In-Vehicle Sources of Radio Interference

[teotwaki]

I just tracked down a most annoying RFI source that would land in the 700 to 900 KHz range and interfere with the AM radio reception in my 4Runner. It is possible that multiples of this spur were getting into my HF radio also. I had the AM radio on and the truck was parked and engine running but the Engel Fridge was still running off of an AC cord at the time. When I unplugged the AC I suddenly heard the heterodyne of the spur landing near 790 KHz. I need to open up the fridge and add some RFI solutions onto the DC cord and also the electronics that switch between AC & DC if need be.

Has anyone else had an issue with a DC powered fridge?

 

[DaveInDenver]

Never noticed much RFI my Engel MT45 but the circuits in my old junk are much less complex than in your 4Runner. The fridge circuit is isolated by proximity, there is 15 feet of physical distance and #6 wire pair that feeds it in the bed of my truck, away from just about everything else electrical. But it would not surprise me to find out the motor in one would cause a lot of interference, it makes sense. In general ferrite beads should be your close, close friends in vehicle and HF.

 

[teotwaki]

I'm not sure that the complexity of the wiring is it unless you are thinking that it helps to reradiate the RFI. I don't believe it to be a factor because the Engel is in the back of the 4Runner and the radio's dual/diversity antennas are part of the rear glass windows. Nice coincidence, eh? I'm thinking I'll be applying ferrite beads as well as some high quality capacitors to help shunt the RFI to ground. I have a lot of RFI suppression parts available in my lab.

In your case it may be that the steel truck cab is helping to shield your radio and antenna from the fridge at the back of the truck?

 

[DaveInDenver]

It's not complexity as such but density and proximity. It could be re-radiating, all wires do to some extent. Physical separation between conductors is one of the best ways to solve RFI problems, particularly keeping RF conductor away from each other and power.

I seem to remember something about the commercial radio antenna being in the back glass and with your fridge right there it might be a tough one to solve. The fridge motor itself will generate a field, so you might have to filter the antenna itself rather than the power to the Engel. Another way to approach it would be to open the fridge and put in bypass caps and some strategic shielding, e.g. foil-backed insulation for example.

In my truck I have placed the fridge in the right rear corner of the pickup bed and ran power from the engine bay back to it (I assume you are using the Toyota-provided 12VDC outlet, which means RFI could be more easily coupled into your vehicle harness). There are two other vehicle circuits anywhere near it, the fuel pump (which is bypassed) and the tail lights (100% DC). There are maybe 6 wires running in the bundle and all but one come from the main fuse box in the engine bay.

So yup, the pickup box and topper are surely a huge Faraday cage around my fridge. My V/UHF antenna(s) are on the roof and when I still had HF in the truck the screwdriver was on the front bumper, 20+ feet from the fridge. My AM/FM antenna is the standard fender mast, also nowhere near the fridge. Not to mention that is barely a dummy load anyway.

 

[teotwaki]

For the fridge I took apart the rear cargo area and ran two double 10 gauge wires from the secondary battery

to two new outlets next to the factory outlet which was modified for constant power

 

[1911]

I need to open up the fridge and add some RFI solutions onto the DC cord and also the electronics that switch between AC & DC if need be.

Possibly a simple ferrite choke (of the appropriate mixture for 700-900 KHz) externally on the DC power cord might do a lot for you, without having to delve into the fridge electronics. The switch might still be a problem though.

Has anyone else had an issue with a DC powered fridge?

I am amazed at how well shielded and electrically-quiet my 2007 FJC has been straight from the factory. No problems at all there with any of my radio equipment, though I don't operate HF mobile - only in camp and outside the truck, but I still have my ARB fridge running from the same battery as the radio, w/o any problems.

The FJ40 is a whole 'nother story, it was extremely noisy until I bonded everything in sight; exhaust to frame, frame to body, bumpers to frame & body, engine block to frame and battery, etc. etc. Now it is almost quiet, at least to where the road noise is more of an impediment than RF interference. But like the FJC, I operate HF in camp, using the same battery as the fridge, without any RF noise problems.

Edit: Reading Dave's posts, I did take some pains to route my fridge power lines well away from any radio power or coax lines in both trucks.

 

[teotwaki]

Possibly a simple ferrite choke (of the appropriate mixture for 700-900 KHz) externally on the DC power cord might do a lot for you, without having to delve into the fridge electronics. The switch might still be a problem though.

I am amazed at how well shielded and electrically-quiet my 2007 FJC has been straight from the factory. No problems at all there with any of my radio equipment, though I don't operate HF mobile - only in camp and outside the truck, but I still have my ARB fridge running from the same battery as the radio, w/o any problems.

The FJ40 is a whole 'nother story, it was extremely noisy until I bonded everything in sight; exhaust to frame, frame to body, bumpers to frame & body, engine block to frame and battery, etc. etc. Now it is almost quiet, at least to where the road noise is more of an impediment than RF interference. But like the FJC, I operate HF in camp, using the same battery as the fridge, without any RF noise problems.

Edit: Reading Dave's posts, I did take some pains to route my fridge power lines well away from any radio power or coax lines in both trucks.

If I recall correctly, the FJC has a standard 3 foot antenna on the front of the truck and assuming that your fridge is in the back you've got some distance and cab shielding. I don't think that fridge is radiating the RFI through the wiring but is radiating directly into the antenna's in the 4Runner where I am picking it up on the truck's AM radio.

 

[DaveInDenver]

It would make sense that the inverter would be noisy, I agree with that. I have no idea about their design, but I doubt it runs at 700KHz, so that might be a harmonic. You need to filter the fundamental of the switching frequency first. Anyone know the frequency the swing motor runs at? I'd guess it's around 60Hz. Is there anything more than a voltage shift running on AC? I'm assuming that they work on low voltage AC, thus my assumption is the DC side is the more complex power supply.

 

[DaveInDenver]

It's possible, though, that they have a single motor driver that creates the AC regardless of which you are actually using to power the fridge. That would mean that there is a supply for both the mains and battery, one being an AC/DC supply and the other a DC/DC. In this case the AC supply would not have to be terribly complex but would eliminate a potentially noisy DC/DC converter while on shore power. That would also very much explain the high frequency noise, a DC/DC converter would run at tens or hundreds of KHz.

You'll definitely need physical shielding in the form of an enclosure or the equivalent, ferrites won't be enough. Also the noise going into the motor driving inverter would have a lot of common mode noise, so take care to balance currents rather than just choking and shunting. Brute force with ferrites & caps is unnecessary if all it takes is providing a controlled return for a sneak path.





And, yes, reading your original post again with coffee on board I'm up to speed. Duh.

 

[teotwaki]

I started Googling and it turns out that I am not alone with the issue:

http://www.pradopoint.com/showthread.php?16398-Prado-radio-interference
http://www.exploroz.com/Forum/Topic/90201/AM_FM_Radio_interference.aspx
http://www.exploroz.com/Forum/Topic/90201/AM_FM_Radio_interference.aspx

and look in the right of this picture :sombrero:

The unit has a grounding point for when radio interference is encountered. Am not keen about having to run a third wire to the fridge but I'll have time to play with it this weekend.

 

[DaveInDenver]

The unit has a grounding point for when radio interference is encountered. Am not keen about having to run a third wire to the fridge but I'll have time to play with it this weekend.

Yes! That's it, they have already provided you the inverter enclosure shielding but you've got it floating. Well there is at least enough impedance between your return and ground that there's a voltage being developed. I bet you measure a slight resistance between that screw and your 12V black. Remember that current flows in a loop, not necessarily to earth/ground. Sometimes you want your return grounded, sometimes not. It's safer having it floating in this case so that you don't inadvertently get an electrically live fridge case (particularly on the mains), but that also means it could be a EMI problem as you've found. I would use a 1/2" or wider braid to the frame or significant sheet metal from that screw and then it'll be nice and quiet on HF.

 

[gary in ohio]

Also check out http://www.k0bg.com/ he has LOTS of information on mobile radios issues. How determine issue and corrective actions.

 

[teotwaki]

Also check out http://www.k0bg.com/ he has LOTS of information on mobile radios issues. How determine issue and corrective actions.

That is a good source and it was a reminder that my 2005 4Runner has something in the anti-lock braking that produces a burst of broadband noise on HF whenever the brake pedal is pushed. Not a big thing but annoying enough that I'll probably track it down too.

 

[4x4junkie]

My Edgestar fridge emitted a lot of crap within the (I'm guessing, as I haven't put an HF rig in the truck) 15-50 MHz range while it was running (it was also detectable on a 2M HT if I held it next to the power cord). It was a very wideband interference, though for some reason it radiated very little that interfered with the AM bc band (530-1700kHz).

My fix was to wrap the DC wires inside the unit through a ferrite toroid, along with adding a pair of .01µF disc caps on each wire to ground right behind the jack. The interference on my 10/11M rig went down from about +10dB over S9 (basically making the rig unusable) to just barely tickling the S-meter needle at times. I should try grounding the fridge cabinet just to see if it might help that last little bit.

Since yours is an AM bc issue, you might up those cap's value to .1µF, just so it is able to shunt the lower frequency interference to ground better.

 

[teotwaki]

I tried some ferrite on the DC and AC cords with no real results. I ran a jumper from the insulated grounding screw that Engel provided over to the truck's chassis and that did the trick for the AM interference.