Curing Interference to Non-Radio Equipment
The ARRL Laboratory staff fields phone calls and letters about interference problems. Those calls about “non-radio” equipment can be complex and the solutions are sometimes elusive. In addition to the "personal diplomacy" involved, you and your neighbors must understand the regulatory and technical aspects of the problem and work together towards a solution. The Audio Interference FAQ Page should help by answering many of your questions.
The Audio Interference FAQ Page
Q: My neighbor bought a new stereo system which “hears” me on 20-meters; my transmitter is picked up by her new stereo. She says it's my fault because the stereo system is brand new. I want to help her, but I need to understand more. Can you help?
A: This is a classic case. We encourage you to contact your ARRL section Technical Coordinator (TC). Much of this information is now available on the ARRL RFI Web Pages and found in The ARRL RFI Book.
Q: I just sent for the The ARRL RFI Book, but I'm still wondering if the problem really is my fault after all. Should I put a filter on my transmitter?
A: No. Since you are interfering with a non-radio device, your neighbor’s stereo is acting as a radio receiver through no fault of yours. The FCC's Interference Handbook 1990 Edition says "Telephones, stereos, computers, electronic organs and home intercom devices can receive interference from nearby radio transmitters. When this happens, the device improperly functions as a radio receiver. Proper shielding or filtering can eliminate such interference." If your transmitter is not putting out illegal spurious signals, her problem is almost certainly not caused by your harmonics.
Q: I guess the law has let me off the hook?
A: As far as the technical and regulatory issues are concerned, you’re in the clear. But she is still your neighbor. Offer her a helping hand, as Amateur Radio is best known for its public-service contributions. Apply your technical skills, or those of your local TC or EMI/RFI Committee, to the problem as a form of public service.
Q: I read the ARRL RFI Book and browsed the ARRL RFI Pages with its downloadable pamphlet about interference. I gave the pamphlet to my neighbor and she sees that this might not be my fault. She is ready to work with me toward a solution. Where do we begin?
A: Do not attempt to “repair” your neighbor's equipment. While this problem can often be fixed with proper internal filtering and shielding, this is something best done by the manufacturer. If you dismantle a new stereo system, you’re setting yourself up for liabilities. You will probably void the warranty and assume responsibility for anything that goes wrong with that system. You may also be breaking the law. Most states have laws that prohibit you from working on your neighbor's electronic equipment (even for free) unless you hold a valid state service license. Your ham license is not a substitute.
Q: Should I tell her there is nothing I can do?
A: No, there’s a lot you can do! Help her contact the manufacturer of the stereo system. The Consumer Electronics Association (CEA) maintains a database of key contact people at each manufacturer. You may discover that the manufacturer already has fixes available or will do the work under warranty. Locate the manufacturer's representative to repair the stereo. Help that representative understand the technical issues involved. Manufacturers are usually willing to assume their responsibilities fairly, but they do not always understand the fundamental causes of the problem, nor all the solutions. Read The RFI Book chapters on fundamentals and curing audio equipment interference and show them to the technician. Make your station available for testing when the service person arrives. You need to be there to put the radio signal on the air, to make sure the “cures” actually worked. Be helpful.
Q: The manufacturer has agreed to send a service person to investigate the problem. I’d like to be helpful. I've read the "ARRL RFI Web Pages" and learned some troubleshooting tactics. What's the best way to start?
A: Simplify the problem. Disconnect all inputs to the stereo system, one by one, and see if the interference suddenly goes away. Now you know where to apply the cure. Try disconnecting portable units from line power as well.
Q: We disconnected everything, including the long audio cable from the stereo TV. The interference is still there.
A: That's unfortunate. Long cables are suspect since they make a good long-wire antenna and pick up lots of RF. Components don't pick up RF energy—their wires do! Take a look at the wires that are connected to the amplifier.
Q: The only wires are the speaker wires. The speakers are located about 15 feet from the stereo. They are just about the size of my 20-meter dipole. Is that why she gets interference when I operate on 20 meters?
A: Could be! Speaker wires are often 8 to 16 feet long. When you put two of them together, you make an efficient receiving antenna. Try bundling the speaker wires to reduce their effectiveness as an antenna. This procedure has been known to eliminate the interference all by itself.
Q: We moved the speakers and tried bundling the wire. It helped a little, but not enough. "CQ DX" is still interspersed with her favorite songs. What's next?
A: You're on the right track. Disconnect the speaker wires altogether and plug in a set of headphones. What happens?
Q: The interference is gone! My neighbor is reluctant to use her headphones whenever she wants to listen to her stereo, though. What now?
A: Well, you've just learned that the RF is being picked up on the speaker wires and being conducted into the amplifier. (The output transistors are possibly rectifying the RF into audio, and the amplifier's internal negative feedback circuitry is conducting it back to the high-gain stages of the amplifier.) Let's start with common-mode chokes on each speaker wire pair.
Q: What is a common-mode choke?
A: For a detailed explanation, refer to the RFI Book. You can build two of them, one for each speaker output. Wrap ten to fifteen turns of speaker wire onto an FT-140-43 ferrite core. (Use and FT-240-43 if the speaker wires are large, and use -73 material for interference from 80 or 160-meter signals. Alternately, type -31 material is an excellent general purpose choice for HF.) Install them right at the amplifier. (If the system uses amplified speakers, you should install one at each speaker, too.)
Another possibility is to try a few commercially available filters. Here are two companies that sell ac line filters suitable for audio systems:
- Morgan Manufacturing Model 465 or similar series as required. See Morgan's AC Line Protector Page for additional details.
- Corcom caries a wide range of filters. See Corcom's Power Line General Purpose Filter page for additional info and selection.
These companies have a fine reputation, and sell a broad line of other interference-reduction products. Contact them for more information.
Q: I read an old QST article that recommended placing a 0.01 microfarad capacitor across the speaker terminals. Wouldn't that be a lot easier?
A: Don't do it! That was good advice when amplifiers used tubes. Many modern solid-state amplifiers don't work well into capacitive loads and may break into a full-power (sometimes ultrasonic) oscillation. This can result in the near-instantaneous destruction of the output module or transistors. And you thought you had a diplomacy problem before you blow up her stereo!
Q: Thanks for the warning! Where do I get the right ferrite chokes?
A: The ARRL Web Site contains a list of EMI/RFI materials suppliers. You can also refer to the advertisements in QST – or use search phrases like “ferrite cores” or “filters.”
Q: I installed the speaker-lead chokes, and they worked! However, as soon as I hook up the long cable to the TV set, the interference returned. Should I put chokes on that cable, too?
A: Yes. Many interference problems have multiple causes. This is why you want to break the problem down into smaller pieces. Try a ferrite common-mode choke at one or both ends of the long cable.
Q: The chokes helped somewhat. What else can I try?
A: Now we are getting into the more complicated cures. You many need to use an L/C filter in the input lines as well. The ARRL RFI Book contains information about how to build one. Commercially available ac line filters can also be purchased from numerous companies, including Corcom and Morgan Manufacturing as referenced in a previous answer.
Q: We're going to order the filter. In the meantime, my neighbor can’t use her stereo TV connection. Am I going to have similar problems if one of my neighbors installs an intercom or alarm system?
A: You might. Many consumer devices are not designed with EMC in mind. You can apply what you’ve learned to any non-radio interference problem. You can even use those 0.01 microfarad capacitors, putting them across terminals (or from both terminals to ground) of simple alarm systems, or across input connections to intercoms. You can also install some common-mode chokes on long wire runs. Some think a single choke for both wires in a twisted pair works best; others have reported that separate chokes for each wire cured the interference.
In any case, contact the manufacturer through the Consumer Electronics Association (CEA). This assures the best possible source of help and lets the manufacturer knows when people are having problems with their systems. You can also contact Mike Gruber, our RFI expert at ARRL Headquarters to get advice on fixing a problem that has stumped you and your ARRL section Technical Coordinator. Good luck!