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Old 16-12-2014, 15:48   #1
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Electrical noise from NMEA 2000 network

I apparently have a electrical noise problem from my NMEA 2000 network. I've never heard of this before.

I have a newly installed Vesper AIS that was only seeing large ships out to about 15 miles. Vesper support (who were great) suggested it might be electrical noise and sure enough, pulling the fuse for the NMEA 2000 power made ships 50 miles away visible (the RSSI went from about -100 to -140).

The NMEA 2000 backbone cable with multiple drops does run just a few inches from the back of the Vesper but NMEA 2000 is NOT plugged into the Vesper. I'm just using NMEA 0183 to a Garmin chartplotter for now.

Besides moving the cable (a bit of a pain), any suggestion for reducing the noise? Have others run into this?

Carl
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Old 16-12-2014, 15:53   #2
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Re: Electrical noise from NMEA 2000 network

Start by finding if the noise is being picked up by the AIS antenna or the unit itself. Unplug the antenna. If the noise stops you may be able to reduce or eliminate the noise by relocating the antenna.

RF noise can be very difficult to deal with. There are filters but you have to know what filters and where to install them.
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Old 16-12-2014, 17:49   #3
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Re: Electrical noise from NMEA 2000 network

I wouldn't operate the AIS without the antenna unless you turned off the transmit function, and that may not help with diagnosis.

Temporarily move the AIS unit away from the N2K cabling, or move the cabling away from the AIS, to see if that really is the problem.

Does the power for the AIS come from the same circuit as the N2K?

Are there any N2K transducers near the AIS antenna?

Can you connect another antenna to the AIS using a short pigtail and see if you have noise issues?

Mark
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Old 16-12-2014, 18:14   #4
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Re: Electrical noise from NMEA 2000 network

Maybe a cable with better shielding to the AIS antenna.

The vesper unit and the VHF cable should not be picking up interfering signals, only the antenna element.
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Old 17-12-2014, 04:53   #5
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Re: Electrical noise from NMEA 2000 network

Maretron makes a great nema 2k tester. Singles out the problem easily. See if local electronics guy has one . Worth a try. We use the tool on evrry install regardless. Saves headaches .

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Old 17-12-2014, 05:45   #6
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Re: Electrical noise from NMEA 2000 network

It is more likely one of the instruments on the 2K bus than the bus itself. Many instruments get their power from the bus. I'd remove each item attached to the bus one at a time and see if you can determine which is the culprit.
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Old 17-12-2014, 05:51   #7
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Re: Electrical noise from NMEA 2000 network

Is the AIS GPS antenna outside or close to a N2k transducer?
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Old 17-12-2014, 07:31   #8
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Re: Electrical noise from NMEA 2000 network

A cheap and simple instrument for locating the source of RF interference is an ordinary old time pocket sized transistor radio set on AM and tuned between stations. The static will get louder as you approach the source.

Interference can be broadcast through the air and be picked up by the antenna or any wires entering or leaving the unit or it can be carried on the power leads. It helps to figure out where the interference is coming from and how it is getting into the unit. Once you figure that out, then you can figure out how to eliminate it.
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Old 17-12-2014, 07:35   #9
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Re: Electrical noise from NMEA 2000 network

This is why we use the maritron unit. Transistor radio sounds like a cool.odea too.

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Old 17-12-2014, 09:56   #10
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Re: Electrical noise from NMEA 2000 network

(nice install, B17. Some installers get too anal and don't leave enough slack to permit disconnection/reconnection and diagnostic patching)
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Old 17-12-2014, 20:14   #11
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Re: Electrical noise from NMEA 2000 network

Using the AM radio to find the RF source you can reduce it by shielding the item and the cable run. A cheap way is to use aluminum foil as a shield and then to ground the shield. If that works you can design something that fits better with your wiring job.
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Old 17-12-2014, 20:34   #12
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Re: Electrical noise from NMEA 2000 network

Thanks for all the suggestions. The fuse only powers the NMEA 2000 network. I think all of the instruments have their own fused power - but will double check. Pulling any other fuse has no impact on the Vesper reading - just the NMEA 2000 bus fuse. The coax to the antenna is LMR400

I like the idea of the AM radio as a Geiger counter. I'll experiment. Aluminum foil with a grounded drain is also interesting.

Unfortunately, I'm off the boat now and can't fiddle more until after the holidays.
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Old 17-12-2014, 20:49   #13
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Re: Electrical noise from NMEA 2000 network

Carl,

The NMEA cable should be 2 twisted pairs in a shielded cable. These type cables should not radiate any significant energy in the AIS frequency band. So I doubt that is the root cause.

A more likely suspect is one or more NMEA2000 devices putting noise onto the cable. This is not uncommon. To find the offending device go to each drop off the NMEA2000 backbone and unplug all the instruments. Then plug each instrument into the backbone to see which causes the noise to increase. It could be more than one so check them all individually.

If you find an instrument causing problems you can try one of the snap-on ferrites (RadioShack® Snap-Together Ferrite Choke Core - Radioshack) placed around the NMEA2000 cable close to the offending device. As close to the device connector as you can get it is best. Sometimes two ferrites are better than one and cannot cause any harm.

The AIS VHF antenna is a more likely entry point of the noise than the AIS unit itself. Take extra notice of any NMEA2000 devices and cables located close to the AIS antenna.

Good luck and let us know what you find out.

Dan
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Old 17-12-2014, 21:30   #14
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Re: Electrical noise from NMEA 2000 network

The NMEA 2000 network is properly terminated at both ends, isn't it?
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