A Guide to Finding Radio Frequency Interference (RFI) Noise
Finding and correcting radio frequency noise (RFI) requires a process and the tools you will need to resolve it. In some cases, you’ll need help, especially if the noise is coming from outside your home. You can click on links below to get more information about how to find and fix your noise interference. A summary of the purpose and process contained in the guide appears in this post.
Once you are sure that what you’re hearing is RFI, make sure that you start the process with the logic tree near the bottom of the page. It will guide you and keep you organized as you work!
Do Some Easy Things First – Looking for Clues
Before doing anything else, ask yourself whether the noise you hear is something new. If so, ask yourself if you have just started up some new electrical device in your home, It could be anything from a new major appliance to a light bulb. Noise can also come from older equipment that’s degrading as well so look around for older electrical devices, especially those that aren’t fully protected like those in garages. Listening with a battery-operated transceiver while you open the main breaker will tell you if the source is inside or outside your home. Don’t forget that battery-operated devices will need to be disabled since they may also cause RFI. Inside, it’s simple to turn off or unplug any devices you suspect and see if the noise goes away, keeping track of what you check by writing it down in case you need outside help. A list of potential RFI sources has been compiled and will be updated as necessary. You can see some examples of RFI generators and the sounds they make. These may give you more clues. You can then begin to look further if the problem is elsewhere.
If you have a directional antenna, turn it and see what direction the noise is strongest. If you know of another ham in your vicinity with a directional antenna, see if you can work with him or her to tune to the same frequency of the noise and point their antenna at the direction where it is strongest at their location (the source will be close to where the directions intersect.) Note also what times of day and day of the week you hear the noise and when you don’t. In addition, observe whether the noise is weather-dependent or sunlight dependent. Temperature can also cause variations in noise as mechanical connections get tighter or looser. Note the lowest frequency you hear the noise and where it also is heard on any higher frequencies (harmonics.) Keep in mind that the “where” is usually more important than the “what,” especially for hunting in your own home so don’t delay your hunt there after doing what’s easy. You can always make recordings later if simply opening and closing circuit breakers doesn’t identify the culprit.
You may hear noise from more than one source. Take these one at a time, stating with the loudest first. Repeat the process for each noise you want to deal with.
One more very important RFI behavior to know: it often starts low in frequency, say 120 Hz for power line noise, and continues to generate harmonics up the entire MF and HF spectrum, getting less and less intense, up into VHF and even UHF region. This is why we start to hunt with a small HF loop to find the location of strongest RFI and finish with a directional VHF antenna to locate the precise source. Another example is switching power supplies though the noise may start higher in frequency.
Get Some Simple Tools Together and Start Searching
The decision tree below will help you to locate the source of the noise. If you still hear noise on your battery-operated receiver after opening the main breaker, the noise is outside the house and you can start hunting there. If the noise dissipates, it’s internal to the house – open all the feeder circuit breakers then turn the main breaker back on and then turn on each circuit beaker one by one, listening for the RFI, until the noise re-appears to determine which room the noise is in. You can then start unplugging items in that room. Once you identify the noise, you’ll have to decide what to do about it: replace, repair, throw out, filter it out. Let your RFI team know the source regardless so we build up our knowledge of these noise-makers.
Searching In the Home:
The tools you’ll need for finding noise sources are:
- Your receiver on battery power
- Basic knowledge of your home’s electrical system
Searching Outside the Home:
The tools you’ll need for finding noise sources are:
- A portable AM or shortwave radio
- A VHF radio with AM capability (i.e. aircraft band)
- A directional antenna (a cheap tape measure beam is also good for fox hunting} and a “sense” antenna for HF (really just a length of wire.)
- A car (and a friend to drive it) for checking beyond walking distance
Warning – Warning – Warning
Be careful where you go! DO NOT TRESPASS! Only you can judge the safety of a neighborhood and how safe it may be for a number of reasons to be driving the neighborhood, parking safely if needed and getting out of the car.
DISCLAIMER: This information is provided onlyto assist radio amateurs. All actions taken are the sole responsibility of the radio amateur. The author and/or any RFI Team members are not responsible for any adverse consequences resulting from this information.
The web-based resources you may need are:
- list of AM radios and antennas you can purchase (or build) for troubleshooting
- power company contacts if you know that your noise is a power line issue (and not just coming from a pole that has several services on it.)
- audio files for several different commonly encountered sources
- spectrum files for several different commonly encountered sources
- Simple antennas and attenuators for finding outside noise (that you can build or buy)
RFI teams in each section will help you if you get stuck. They will likely have the equipment and expertise to find the “tough ones” and deal with the communication issues as well. If all else fails, they can refer the process to ARRL HQ and HQ to the FCC if necessary.
One way YOU can help is to let the local RFI Team know about problem devices you discover, even if you fix the problem. Building up our body of noise cases will help HQ when we need to approach manufacturers with the issues we uncover.
Select the choices below in the Logic Tree that will guide you through the process.
RFI on Repeaters is resolved with the assistance of the coordinating body and, if necessary, the amateur auxiliary and the FCC. NESMC is the coordinating authority for New England except for CT that uses Connecticut Spectrum Management Association and VT that uses the Vermont Independent Repeater Coordination Committee.
The New England Division wishes to acknowledge the assistance of Ed Hare, W1RFI, head of the ARRL Laboratory as well as the other members of the Spectrum Protection and Use working group in the preparation of this guide.