RFI Sources in Recreational Vehicles

RFI Sources in Recreational Vehicles
by Bill Bytheway (K7TTY)
April 2005

There are many sources of RFI in trailers these days. Attempting to communicate via ham radio at a remote campsite on the high frequency (HF) bands can be very challenging if there is a large amount of Radio Frequency Interference (RFI) present. Here are some of my findings and recommended solutions.

Power Converter
The switching power supply found in most newer recreational vehicles was found to be the largest contributer of RFI.

Power Converter Design
Ideally the manufacturer would design the unit such that they would not interfere with radio receivers. Here is how the typical swiching units work, reference the following figure. Electronic converter/chargers first convert the 120 VAC 60 Hz from the outlet to 120 VDC after it passes through Diode Rectifier #1, the Capacitor then filters the ripple voltage. This 120 VDC voltage is then fed to an Electronic Switching Circuit that converts it to back to AC and increases the frequency from the original 60 Hz to 3,500 HZ. This high frequency AC voltage is now fed to a step-down transformer where the output voltage reduced to approximately 13.6 volts AC and Diode Rectifier #2 converts the AC to 13.6 Volts DC and the Capacitor filters out the ripple.

Adding Filters
In the 1998 AF-26X trailer the inverter is under the refrig and the circuit breaker panel is under the bed in the rear of the rig. The wires between the battery-to-inverter and the wires between the inverter-to-breaker act as an antenna and radiate. I found that installing filter capacitors at the 12V lines at the inverter and at the breaker panel solved most of the problem. Installing filter capacitors at each of the cigar lighter outlets also helps, as each wire also acts like an antenna when nothing is plugged in.

Power Supply RFI makes BPL Seem Lame
After adding filters to your own rig, you wold think that now you are ready to hit the road and ham it up from within any campground. So you pull into a campground, hook up, turn on the radio, and the hash is so loud that it incompacitates your receiver. So it's not good enough to filter the power supply in your own rig. The problem seems to propagate back down the input AC power lines and through the entire campground. Every new camper that hooks up to AC power using a switching power converter is now contributing to broad band over power line RFI.

This was demonstrated on our last visit to a local Washington State park. We were one of the first campers to the electrical hook-up area, and prior to connecting my radio, RFI was undetectable. After connecting my unit to AC power, I started noticing the hash noise. By evening, the park was full of not only campers, but 20 dB over S9 hash noise, thus making radio communications unusable. Upon leaving the campground, it was easy to locate each of the AC power hook-up station by the noise level.

Disconnecting the RV from campground AC power does help with reducing RFI. I may need to experiment with brute-force AC filters at the campground power utility box to eliminate RFI being propagated inside the RV. The RFI comes from both AC and DC lines and radiates.

Manufacturer Claims
A quick look at manufactures sold by a local RV parts store and their claim is as follows:

Every Parallax Power Supply converter, automatic transfer switch, and AC/DC distribution panel is listed with Underwriter’s Laboratories and complies with all applicable requirements for the applied listing mark. Extra effort in engineering results in FCC Class B certification for all 555 and 7300 series products, which certifies these models produce minimal interference. http://www.parallaxpower.com/.
Documentation for the 40 Amp Electronic Power Converter manufactured by Progressive Dynamics, Inc makes no mention of FCC type acceptance at all. The service center says they have no FCC type acceptance. However, they claim their units are quieter than many competitors who claim to have FCC acceptance but noisier than the one competitor who actually does, which they say is Magnetek. All switcher type converters emit RF and cause interference on AM radios, Ham radios, and low band TV frequencies. Progressive Dynamics can be contacted at http://progressivedyn.com/. They recommend that :
1. Make sure your converter is not near or around antenna wires.
2. Some customers have found that turning the converter 90 degrees helps.
3. Some have gone as far as to build a cage over the unit to catch RF.
4. Really the only sure fire cure at this time is to disconnect the converter from AC and run on just battery power when using your radio and connect to AC when done.
MFJ's sells the MFJ-4245MV Adjustable Voltage Switching Power Supply they claim are clean and you won't hear any RF hash on your signal or receiver. Their super clean MFJ MightyLites Switching Power Supplies meet all FCC Class B regulations. I've been running one here at the radio shack, and indeed they are clean. For use in recreational vehicles, one would have to provide diode isolation, as they are not designed to charge batteries. The point of mentioning it here is that clean switching power supplies can be produced commercially. http://www.mfjenterprises.com/ .

Other manufacturers of switching power supplies can be found at:

Propane Detector
An additional source of RFI is the propane detector near the floor. It generates an RF whooping sound on an AM broadcast receiver while listening to weak signals. I have tried filter capacitors with some success, but not total removal of this signal source.

Brake Controller
Another source is the brake controller on the tow vehicle. On my unit, it sends a periodic signal to test the circuit. This can be heard on an AM radio. The fix is easy, simply disconnect your RV from the tow vehicle.

When the brakes are applied, a loud buzzing noise can be heard on the HF receiver. At this time, I can't recommend attempting to filter the brake controller output. Any attempt to apply filter capacitors may interfere with the current sense circuits in many controllers. Ferrite beads may reduce some of the RFI.

Florescent Lights
The DC-DC inverter in florescent lights also radiate, but turning the light off was easier than adding a filter capacitor.

The filter capacitors were in the order of .05 microfarad disks, but I used whatever was in my junk box up to several microfarads. Condenser caps removed from old distributors work just fine (~.05mfd). They usually have a small tab for mounting to ground with a short wire to connect to the +12V line.

TV Antenna Preamps
It was discoverd that the TV amplifier sold in many units around 2000 by Winegard Manufacturing were generagting RFI From self-oscillating amplifier. They were found to be transmitting from 438 to 483 MHz and drifting up and down the band with spurs in the 900 MHz range and getting into getting into 440repeaters11+ miles away.

I also discovered that under unknown conditions, the TV preamplfier inside the wallplate of my 1998 Arctic Fox suffered oscillations.

HF Transmitters
Another source of RFI is your CB or HAM radio. Since my Arctic Fox is fiberglass, there is no shielding between the exterior antenna and the inside wiring. I successfully crashed my wife's laptop computer with only 100 watts RF, not a good idea . Untold wonders to the TV reception as well, the wife voiced her opinion quite clearly !

Locating the Source
To locate RFI in your rig, use a cheap pocket-size transistorized AM radio, the ferrite core antenna is usually at the top of the radio. To sniff RFI, tune to where there is no station, put on headphones, and use the radio like a stud-finder. You should be able to isolate the interference to within inches of the source. Apply filter capacitors where required.

Summary
The 1998 AF 26X RFI problem has been solved, we traded it in for a recycled 2002 AF 23-5A fifth wheel. At least, I thought the problem was solved. Instead, what was discovered is that most manufacturers buy from the same core group of power supply manufacturers. The year of the RV doesn't seem to make a difference, but what is known is that doing ham radio from an RV campground has a high risk of unavoidable RFI from multiple sources..

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Bill (K7TTY) and Eva (KC7WOI) Bytheway
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2002 Arctic Fox 23-5A
1993 Dodge Ram 2500 Cummings