ozzyb
Member
I seem to be picking up a signal interference via my instruments (NMEA) on my VHF radio. Any ideas as to what may be causing it ?. I've checked connections behind the panel nothing seems to be loose. Any help would be great.
Radio Interference
- Thread starter ozzyb
- Start date
chippie
New member
As a wildcard guess,make sure that no flouro light is sited near to a potential receiver.
ozzyb
Member
As additional info the interference is only from the intruments and can also be picked up with a hand held radio am/fm nearby the panel. Thanks.
HaraldS
New member
Some more info would help making some suggestions on what may help. I understand that you pick up the interference with a standard FM radio, that is self ocntained, running on it's own batteries, with no wire connection to anything else-right?
If that is true, your instruments could radiate via the interface cables or the power cables, or directly from themselfes. In the later case they shouldn't pass CE approval on EMI testing.
In the other cases there are some remedies, like checking the wireing and applying supression ferrites.
So: What type, make of instruments, what interconnections, transducers etc?
If that is true, your instruments could radiate via the interface cables or the power cables, or directly from themselfes. In the later case they shouldn't pass CE approval on EMI testing.
In the other cases there are some remedies, like checking the wireing and applying supression ferrites.
So: What type, make of instruments, what interconnections, transducers etc?
Lynette
Member
Another wild guess - depth sounder. That and fluorescent lights are the main sources of radio interference on my boat.
ozzyb
Member
Yes the interference is picked up by independant self contained battery radio. The problem started during last season.
Instruments are Stowe dataline with a central display and data box (bus) behind panel. Interference is there without any lighting ie. instruments only. The system seemed OK for years before.
Instruments are Stowe dataline with a central display and data box (bus) behind panel. Interference is there without any lighting ie. instruments only. The system seemed OK for years before.
byron
RIP
Hmmm! I had something similar on my VHF on channels 6.13. & 14 only. It proved to be caused by the Plotter. I fixed it by wrapping the lead from the Plotter to the GPS in plain old ordinary BACOFOIL. Not a professional way of doing things I know but it worked.
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HaraldS
New member
If you didn't change anything on the system or connected any new devices to it, then a likley cause could be that some connection to cable shielding has gone bad, (corroded). Check out those cables between display and box, and whatever goes to transducers or other electronics. What's the sound like?
ozzyb
Member
Thanks Harald, I did not connect anything new to the system, but it may have been disturbed when I investigated the possibility of connecting to the GPS chart plotter. There are no external signs of corrosion or pinched wires, the shielding stops approx 25mm short of the connectors but this has always been the case.
Someone else suggested it may be a bad earth, if so how could this be checked.
The depth sounder excess cable is looped in a bundle behind the panel, I did move it to one side and cable tie it to the side away from the radio, to tidy up. I assume that the excess is just because it comes with x amount of cable, can this be cut back to minimise the excess or must it be left alone. The sound is similar to Data / Fax trans. but not so intense. More like very fast morse.
Someone else suggested it may be a bad earth, if so how could this be checked.
The depth sounder excess cable is looped in a bundle behind the panel, I did move it to one side and cable tie it to the side away from the radio, to tidy up. I assume that the excess is just because it comes with x amount of cable, can this be cut back to minimise the excess or must it be left alone. The sound is similar to Data / Fax trans. but not so intense. More like very fast morse.
HaraldS
New member
Fast morse sounds like echo sounder. I don't know the Stowe system, but if I assume you have a regular passive depth transducer, then it would be the actual signal that is fed over your cable, and that would sound a bit like very fast morse.
Active transducers send NMEA or some proprietary data on the line and need a power feed.
Assuming you have two wires plus shield, it could be that the swinger is fed just fine via the two inner wires, but the shield is not connected to ground anywhere, and then the noise could be explainable. (I presume your sounder is working fine. )
When you talk about the shield that ends 25mm before the connector, is this also true for the sounder transducer? -- If the shield isn't grounded on either end, you should do that and see if your interference goes away.
Avoind grounding both ends, or making multiple ground connections especially if one or more of them connect to DC ground (12V-), because that might give a differnet problem (potential for electrolysis).
You might as a first try to disconnect the transducer altogether from the box, and see if you still have the interference.
Active transducers send NMEA or some proprietary data on the line and need a power feed.
Assuming you have two wires plus shield, it could be that the swinger is fed just fine via the two inner wires, but the shield is not connected to ground anywhere, and then the noise could be explainable. (I presume your sounder is working fine. )
When you talk about the shield that ends 25mm before the connector, is this also true for the sounder transducer? -- If the shield isn't grounded on either end, you should do that and see if your interference goes away.
Avoind grounding both ends, or making multiple ground connections especially if one or more of them connect to DC ground (12V-), because that might give a differnet problem (potential for electrolysis).
You might as a first try to disconnect the transducer altogether from the box, and see if you still have the interference.
robp
Active member
Think Harald is on the right track. If the shielding stops 25mm before the connector, does it connect fully at the other end? Most likely disturbed earth or ground connection. If you can manouver wiring behind the panel whilst it's all working, you might try gently untying the sounder cable and moving it around in different planes. Or even not looped. At least you will hear a variation if it's that. I don't think you should shorten the sounder cable.
Bergman
New member
You might try disconnecting the power to each instrument in turn (unscrew the fuse) to see if you can isolate the source to a single item. Like Harald my bet would be echosounder.
Once you have done this you are almost certainly into the problematic earth syndrome.
Ideally screened cable should have the screen earthed at ONE point only. So check that: a- it is earthed and b- earthed only once.
If this fails then Byrons bakofoil is certainly worth a try, perhaps you could contrive a screen around the instrument mountings. The difficulty with foil is getting a good connection to it. Soldering doesn't work and it tears from under screw connectors, perhaps big washers may do it, but it does need to be earthed to work properly.
Best of luck
Once you have done this you are almost certainly into the problematic earth syndrome.
Ideally screened cable should have the screen earthed at ONE point only. So check that: a- it is earthed and b- earthed only once.
If this fails then Byrons bakofoil is certainly worth a try, perhaps you could contrive a screen around the instrument mountings. The difficulty with foil is getting a good connection to it. Soldering doesn't work and it tears from under screw connectors, perhaps big washers may do it, but it does need to be earthed to work properly.
Best of luck
G
Guest
Guest
There is as always an other way to look at this The aerial to the VHF could have a poor screening . Check the aerial cable.
Happy Sailing Paul
Happy Sailing Paul
graham
Well-known member
Re:uneducated guess.
I am not a sparks and only speak from personal experience.
Are any of your cables going to your instruments wrapped up with your vhf ariel coax?? I once had a problem similar to yours cured by seperating all the wires cables ,coaxes etc.
Hope this helps or someone elses advise sorts it for you .Best Regards Graham..
I am not a sparks and only speak from personal experience.
Are any of your cables going to your instruments wrapped up with your vhf ariel coax?? I once had a problem similar to yours cured by seperating all the wires cables ,coaxes etc.
Hope this helps or someone elses advise sorts it for you .Best Regards Graham..
ozzyb
Member
Re: Radio Interference, Thanks
Thanks Harald (and others) I'll check out your advice when back on the boat in a couple of days.
Thanks Harald (and others) I'll check out your advice when back on the boat in a couple of days.
bedouin
Well-known member
Don't think it's the echo sounder - that probably only 'pings' about once a second. 'Fast Morse' sounds like a genuine serial digital signal (rather like a fax machine, or computer modem) - in other words the NMEA signal itself
HaraldS
New member
Think you are right Bedouin! I just put the NMEA signal on the speaker and it does indeed make an fax like sound, just a bit lower and pretty awfull. Depending on how I space the sentences I can make it sound like fast morse.
It looks like Bernard will need to have a close look at his NMEA connections. Think he mentioned something that he once tried to interface a GPS, but in th end didn't.
So he might have some of that wireing left or might have blown a NMEA differential driver, or indeed lost a shield like we originally suspected.
Lots of these problems come from people tieing RS232 to the NMEA RS445. That grounds one side of the ground free NMEA signal. It usually works, but depending on how the power sources work inside, could lead to a short. Also if on such a connection the shield gets used as the ground connection, it would carry the NMEA signal and radiate it.
It looks like Bernard will need to have a close look at his NMEA connections. Think he mentioned something that he once tried to interface a GPS, but in th end didn't.
So he might have some of that wireing left or might have blown a NMEA differential driver, or indeed lost a shield like we originally suspected.
Lots of these problems come from people tieing RS232 to the NMEA RS445. That grounds one side of the ground free NMEA signal. It usually works, but depending on how the power sources work inside, could lead to a short. Also if on such a connection the shield gets used as the ground connection, it would carry the NMEA signal and radiate it.
bedouin
Well-known member
What is odd, of course, is that it is producing interference on a Radio. I would suspect that it is not being picked up by the radio tuner but rather by the low-level audio circuit.
My Garmin GPS has NMEA signal ground tied to 0V so it is very difficult to shield effectively - the best solution is probably to use two core shielded wire and connect it at one end only - although UTP might actually work better.
My Garmin GPS has NMEA signal ground tied to 0V so it is very difficult to shield effectively - the best solution is probably to use two core shielded wire and connect it at one end only - although UTP might actually work better.
HaraldS
New member
Quite puzzling indeed. I never had a NMEA signal interfere, but I ran all ground free and differntial on standard cat 5 UTP. That's why I didn't think it was NMEA.
By the way my echo sounders seem to go off several times a second, sounds like a machine gun.
Also why don't we pick up SeaTalk, which is also 4800 baud and simple open collector against ground?
I could imagine a shorted NMEA driver feeding noise back over the supply line as one option. NMEA drivers can be quite powerfull.
If the radio picks up the signal with it's LF circuit, like ypu suspect, then I would still think the much better shielded VHF, with usually a metal case and a coax antenna, shouldn't pick it up, or possibly only if the noise is on its power feed.
So maybe both is true, radio gets it via LF circuit and VHF via power feed.
By the way my echo sounders seem to go off several times a second, sounds like a machine gun.
Also why don't we pick up SeaTalk, which is also 4800 baud and simple open collector against ground?
I could imagine a shorted NMEA driver feeding noise back over the supply line as one option. NMEA drivers can be quite powerfull.
If the radio picks up the signal with it's LF circuit, like ypu suspect, then I would still think the much better shielded VHF, with usually a metal case and a coax antenna, shouldn't pick it up, or possibly only if the noise is on its power feed.
So maybe both is true, radio gets it via LF circuit and VHF via power feed.
bedouin
Well-known member
As the interference can be heard on a battery-powered am/fm radio it can't be transmitted via the power feed - so it must be air-borne EM interference.
Thinking it through - I suspect that a fault in the cable, or a poor connection may be to blame. Either a near-short, causing much higher signal current, or a poor connection (which I suppose could cause broad-bandwidth RF interference?).
Either way the first action is to check the cabling and connections!
Thinking it through - I suspect that a fault in the cable, or a poor connection may be to blame. Either a near-short, causing much higher signal current, or a poor connection (which I suppose could cause broad-bandwidth RF interference?).
Either way the first action is to check the cabling and connections!
