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AIS transponder - SOTDMA vs CTDMA  

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Steve Mitchell
(@stevemitchell)
Member Moderator
Joined:5 months  ago
Posts: 47
April 21, 2018 8:29 pm  
Posted by: Saffy The Pook

From a spec standpoint, SOTDMA may be better than CTDMA but with a good install, even with long cable runs and a shared antenna, CTDMA works great. I'd be willing to bet that most AIS installations are limited by compromised cables/connectors/antennas rather than the technology of the AIS unit itself.

I had a similar setup, same Vesper unit, active splitter with cable inside, and then 50' to the top of the mast with a generic Shakespeare VHF antenna. I could see things for a decent length, but not to Port Townsend.

Switch to the same AIS unit, but a stern mounted, correctly tuned AIS VHF antenna, no splitter, and shorter cable of 15' and I could see further than the mast mounted antenna, and boats locally close in far more accurately than ever before. 

Cabling and antennas matter a heck of a lot! I still think the SOTDMA and higher transmit power of the AMEC unit has helped me appear more frequently locally.

Editor, SailBits.com


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Saffy The Pook
(@saffythepook)
Active Member
Joined:4 months  ago
Posts: 5
April 22, 2018 10:55 am  

I don't disagree that with an apples to apples setup, SOTDMA will be better. With SOTDMA instead of CTDMA, maybe I could see all the way to Friday Harbor! 😀 However, for my purposes a 30 mile range is more than I need even with fast traffic. I also agree that a dedicated AIS antenna is ideal, assuming you have a good place to mount it (which I don't, unfortunately).

I suspect that the "out of sight out of mind" nature of cabling and antennas leads to a lot of AIS and VHF performance problems. My advice to my dockmates who ask me about AIS is to worry less about which AIS unit you buy and more about whether the antenna and cable are in good shape. However, for folks going into blue water I will make a plug for the Vesper wideband AIS/VHF antenna even if you're dedicating it to your AIS. If something should happen to your masthead VHF antenna, having the wideband secondary antenna would allow you to use it with your VHF in an emergency. 


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Richard Cassano
(@rcassano)
Active Member
Joined:4 months  ago
Posts: 3
April 22, 2018 11:32 am  

I find the comments on the Vesper splitter interesting. I have the XB8000 and the Vesper splitter with LMR 400 to a Vesper AIS/VHF antenna on the masthead. There is a cable to cable PL259 at the base of the mast and all connectors were soldered by me, (I thought I did a fine job, but you never know). When first installed I was just slightly in the Red on the SWR which was unacceptable. So to correct this I replaced all the cable with factory installed PL259's but kept the splitter in the system and the masthead antenna. This only slightly improved the SWR to just acceptable. If I eliminate the splitter and switch to a bimini mounted, plain VHF antenna and cheap coax my SWR drops significantly, (sorry this is not a scientific evaluation with recorded SWR numbers). The question is, am I better off with low SWR and an antenna 10 feet off the water without a splitter or just acceptable SWR at the masthead with a splitter?


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Steve Mitchell
(@stevemitchell)
Member Moderator
Joined:5 months  ago
Posts: 47
April 22, 2018 1:06 pm  
Posted by: Saffy The Pook

I don't disagree that with an apples to apples setup, SOTDMA will be better. With SOTDMA instead of CTDMA, maybe I could see all the way to Friday Harbor! 😀 However, for my purposes a 30 mile range is more than I need even with fast traffic. I also agree that a dedicated AIS antenna is ideal, assuming you have a good place to mount it (which I don't, unfortunately).

I suspect that the "out of sight out of mind" nature of cabling and antennas leads to a lot of AIS and VHF performance problems. My advice to my dockmates who ask me about AIS is to worry less about which AIS unit you buy and more about whether the antenna and cable are in good shape. 

I guess it depends on what your goals are. I want to be able to see things in short range, preferably a 5-10NM range around me, but also ensure that my location is broadcast a similar range very accurately.

So in my case, the antenna being on the radar arch limits my line of sight compared to the masthead, but I only really want the local data anyhow.

The 5W power along with SOTDMA puts my signal going out higher up in the probability of being seen by someone else, which is meeting that second criteria.

The Vesper wide band antenna is good - I had it on my last boat or on a test rig on this boat (I forget) and it was great. It is not as good as a specifically tuned AIS antenna, but even my AIS tuned antenna could function as a standard VHF antenna in a pinch. If I were going offshore I would probably have dual antennas that were both similarly tuned like the Vesper, or one Vesper, and one tuned to VHF only.

Editor, SailBits.com


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Steve Mitchell
(@stevemitchell)
Member Moderator
Joined:5 months  ago
Posts: 47
April 22, 2018 1:09 pm  
Posted by: Richard Cassano

I find the comments on the Vesper splitter interesting. I have the XB8000 and the Vesper splitter with LMR 400 to a Vesper AIS/VHF antenna on the masthead. There is a cable to cable PL259 at the base of the mast and all connectors were soldered by me, (I thought I did a fine job, but you never know). When first installed I was just slightly in the Red on the SWR which was unacceptable. So to correct this I replaced all the cable with factory installed PL259's but kept the splitter in the system and the masthead antenna. This only slightly improved the SWR to just acceptable. If I eliminate the splitter and switch to a bimini mounted, plain VHF antenna and cheap coax my SWR drops significantly, (sorry this is not a scientific evaluation with recorded SWR numbers). The question is, am I better off with low SWR and an antenna 10 feet off the water without a splitter or just acceptable SWR at the masthead with a splitter?

This is almost exactly what I did. I had an unknown vintage VHF antenna with cable going up the mast. I put my XB-8000 on the boat when I bought here a couple years ago, added the Vesper VHF splitter, and got not-so-good SWR readings. Replacing ends helped, but it still never got to where I wanted it to be. VHF from my DSC radio was also not good.

Moving the AIS antenna to a radar arch above the bimini, and having only 15' of cable plus an AIS tuned antenna has brought my SWR down to 1 or less. I also "see" more ships, and my class B stuff seems to be broadcast better, even before the AMEC SOTDMA transponder.

Now, I am finding out that my VHF cabling and antenna on the mast are pretty messed up, so that could very well have been the root of the problem, but if you can, I would recommend separating them.

Editor, SailBits.com


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Geoff Roddick
(@geoff-roddick)
Active Member
Joined:4 months  ago
Posts: 3
April 23, 2018 12:07 pm  

Just thought I'd add to this discussion that I just spoke with Digital Yacht and they are coming out with two SOTDMA transceivers the AIT2500 and AIT3500 which are upgrades to the AIT2000 and AIT3000 and will look identical.  Pricing is expected to be about $300CND more and be out in 1-2 months.

What was interesting is that they are claiming that there really is no benefit to the SOTDMA vs CSTDMA unless you are a fast boat which will gain due to the increase in update rate on the transmissions.  He didn't think the power nor the self organizing made a difference!?  (not that I agree although my knowledge is theoretical at this point)

 


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Ben Ellison
(@ben-ellison)
Member Admin
Joined:6 months  ago
Posts: 46
April 24, 2018 10:53 am  

While I'm cautious about splitting a VHF antenna with AIS, I've been using the Vesper model for years and have not been able to detect a significant difference when I bypass it. I have not tried a specific AIS tuned antenna in a while though, but I have one and will try to A/B test that too.

Actually, anyone can see Gizmo's AIS reception right now and it's pretty cool how that's happening. I'm beta testing a new version of the FloatHub software that allows a user to forward AIS info to Marine Traffic and other sites:

https://www.marinetraffic.com/en/ais/details/stations/937


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