AIS Reception Issue

[twistedtree]

I'm having troubles with AIS reception and would welcome any guidance on how to best diagnose the problem. All the electronics are new, so this is not something that has become broken over time. Right now, my belief is that it's antenna location/interference.

Here's some more background/detail:

- The biggest clue is that if I have the AIS on, but both VHFs turned off, the AIS appears to work well picking up plenty of targets in all directions.

- With the VHFs turned on, I lose all the targets, with only occasional detection of targets behind the boat. Note that I lose the targets even though I am not transmitting at all on the VHF. All it's doing is receiving.

- I've tried the VHFs individually, and having either one powered on causes the AIS targets to be lost.

- The antenna layout is not optimal, but I think it's rare that optimal layout can be accomplished. Realistically, it's an exercise in finding the best tradeoffs, but so for I haven't found that happy place. Here are the details on the layout.

- First, this is a power boat with an arch. In the middle of the arch is a mast about 3' tall housing the radar, and over that a Sat dome.

- There are two VHFs, each with an 8' antenna on either side of the flybridge about mid-ships. The antennas are about 10' apart, but in the same horizontal plane (which ideally they shouldn't be). Adjacent to each antenna, going up from the mount, the first two feet is the flybridge side (glass), the next 4 feet are bimini frame tubes, and the last 2 feet are in clear space. I know this is sub-optimal, and my plan is to raise the antennas using extension poles, but I don't have a good way to support the extensions, and until I do my mounts are only rated for the 8' that I have.

- The AIS antenna is a 4' Shakespeare AIS antanna mounted on the port side of the arch. It is about 8'-10' aft of the port VHF antenna, and it has a 4' extension pole. It's mounted such that the base of the antenna (not the base of the extension pole) is level with the top of the VHF antennas, so they are not in an overlapping plane.

Questions and thoughts so far.....

- With no transmissions on the VHFs, how could this be interference between the VHF and AIS antennas?

- In looking at it yesterday, I realized my bimini frame is not grounded, yet it's clearly in the line of fire of the VHFs. Could this be setting up some sort of Faraday cage that shields the AIS, even though it's all below the AIS antenna? The bimini, after all, is all forward of the AIS antenna, and the only place I pickup targets with the VHF on is aft. Have I inadvertently created an AIS cloaking device?

- Even though the VHFs are on a dedicated, and separate breaker from the AIS, could there be interference carried across the DC power lines?

- I'll have to double check, but I don't think the AIS antenna cable runs alongside either of the VHF antenna cables.

Again, I'd welcome any thoughts on how to diagnose what's going on, experiments to run, etc. So far my next step is to rig a ground wire to the bimini to see if that has any effect.

Thanks in advance. I'm an electrical engineer, but to me all this radio stuff is still witch craft.

[Paul Elliott]

For the moment at least, don't worry about the antenna placement, shielding by the bimini, etc, since you are getting decent AIS reception when the VHF radios are turned off.

You report being able to only receive AIS signals from the aft direction when the VHF transceivers are on, and from all directions then the VHFs are off. This would be very surprising, although it's possible that your antenna system is directional towards the stern and only strong signals can punch through the (suspected) interference. Can you swap antennas, or put up a temporary antenna in a different spot? This might help in understanding the problem.

As described, the problem may be due to emissions from the oscillators in the VHF radios. Usually these signals are very weak, and I wouldn't expect them to be an issue, but it's always possible. Are the VHF receivers the same make/model?

Here are some things to try:

Does the problem go away when you switch the VHF to a different channel?

Are you turning on / off the VHF radios with their front-panel power switch, or with a circuit breaker (or switch) on your power distribution panel? Is there any other equipment connected to the same power circuit as the VHF units? Make sure that you are turning on and off only the VHF transceivers, and not something else sharing the DC circuit.

Monitor the AIS frequencies with another VHF -- a handheld VHF is good for this. You can also use one of your existing VHF transceivers, but I like the handheld because it lets me point it and move it around as I search for the interference, and because it eliminates power-circuit conducted signals.

AIS uses two frequencies: 87B (161.975 MHz) and 88B (162.025 MHz). Turn the handheld squelch off (so you hear static) and tune to one of the AIS channels. Turn your offending VHF transceivers on and see if the noise on the handheld changes (quiets). Repeat this test on the other AIS channel. If the static is unchanged when you power-up the VHF units then we are not dealing with radiated interference.

There are other possibilities, but it's good to narrow things down as soon as you can. Please report back!

[rodney_k]

Wow, what an "interesting" problem. Couple of questions though: 1) Are both VHFs the same make and model? 2) Do you have a handheld VHF? If so, have you used it as a "sniffer" to see if there is a signal being radiated with the VHFs on? By this I mean to turn the handheld to VHF channels 87 or 88 and hold near the VHF antennas and see if the squelch opens or turning the squelch all the way off, listening for any kind of signal. Ok, and a 3rd question; With both VHFs on, have you tuned one of them to channels 87 or 88 and listened for any kind of signal? Your looking for any kind of steady signal. BTW, channels 87 & 88 are the AIS channels. Normally, when listening on these channels with an ordinary VHF, the squelch opens for very short periods when an AIS burst is received.

[rodney_k]

Hey Paul, great minds think alike :-)

[Paul Elliott]

Originally Posted by rodney_k

Hey Paul, great minds think alike :-)

Definitely! I can't wait to see how this one turns out -- just enough weirdness to make it really interesting.

While the weird stuff usually ends up being not germane, it's best to keep it in the mix until you are sure. Once in a while it's the weird symptom that holds the key. Those are the fun ones!

[s/v Jedi]

Connect one of the 8' VHF antenna's to the AIS and try again :-)

Tell us what brand/type VHF and AIS we're talking about.

ciao!
Nick.

[fairbank56]

Try turning the VHF's on with their antenna's disconnected. This will help rule out antenna radiated interference from them. Listening with an unsquelched handheld may not prove anything. The noise your hear with the squelch off is internal receiver noise, not atmospheric static. The suspected antenna radiated interference from the VHF could be weak enough to not cause quieting on the handheld but still strong enough to cause a problem with the AIS receiver, especially if it is an oscillator frequency far removed from the AIS channel frequency. Interesting problem.

Eric

[Paul Elliott]

Originally Posted by fairbank56

Try turning the VHF's on with their antenna's disconnected. This will help rule out antenna radiated interference from them.

This is a very good suggestion.

Listening with an unsquelched handheld may not prove anything. The noise your hear with the squelch off is internal receiver noise, not atmospheric static. The suspected antenna radiated interference from the VHF could be weak enough to not cause quieting on the handheld but still strong enough to cause a problem with the AIS receiver, especially if it is an oscillator frequency far removed from the AIS channel frequency.

I was considering the possibility of a strong off-frequency interfering signal, and while it's true that different receiver designs will have different behavior in this case, it seemed very unlikely that two VHF transceivers would be generating similar strong signals when receiving. This would be a serious design defect, and such a transceiver would not be in compliance with regulations. What would the chance that both of the VHF transceivers would be out of compliance, or have identical defects?

AIS receivers should be reasonably immune to off-frequency interference, and it would take a really strong spurious output from the VHF transceiver to cause this sort of problem.

I still think that the first thing to do is to monitor the AIS frequencies with a handheld. Yes, the unsquelched noise is internally-generated, but even an extremely weak external signal will cause a noticeable amount of quieting.

Of course we are working from the reported symptoms, and these may not be exactly true. I know that I can get myself pretty confused in the early stages of troubleshooting, so I mean no disrespect to twistedtree.

[twistedtree]

Yes, this one's a real head-scratcher.

I totally agree about getting one's self confused while debugging. I'll do my best to keep it all straight.

You asked about the equipment. Sorry I forgot to mention it before. The AIS and chart plotter are Furuno (AIS-50 transponder, not just a receiver). One radio is an ICOM 504, and the other is a Ray 218. I think it's all quality equipment, but welcome feedback to the contrary.

I'll add the antenna disconnect test to my list of things to try. My understanding is that if I disconnect the antenna, it's important that I not transmit on the VHF? The instructions have all sorts of warnings about not operating without an antenna connected.

I can't easily switch which device uses which antenna since they are all in different locations and I don't think the cables will reach.

I tried tuning into 87 and 88 on my handheld. 88 is silent, and 87 squawks periodically which I'm presuming is the result of AIS transmissions. There was no steady noise on either channel. That said, I did not play with the squelch settings, so it probably warrants some more checking.

The biggest problem right now is that I'm home on my mooring, and nobody around here uses AIS, so targets to experiment with are almost non-existent. I was able to track one coming into the harbor with my VHFs off, and after turning only the ICOM back on (ch 16) I was still able to follow the target, but by then is was off my stern which is where I've generally still been able to pick them up, so I'd say the experiment was inconclusive. I originally narrowed the issue down to the VHF interaction in Provincetown where there were multiple targets including one vessel at a dock nearby. I then experimented some more in Boston harbor where targets are plentiful.

Someone asked about breakers and what else is on them. The two VHFs are on the same breaker, and they are the only things on it. The AIS comes on with a bunch of other things including 2 chart plotters, a fish finder, sat weather, and a network hub (I think that's it).

The AIS unit itself isn't reporting any errors and its self test works fine, so it thinks everything is hunky dory.

[fairbank56]

If it's an "off-frequency" interfering signal, it would have to be an extremely strong one to cause any quieting on the handheld monitoring on-frequency.

I do know the recommended AIS antenna installation guidelines are pretty difficult to comply with on a small vessel. They call for the AIS antenna to be installed either directly above or below, with 2.8 meter vertical separation, the other VHF antenna with no horizontal separation. If they are on or near the same horizontal plane, then they call for a 30 meter horizontal separation. I suspect this is primarily to prevent interference during transmission vice receiving, but who knows.

Eric

[Paul Elliott]

You do not want to transmit when the antenna / cable are unplugged. Modern radios are pretty rugged, but it is possible to damage the transmitter output stage.

You should be hearing very brief bursts on both of the AIS channels. An AIS transponder transmits on alternating channels. There can be 30 seconds between transmissions. The AIS transmission may not break squelch on a receiver designed for voice, I have better results by turning off the squelch and listening for brief intervals of quieting caused by the AIS burst transmissions.

It is very difficult to diagnose receive problems when you only have sporadic signals to test with. Be careful about drawing conclusions!

[twistedtree]

Originally Posted by fairbank56

I do know the recommended AIS antenna installation guidelines are pretty difficult to comply with on a small vessel. They call for the AIS antenna to be installed either directly above or below, with 2.8 meter vertical separation, the other VHF antenna with no horizontal separation. If they are on or near the same horizontal plane, then they call for a 30 meter horizontal separation.
Eric

Exactly. Good luck meeting those requirements. Perhaps when I buy a proper Yacht that has separate municipalities forward and aft I'll be able to comply, but for now....

Do you suppose AIS is super sensitive? I'd think if I can make out what someone is saying on the VHF then AIS should be able to decode the text messages. It doesn't make sense to me.

[s/v Jedi]

You can also try one of those emergency VHF antenna's (many boats carry one). It'll probably have a different connector than what the AIS wants but there are adapters for that.

These have 20' coax or so and allow you to experiment with antenna position. It also rules out a problem with the AIS antenna you have.

ciao!
Nick.

[fairbank56]

Another thing to consider is the data interface between the FA50 and the display. Maybe the AIS is receiving just fine but nothing is showing up on the display because of a problem with the interface when the radio is on. You can monitor activity on the FA50 by watching the RX led. When your in an area with lots of AIS traffic, it should be blinking on/off a lot. Then turn on the VHF and see if the blinking stops or slows considerably. If it doesn't change, you may have an interface issue. Then you have to consider are you using the Furuno network interface or NMEA? Is it shared with other devices? If network, is it direct or through ethernet switch?...etc. Keep us posted.

We use the FA50 and Icom M504 on the Navy 44's at the Naval Academy without issue, however we share the masthead VHF antenna by using an active antenna splitter.

Eric

[twistedtree]

Originally Posted by fairbank56

Another thing to consider is the data interface between the FA50 and the display. Maybe the AIS is receiving just fine but nothing is showing up on the display because of a problem with the interface when the radio is on. You can monitor activity on the FA50 by watching the RX led. When your in an area with lots of AIS traffic, it should be blinking on/off a lot. Then turn on the VHF and see if the blinking stops or slows considerably. If it doesn't change, you may have an interface issue. Then you have to consider are you using the Furuno network interface or NMEA? Is it shared with other devices? If network, is it direct or through ethernet switch?...etc. Keep us posted.

We use the FA50 and Icom M504 on the Navy 44's at the Naval Academy without issue, however we share the masthead VHF antenna by using an active antenna splitter.

Eric

That's an interesting possibility, and another experiment to add to my list. The FA50 is networked to the MDF12 chart plotters via a Furuno HUB101. Other than the VHFs, it's all Furuno so there is minimal opportunity for multi-vendor-mangling, but you never know. I can access the FA50 configuration pages from the MFD which suggests communications is working OK, but it's a straight forward experiment to try.

All I need now are AIS targets. Want to bring one of those Navy 44s and anchor in Gloucester for a few days so I can experiment?