount and a 1m or so extension. That will get it above the bimini without putting added stress on the bimini frame.

[svmobert]

This is good advice.. OP says there is a stubby antenna at bimini.. but without details such as the antennas center frequency and gain, and the cable type and length between the antenna and the antenna, it's hard to know what is expected.. A small diameter RG cable and a low gain stubby antenna at bimini height may completely explain the 8nm range. A standard stainless whip alone may improve things, or larger diameter coax..

A quick search of stubby AIS antennas indicates at least some of them are around 1db gain, where a standard stainless whip antenna will be more like 3db. 3db gain = doubling of signal.

RG58 at 20ft will insert about 1.4db loss. LMR240 = 0.7db, RG213 = 0.6db, and LMR400 = 0.4db.

So assuming a 20ft RG-58 cable, and 1db stubby.. Switching from that to LMR240 and Whip antenna with same cable length and mounting location you go from -0.4db total signal gain at the AIS transceiver antenna port to +2.3db total gain, pretty much double the signal.

As mentioned below, LED lights and other emitters close to the AIS antenna may impact SNR and reduce range as well. Old, corroded connectors, poor soldering, etc can all impact it as well.

Start with identifying a few key things.. specs on your stubby antenna and type and length of the cable to start. Ensure that the cable and connectors are in good shape. Then make a plan..

Note also, if you have small RG cable up the mast for VHF, you should consider upgrading to RG213/LMR400 next time you service the mast or rigging, and then upgrade to a GAM SS-2 Split antenna at the masthead, add a zero loss splitter and all new connections at radio, splitter, and AIS and you should see good performance overall.

Originally Posted by donradcliffe

I agree that you must be having an installation problem. I can pick up most ships 30 miles away with a stainless whip antenna mounted at bimini height. The other consideration is you may have some big noise generators on your boat. Try turning off all other electronics and led lights an checking your receive range again.