AIS Reception Issue

[donradcliffe]

Could be a network problem. Can you hook the AIS up to a laptop running proAIS or OpenCPN and separate it from the network?


The transponder has its own internal GPS, so it doesn't need a position feed.

[CarinaPDX]

My antenna, designed specifically for the AIS frequencies, is 4.5db, so there is no 3db limit on AIS antennas. I can pick up the ships at the mouth of the Columbia River from Portland - about 100 miles - and my antenna is mounted about 10ft off the water, so when the system is working right you should be getting a flood of targets.

The first antenna I bought didn't work very well (<-understatement); it turned out to have an SWR of 5:1 from the factory (i.e. way out of spec). You should insert an SWR meter at each radio and test the antenna system. Then use a radio in place of the AIS to test its antenna. That should set your mind at ease re: the antenna.

[Paul Elliott]

Tune one of the VHF radios to the AIS channels, with the squelch off as we have discussed, and turn the AP on / off / on / etc. Listen for any change in the unsquelched noise. The goal is to see if the (main?) problem is due to interference radiated from the autopilot system.

If you hear evidence of interference, the solution may be as easy as putting some ferrite cores on the cables to/from the AP. Or it may be more complicated than this, but let's take it one step at a time.

By the way, congratulations! You have made great progress on what was a strange problem.

[DeepFrz]

You are getting better reception using one of the VHF antennas because it is a better antenna

Sorry, I used sloppy language. I meant higher gain.

I still think its a networking problem of some sort. I just don't see how powering up a radio that is not transmitting can affect the reception of a device on a different antenna. Also now the plot thickens because the autopilot has been added to the mix. Powering up the autopilot will add traffic to the network. Maybe the autopilot is not playing nice with the "Ethernet" network in some way.

Another possibility is something is corrupting the GPS signal in some way, or perhaps overloading the GPS signals on the network. (likely a different network)

I think you should draw out your system and all of its connections. That may help you understand what the problem is.

[fairbank56]

You did not indicate as to the status of the yellow rx led on the AIS unit when you start losing targets. If the blinking of the led does not change when you start losing targets, you have an interface problem. If it does change significantly, you have an antenna/reception problem. For every AIS packet that you receive, the led will blink.

Are you using the NMEA port of the AIS for anything? You could connect that directly to a laptop and monitor the data.

The only link between the AIS unit and the MFD12 is through the ethernet cable. The AIS has it's own GPS antenna and receiver. The autopilot and VHF's are getting their data via one of the NMEA ports of the MFD, not the ethernet cable.

Eric

[twistedtree]

I'll post more info later today, but I'm now focused on the AIS antenna as the problem. It appear to me that using an SWR meter is the most quantitative way to measure and tune antenna performance. I picked up a cheap Radio Shack SWR meter today but discovered it's only good up to 30Mhz.

Can anyone recommend a reasonably priced (under $200, and preferably under $100) SWR meter suitable for Marine VHF bands? If it also covered cell phone bands, that would be a bonus and justify a higher price.

Thanks, and I'll report later today on my experiments.

[Paul Elliott]

Here's an SWR meter for under $40 that covers the marine VHF range (not cellphones). It's not a laboratory-grade piece of precision gear, but it should be good enough for troubleshooting: MFJ-812B VHF SWR-Watt Meter

However, from the symptoms you are reporting I don't think that the AIS antenna is the problem. I still think that the autopilot is radiating noise, which is interfering with the weaker AIS signals. I recommend you try the test I suggested in http://www.cruisersforum.com/forums/...tml#post711662

[twistedtree]

OK, I'm back with more info, but unfortunately I left my notes on the boat, but I'll do my best to summarize.

I started again with the AIS and M504 antennas swapped, i.e. the specialized AIS antenna attached to the M504 and the 5225 attached to the AIS. Everything else was off at the breaker panel.

Results: Lots of targets all the way to Boston harbor nearly 25nm away.

Turned on the two VHFs and there was no loss of targets

On my handheld VHF I shut off the squelch and listened to 87 and 88, then turned on the AP. There was no change in the background noise on 87 and 88, and no loss of targets.

I then played with the SWR meter that I bought, but it indicated infinite SWR. I attribute it to operating well above it's 30Mhz limit, and disregarded the results.

Next I went to swap back the antennas, and this time I still had the AIS powered up, so I took another look at the LEDs. There were no errors indicated (and never have been), and the RX light was flashing every second or so which seems about right for the dozen or so targets I was picking up. I also observed a TX flash as my unit broadcast.

When I swapped the antennas putting the AIS antenna back on the AIS box, the RX flashes dropped way off. I still got a few, but I'd say it was 80% less than with the other antenna. When I got back to the plotter, targets were dropping, and the only one I ended up with was the closest one about a mile or 2 away.

I then measured the SWR on the 5225 antenna and got infinite SWR which is when I concluded the meter was operating outside it's range.

I then switched the antennas back (AIS antenna on M504, 5225 antenna on AIS) and all my targets came right back.

So today the only variable that impacted targets was which antenna I used on the AIS.

My thinking now is that my AIS antenna, cable, or connector is faulty and that I'm operating on a half-baked antenna that is so weak that the heal of the boat impacts it. Also, when I look back at all the things I did installing this system, there was plenty of room for error rigging the antenna cable. I installed the PL-259 plug, and also had to splice some extra length of RG8X to reach the AIS box. I could have screwed up either of those. I'm guessing that with the AIS antenna, it's just dumb luck when I pick up any targets, and dumb luck when I lose them. First it's only targets behind the boat, then it's the VHFs that kill targets, then it's the AP - I don't buy it anymore. I'll bet I could use the head and drop targets (no pun intended).

The behavior reminds me of a digital circuit when there is a floating input and the circuit's behavior goes psycho. You chase signal integrity, power supply noise, timing, and all sorts of esoteric theories only to find someone forgot to tie off an unused input. Once fixed, sanity returns and the laws of physics once more apply.

A few other things:

I'm not pursuing any data communications theories for now. Nothing is reporting any errors, and I can access the AIS via the plotter over ethernet without any issues. If those packets get through without issue, I have to believe that the target reports are too.

I have a detailed schematic of the whole system, both communications and power. The engineer in me couldn't resist. The system is actually quite simple. All the furuno stuff is on Ethernet, and everything else is on N2K. The N2K attaches to one of the plotters, and it repeats all the data across the ethernet. The only place 0183 exists (good riddance) is for the VHFs since they don't support N2K, and that's handled by an N2K to 0183 converter. The N2K bus appears quite happy with zero transmission or receive errors.

I re-ran the transceiver tests and GPS tests on the AIS, and all pass with flying colors. Today I was running only on the AIS GPS, and it pegged my location and the other boat locations without problem I could see a couple of tanker/freighters running the shipping lanes in/out of Boston and they were right where they belonged. I also believe the error LED lights or blinks if the AIS loses GPS lock.

[Paul Elliott]

OK, with this information I'm liking your "bad AIS antenna / cable / connector" theory. Watching what the AIS receiver light is doing is definitely a good way to monitor the effects of your changes. You're right about random-appearing behavior when you've got weak or sporadic signals. It can get extremely confusing.

[s/v Jedi]

Looks like I'm winning cans of beans :-)

I still recommend using a VHF emergency antenna to test this out. They cost just over $30.- and come with 12' cable and suction cup mount: ideal to do this test and have around in general:

ciao!
Nick.

[SoonerSailor]

How did you add extra length to the AIS ant cable? I presume with a barrel connector and extra PL-259 plugs. You can't just splice coax together without really screwing up impedances.

Even with carefully installed PL-259 plugs, it is easy to end up with a bad connection once in a while.

Disconnect your AIS cable at both ends. Measure with a multimeter the resistance between center conductor and shield. It should be infinite. Now short the far end. Should be just a few ohms. That is at least a basic test of your coax, but it would't detect an impedance discontinuity if you just "spliced" the ends of two coaxial cables together.

Of course that still wouldnt fully explain why you were getting targets ok until the other devices were switched on.

Chip

[DeepFrz]

Beans are in the mail, with protest that not all relevant information was forthcoming. (oh, sorry, Canada Post is on strike)...

[Paul Elliott]

Originally Posted by s/v Jedi

For what it's worth, my experience with this emergency antenna is that it is pretty inefficient. I tried one with my AIS and comparing it to a regular short whip mounted on the stern rail, the emergency antenna on the cabin top wasn't picking up the more distant signals. The difference was quite noticeable. Still, it did pick up everything within a few miles. As an emergency antenna, it's way better than no antenna at all.

[twistedtree]

I spliced the cable using one of the Shakespeare splicing devices, not a barrel and two PL-259s. Since we are betting beans, I'll place my can on that splice connection as the source of the problems.

It's going to be a week or more before I can get back to working on this, but in the mean time I've ordered an SWR meter which I expect will tell me what kind of shape my cable/antenna combination is in, and give me confidence that it's working as well as possible when I'm done.

[SoonerSailor]

Im sorry im not nearby, it would be interesting to me to put the antenna analyzer on it.