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Old 18-06-2011, 16:21   #16
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Re: AIS reception issue

Let me go back to the bimini theory again. It's a large metallic structure (the frame) forward of the AIS antenna, and the two VHF antennas run within a few inches of it's sides.

Can anyone envision how the framework could be creating some sort of sink or shield for the AIS signal? Could it be picking up and radiating some lesser signal enough to make a problem? As I mentioned earlier, the frame is currently ungrounded.
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Old 18-06-2011, 16:28   #17
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Re: AIS reception issue

Are the AIS receiver and VHF radios on the same fuse? Does the supply voltage to the AIS radio drop when you turn on one of your VHF rigs? It's the only thing I can think of. Good luck with the problem.
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Old 18-06-2011, 18:43   #18
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Re: AIS reception issue

Regarding the bimini and the other antennas, I really doubt that there will be a major antenna (radiation) pattern issue. If the dimensions were exactly right you might have some deep peaks and nulls, but for an essentially random configuration you might see a 3-6dB effect. I'm totally guessing about the number, but it shouldn't be bad enough to cause a "all or nothing" pattern.

The typical bimini has large openings, larger than one wavelength at VHF, so it won't make an effective shield. The only way it would have a significant effect would be if the sections of pipe were spaced and dimensioned in a very specific manner. If they aren't dimensioned just so, it will be fairly invisible.

Is there any way you can connect the AIS antenna to a VHF receiver? If so, you can tune to a weather beacon and swing the boat, listening for dead spots. The WX frequencies are about 5 MHz above the AIS channels, but it should be close enough for a rough check.
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Old 18-06-2011, 19:45   #19
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Re: AIS reception issue

I think I probably can swap the AIS and ICOM 504 antennas. I think it's mostly going to be a matter of undressing all my neatly arranges cables. Oh well.
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Old 18-06-2011, 22:31   #20
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Re: AIS reception issue

I'm betting it has something to do with the communications between the AIS receiver and the plotter. Try disconnecting the VHF transceivers from the plotter. I can't see any way that the antennas or the bimini can be causing this problem.

But, I have been wrong before...
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Old 18-06-2011, 23:51   #21
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Re: AIS reception issue

As mentioned earlier, possible voltage drop to the unit, most of these have a very sensitive nature with regards to voltage flucuations.
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Old 19-06-2011, 07:13   #22
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Re: AIS reception issue

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Originally Posted by DeepFrz View Post
I'm betting it has something to do with the communications between the AIS receiver and the plotter.
yes, but that is Ethernet in this case... which is proven to keep working as he can access the AIS from the plotter with VHF radios switched on...

my bet is a faulty AIS antenna or coax/connection

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Old 19-06-2011, 09:10   #23
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Re: AIS reception issue

Ante's are in and I raise a can of beans...
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Old 19-06-2011, 10:12   #24
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Re: AIS reception issue

Here's a paragraph from my unwritten book "The Philosophy of Troubleshooting":

1) Replicate the problem. You can''t know if you've fixed it until you can make it happen, repeatedly.
1-a) Do you really have a problem? See (1).

2) Do Something. See what happens. Take notes.

3) Do the easy stuff first. It might be a "stupid" idea, but since you don't know what the hell is actually going on, maybe it's not so stupid. At worst, you have quickly eliminated a possibility.

4) Simplify. Turn everything off. Disconnect the power. Find the absolute simplest configuration that demonstrates the problem.

5) If the easy stuff doesn't take care of it, spend some time reviewing what you have learned. Design a proper experiment before you begin the difficult tests.

6) If the difficult tests don't solve the puzzle, and you've run out of ideas, perhaps you discarded one of those "stupid" ideas too quickly. Start over at Step (1).

7) Once you have solved the problem, start turning the non-germane gear back on and make sure there's only one problem. Usually there isn't, but sometimes there are multiple problems, each with similar symptoms.

8) Sometimes the problem is sealed inside a Black Box, and you just can't get at it. The best you can do is substitute (or test the Black Box independently if appropriate).

There's more, but keeping these steps in mind has been very helpful for me.

I would characterize the "swing the boat to test the bimini effect" experiment as a difficult test. It's simple enough in concept, but will be tedious and require careful data-taking to be of any real value. Go ahead and do it if you're curious.

Unplugging the various pieces of gear is usually an easy sort of test. You should try some gear substitution where possible.

Some of the symptoms you have reported are quite puzzling. For example, being able to receive AIS from forward, but not aft, when the VHF radios are turned on. Refer to my Philosophy (1-a): "Do you really have a problem?" If you are in an area where there are few AIS signals you might be merely seeing the effects of few targets and varying propagation.

A friend has been working on his AIS transponder, and thinks that he might not be transmitting well. He has measured his cables and connectors, measured SWR at various points, and swapped antennas around. I believe that it's entirely possible that his system is working fine, and the "problem" is that there are few AIS stations in his area, and fewer who are willing to give him a signal report. Still, he's obsessing about a 1.25 SWR and potential 0.1 dB connector losses. He could have a problem, but I kind of doubt it.

Speaking of AIS receivers, where are you located and what is your vessel MMSI (or name, callsign)? We may be able to see your transmitted signal on Live Ships Map - AIS - Vessel Traffic and Positions.
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Old 19-06-2011, 11:26   #25
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Re: AIS reception issue

I'm with DeepFrz on this one, sounds like an interface / data problem to me.

I can't believe that the bimini / other antennas could be causing any significant kind of detuning effect

Are the VHF radios connected to the chart plotter (NEMA DSC) ? I think the ICOM 504 has a two way data interface at 4,800bps could this be interacting in some way ?

You mention a Sat Dome, I assume this is a one way (TV) device, or is it a two way device which is switched in tandem with the VHF ?

Duncan
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Old 19-06-2011, 16:44   #26
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Re: AIS reception issue

Replies to a few recent posts:

Thanks for the debug philosophy - I completely agree.

The VHFs are both set up for DSC, more or less. Both are NEMA 0183 and connect to a Simrad AT10 0180 to N2K converter. I say "more or less" because the ICOM both receives GPS position and transmits DSC alerts, where the Ray only receives GPS. The GPS is directly on the N2K bus, so the VHSs will work with everything else turned off (except the N2K bus). The N2K connects to the chart plotter, but the AIS does not touch the N2K bus.

The Sat dome is receive-only for TV, but it's a red herring. It's not even hooked up and is nothing more than an inanimate object. It's one of those things that came with the boat, but I have very little interest in using.
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Old 19-06-2011, 17:27   #27
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Re: AIS reception issue

I ran a few experiments today, and here are the raw results:

- Connected the AIS system to the ICOM antenna, and left both VHFs disconnected from any antenna and powered off.
-> Results, LOTS of targets. I'd say more and further away than ever before. I picked up several boats 12+nm away, and even picked up the stationary target on Deer Island at the entrance to Boston outer harbor. The targets were in a wide variety of directions.

- Reattached the VHF antennas. Ray on it's antenna, and ICOM on the AIS antenna. Ray powered on, ICOM powered off. Ray on 13 to hear the bridge chatter.
-> Results, after a few minutes all the targets were still active. The MDF12 puts an X through the target when it doesn't report in as expected, and eventually drops it from the display, but I have to wait a few minutes before they start going MIA. CH 13 chatter is clear.

- Turned on the ICOM, set Ray to 16 and ICOM to 13.
-> Results, still no loss of targets. But in some cases I was only hearing the bridge operator and not the boats he was talking to. But I wasn't paying a lot of attention to the chatter and can't really say if it was an isolated case.

At this point I was running out of time so from here on I was a bit rushed, but here's what happened.

- I turned on the Autopilot breaker. This breaker powered the AP brain (Simrad AC42) and the hydraulic pump. The control panels (2 x AP24) are connected to and powered by the N2K bus, but both were turned off.
=>Results, after a minute or so targets started dropping.

- Shut off the AP breaker.
=> Results, Targets all came back

- Turned AP breaker back on
=> Results, targets start dropping again.

So, what to make of all this? Here's my interpretation so far:

1) I think I have a problem with the AIS antenna or cable. I conclude this because after placing the AIS on the ICOM antenna I picked up targets much further away than ever before, with or without VHFs on.

2) I'm not sure what to make of the AP behavior. It might be that the AP is the real source of interference, but I think it's more likely that it's complicit along with the VHFs and antennas in some way. When I first discovered the VHF impact on target reception, I can't say for sure if the AP was on or off, but I know turnign the VHFs on and off impacted reception of targets, so I think there is some combination if things at play.

The good news is that now I know how to pickup a handful of targets which of course if critical to narrowing down the minimum set of circumstances to reproduce the problem.

My next step will be to try various combinations of AP and VHFs to figure out the minimum number of things that need to be powered on to reproduce the problem.

This remains a real puzzler, but having a reproducible set of circumstances makes me confident I'll sort it out soon.
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Old 19-06-2011, 18:09   #28
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Re: AIS reception issue

Is the autopilot connected to the same bus as the AIS?

You are getting better reception using one of the VHF antennas because it is a better antenna, but that doesn't mean you have a problem with the AIS antenna.
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Old 19-06-2011, 18:53   #29
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Re: AIS reception issue

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Is the autopilot connected to the same bus as the AIS?

You are getting better reception using one of the VHF antennas because it is a better antenna, but that doesn't mean you have a problem with the AIS antenna.
The AP is on a dedicated breaker, so there is nothing in common with the other circuits except the breaker panel.

On the antennas, "better" in unclear to me. The VHF antenna is a Shakespeare 5225-XP which I believe is 6db gain, but about 60% of it is obstructed by the binini frame and 20% by the flybridge fiber glass. The AIS antenna is 3db gain, but it's higher and 100% clear of the binimi. Also, my understanding is that you are not supposed to use more than a 3db antenna with AIS (I don't know why). So given all this, how much "better" do you think the VHF antenna is? I just don't know how the pluses and minuses tally up.
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Old 19-06-2011, 19:10   #30
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Re: AIS reception issue

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I ran a few experiments today, and here are the raw results:
[...]
The N2K bus is not connected to the AIS. We have the power, Ethernet and the coax with GPS antenna. When you loose the GPS fix on your AIS module, you also start loosing targets; but you should be able to see an error indicator when that happens.

-> I recommend to test-wire the AIS directly to the batteries, skipping all breakers etc.

-> check out the coax and gps antenna. Try to find status indicators (LEDs on AIS unit and/or info via networked plotter screen).

Quote:
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You are getting better reception using one of the VHF antennas because it is a better antenna, but that doesn't mean you have a problem with the AIS antenna.
I raise your bet with a can of beans as it's not gonna be the Ethernet

ciao!
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