How to Track Down RFI

You turn on the radio and a band that used to be quiet now sits at S7. Or a raspy buzz shows up every evening and goes away at bedtime. That is radio frequency interference, RFI for short, and almost every ham meets it sooner or later.

The good news is that RFI hunting is detective work, not magic. You gather clues, rule things out one at a time, and walk the noise down to its source. Here is how, plus what to do when the interference is coming from you.

Learn Your Noise Floor

Your noise floor is the S-meter reading on an empty frequency with the antenna connected. Write it down for each band you use, at a few different times of day. Without that baseline, you cannot tell whether a noise is new.

Next, listen to the character of the noise in AM mode with the noise blanker off. A harsh buzz with a steady 120 Hz rasp points toward something on the AC power system. A row of carriers or hash humps that repeat every few kilohertz as you tune is more typical of a switching power supply.

One more quick test: disconnect the antenna. If the noise disappears, it is arriving through the antenna. If it stays, the problem is in the radio, its power supply, or the cables in your shack.

The Main Breaker Test

This is the most useful step in any RFI hunt. Run your radio from a battery, or use a battery-powered portable radio that hears the noise, and keep a flashlight handy. Then switch off the main breaker for the whole house and listen.

1. If the noise stops, the source is inside your home. Turn the main back on, then switch off the branch breakers one at a time until the noise goes away.
2. With the guilty circuit found, unplug the devices on it one by one until the noise stops.
3. If the noise does not change with the main off, the source is outside, either at a neighbor’s house or on the power lines.

Warn the household, shut down computers first, and keep the test short. Only flip breakers; never remove the panel cover unless you are qualified to work on live circuits. Anything with its own battery, such as a UPS, keeps running with the main off, and ARRL notes that parts of a solar system can keep making noise while the sun is on the panels.

Walk It Down with a Portable Radio

When the source is outside, a small battery AM broadcast radio becomes your sniffer. Tune it to an empty spot near the top of the dial and walk around the house and down the street. The built-in ferrite bar antenna is directional: the noise is loudest when the bar is broadside to the source and dips when one end points at it.

As you get close, the radio will be swamped everywhere, so tune higher to narrow things down. For power-line noise, ARRL suggests a VHF or UHF AM receiver, such as an aircraft-band radio, with an attenuator as you close in, because HF and broadcast-band radios can show false hot spots along the wires.

The Usual Suspects

• Switching Power Supplies and Chargers: Wall-wart supplies, phone and laptop chargers, and tool battery chargers are frequent offenders.
• LED Bulbs and Dimmers: ARRL warns that efficient LED lighting and grow lights can generate RF noise; old incandescent bulbs did not.
• Solar Inverters and Optimizers: Their switching noise rides out on the DC wiring to the panels and can reach well into HF.
• Doorbell Transformers and Furnace Controls: Forgotten devices that ARRL says are often mistaken for power-line noise.

Once you find the culprit, the cure is often simple. Replace it with a better-made unit, move it, or add ferrite chokes on its cords; our article on Ferrites and Common-Mode Chokes explains how. If the device belongs to a neighbor, explain the problem kindly and offer to help.

Power-Line Noise

True power-line noise is a broadband raspy buzz caused by small arcs across loose or cracked hardware on the lines. It is usually strongest on the lower bands. According to ARRL, it often weakens in rain or humid weather and may flicker when the wind moves the lines, which is a good sign the source is outdoors.

The utility is responsible for noise from its own equipment, so report it to them. Call customer service, ask for an RFI or power quality investigation, and keep a record. ARRL recommends logging these details:

• Frequencies affected and the S-meter reading on each.
• Dates, times and weather, including the times the noise is absent.
• Ticket numbers, plus the name of everyone you spoke with and what they said.

Point the investigator toward the area you found, but do not guess which part is bad. If the utility makes no progress after a reasonable time, contact the ARRL RFI Desk, which works with the FCC on these cases.

Safety First: never climb, hit or shake a utility pole, and never touch or pull on a guy wire. Stay well away from any downed line. Locating the noise is your job; touching the hardware is strictly the line crew’s.

What Part 15 Says

Most consumer electronics are unlicensed devices covered by Part 15 of the FCC rules. Section 15.5 sets the deal: these devices may be used only on the condition that they cause no harmful interference, and they must accept any interference they receive. If an FCC representative tells the operator that a device is causing harmful interference, the operator must stop using it until the problem is fixed.

“Harmful” has a specific meaning: interference that seriously degrades, obstructs or repeatedly interrupts a radio service. The exact wording is in 47 CFR 15.5 on eCFR. In practice the rules are a last resort; a friendly approach and a clear log solve far more cases.

When the Interference Is Yours

Sometimes your signal is the problem, showing up in the neighbor’s speakers, phone or doorbell. Start at home: if your own electronics misbehave when you transmit, theirs probably do too, and you can test fixes on your own gear first.

• Put a common-mode choke at the antenna feed point so the coax is not radiating into the house.
• Mount the antenna as far from houses and wiring as you reasonably can.
• Add ferrite chokes on the affected device’s speaker, phone or power cords.
• Try lower power while you work on it.

When you are stuck, the ARRL Radio Frequency Interference Pages cover specific devices, from solar systems to touch lamps, and explain how to reach the ARRL RFI Desk. Be patient, be neighborly, and work one step at a time.