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Old 16-02-2015, 16:37   #1
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12v RF interference with FM radio

Whenever I turn on a 12 volt plug outlet located at my nav station, I notice that my clear FM radio reception gets replaced by static.
The stereo is located about a foot away from the outlet. I presume the solution for this involves ferrite chokes, but where? Power supply? speaker wires? FM antenna input? Would making some effort to twist the two 12 volt wires be likely to help?
THought I'd ask for some advice before randomly trying different things...
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Old 16-02-2015, 18:41   #2
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Re: 12v RF interference with FM radio

Is something plugged into the outlet? An unused 12V outlet should not generate any noise. Are there any other devices connected to that switched 12V wiring? Do your best to locate the source of interference. An empty socket could generate noise if there is corrosion and leakage, but this seems unlikely, and if so would be a fire hazard and with luck pop the circuit breaker / fuse.

I suppose it's possible that noise on your 12V wiring is being routed to the 12V outlet when the switch (breaker?) is turned on, and your FM radio's antenna is located very close to the outlet. If this is the case, first try re-locating the radio antenna.

You really want to find the noise source and attack the problem there. Ferrites can be a big help if the noise is R.F. noise. It might also be lower-frequency hash conducted by the radio power wiring (presumably coming from something else on the 12V outlet feed-wires). Again, the trick is to find the source.
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Old 16-02-2015, 21:23   #3
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Re: 12v RF interference with FM radio

Quote:
Originally Posted by Uliad View Post
Whenever I turn on a 12 volt plug outlet located at my nav station, I notice that my clear FM radio reception gets replaced by static.
As Paul mentioned, finding exactly what device or circuit is making that static is important.

Another consideration - what are you using as an FM antenna? If it's a marginal antenna or poorly positioned, you may not be getting enough of the FM signal. A well positioned FM antenna is much less susceptible to interference.
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Old 16-02-2015, 23:56   #4
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Re: 12v RF interference with FM radio

The 12 volt plug is connected to a DC/DC converter powering my laptop. There's only interference when it is plugged in and drawing power

The FM antenna of the stereo runs to a splitter, then to the VHF aerial atop the mast. Great reception when that 12 volt outlet isn't drawing amps.
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Old 17-02-2015, 06:15   #5
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Re: 12v RF interference with FM radio

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Originally Posted by Uliad View Post
Great reception when that 12 volt outlet isn't drawing amps.
Actually "you mean great reception when not using that laptop power supply", right? Just pulling amps isn't necessarily the issue (to confirm this, try a 12v searchlight in the same outlet). DC-DC converters are switching power supplies which are often noisy

You can try:
  • ferrites or noise filters on the power lead of the laptop power supply
  • ferrites or noise filters on the power lead of the radio
  • using a different 12v outlet
I would also try a different antenna on the FM receiver, just to see what change that makes.
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Old 17-02-2015, 06:26   #6
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Re: 12v RF interference with FM radio

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Originally Posted by Uliad View Post
The 12 volt plug is connected to a DC/DC converter powering my laptop. There's only interference when it is plugged in and drawing power

The FM antenna of the stereo runs to a splitter, then to the VHF aerial atop the mast. Great reception when that 12 volt outlet isn't drawing amps.
It would have been simpler if you had mentioned the DC to DC converter in the first post. It is the DC to DC converter that you are turning on, not the outlet. Prove this to yourself by unplugging the DC to DC converter and turning on the 12 volt outlet.

Unplug the antenna from the FM radio while it and the DC converter are on and you are hearing static. If the static stops, the converter is broadcasting static (like a mini radio station). If the static continues, it is getting to the radio through the wiring.

Plug the antenna back in.

If the static is getting to the radio through the antenna, try grounding the case of the converter (unless it's plastic). Move the converter and wiring as far as possible from the radio and antenna.

If the static is getting to the radio through the wiring, go to a place that installs car stereos and explain the problem to them and see if they sell filters for the wiring. Generally, the filters go on the device causing the interference (in this case, the converter), but follow the instructions.

I should have put this first, but make sure all the antenna cables are in good condition and the grounds (shields) are intact. Removing the splitter temporarily will help with your troubleshooting.
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Old 17-02-2015, 09:05   #7
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Re: 12v RF interference with FM radio

I'm an electromagnetic Compatibility guy, I earn my money on this kind of stuff, so I may be able to help.

All good advice given, nobody has suggested anything wrong.

Looks almost certainly like it's your converter for the lap-top, but it could also be the lap-top power supply. It may or may not be easily fixable. As others have mentioned, ferrite clamps on the wiring into and out of the converter may be effective. The clamps should be as close to the converter as possible, and all wires should go though the clamp. Looping though the clamp will probably not help, just once though should work best. I usually buy from Digikey.

Good luck.
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Old 17-02-2015, 09:34   #8
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Re: 12v RF interference with FM radio

Ahhh.. The crappy switching power supply in the DC-DC converter.

You may be able to reduce the noise with a ferrite (both in and out), but chances are it won't stop it. Most likely the converter is also leaking RF radiation in a 1 - 1.5 ft bubble (since none of the chips will be shielded in a can). If you have access to a scope you can confirm this by looping the wire on the probe to the probe tip (making a crude wire loop). Then hold this loop at various distances from the converter. I'm guessing you will see a 500khz(ish) signal being picked up (should have the laptop connected and drawing current).

I think your only solution would be to move the converter away from BOTH the VHF radio and any antenna wire going to the radio. You should still use a ferrite on the in and out. Make some bodge wires that will allow you to test various distances till the problem goes away, then move your 12V socket to that location.
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Old 17-02-2015, 09:42   #9
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Re: 12v RF interference with FM radio

Yes, the converter is almost certainly the culprit. Ferrites on the 12V wire, and the wire to the computer should help. Put the ferrites as close as you can to the converter.

Quote:
Originally Posted by ctsbillc View Post
[...] Looping though the [ferrite] clamp will probably not help, just once though should work best. I usually buy from Digikey.
I used to do this stuff for a living, and in my experience taking two or three turns through the ferrite clamp will improve the interference suppression. It all depends on the frequencies involved, the physical size of the clamp and wires, the clamp material, etc, but with typical devices and materials in the 200 MHz range (and below) a couple of turns will be better than a single turn. At FM frequencies (100 MHz), two turns might be best. At SSB frequencies (3-30 MHz) more turns might help. It won't hurt to try anyway.

FYI, running the wire through the clamp one time counts as one turn. Two passes through the clamp is two turns.

(We can get way more technical on the pros and cons of multiple turns if anybody is interested)
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Old 17-02-2015, 11:12   #10
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Re: 12v RF interference with FM radio

I'm a dumb old man. The static is getting in at the audio, AM, portion of the FM receiver. Try shielding the receiver with probably no more than aluminum foil grounded. Or just insure the case of the receiver has a good ground.
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Old 17-02-2015, 13:25   #11
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Re: 12v RF interference with FM radio

This type of RFI from various "power supplies" is sadly normal. And in the US, it is also often flatly illegal but allowed by the FCC. The easiest answer is to return the power supply, if you can, and try one from another manufacturer.

Assuming it is too old and out of warranty, your first resource is to contact whoever made it. Let them know it causes RFI and interferes with the navigational electronics on your vessel. (A slight overstatement.) Ask them if they have a remediation kit, because some actually KNOW their equipment will cause problems and they will send out appropriate chokes for the power cables (on both sides) when requested. If not, find out if they know what frequency their gizmo uses, so you can buy chokes that are correctly matched to that frequency. Just applying random chokes, only has a random chance of working.

Sometimes they will offer to buy back the power supply, even out of warranty, because that's cheaper than taking responsibility and manufacturing them correctly. You've got the tip of a whole iceberg here, there are no simple universal answers beyond "get rid of it".

But starting with the manufacturer, and if their support says "Huh?" asking for a senior technician, is always a good start.
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Old 17-02-2015, 13:58   #12
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Re: 12v RF interference with FM radio

Thanks to everyone for the advice. I will focus on the various solutions suggested and post my results soon.
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Old 17-02-2015, 15:10   #13
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Re: 12v RF interference with FM radio

From left field. You don't have any of those ancient 12 volt flouro lights do you? Great light, but....Some are notorious for radiating spurious EM splatter. If so, give them the old "buoyancy test".
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Old 18-02-2015, 05:18   #14
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Re: 12v RF interference with FM radio

Just out of interest, does anyone know of good DC-DC converters that don't have these kind of problems?
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Old 18-02-2015, 05:44   #15
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Re: 12v RF interference with FM radio

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Originally Posted by billgewater View Post
From left field. You don't have any of those ancient 12 volt flouro lights do you? Great light, but....Some are notorious for radiating spurious EM splatter. If so, give them the old "buoyancy test".
The interference happens when he powers the Dc to DC converter to run his computer. It's not lights unless he's turning them on and off at the same time.
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