Radio frequency interference

Radio frequency interference
Abbreviation	RFI; also called EMI (electromagnetic interference)
Outgoing RFI	A transmitter upsetting nearby electronics such as speakers, telephones, televisions and computers
Incoming RFI	Noise from electrical devices and power lines reaching the receiver
Common cures	Ferrite chokes on feed lines and device cables, AC line filters, antenna placement, finding and fixing the noise source
Rules and help
US regulator	Federal Communications Commission (FCC)
Consumer devices	Covered by Part 15 of the FCC rules
Help for hams	The ARRL's RFI information and technical staff

Radio frequency interference (RFI) is unwanted radio energy that disrupts electronic equipment or radio reception. For hams it runs in two directions. A transmitting station can upset nearby televisions, stereo speakers, telephones and computers, and a growing crowd of household electronics fills receivers with noise. The first is usually curable with ferrite and good station practice; the second, now the more common complaint, often means tracking down a single device nearby.

When your station causes interference

Most interference to consumer electronics is not caused by an illegal signal. Instead, the device can't reject a strong signal from a transmitter nearby. Speaker leads, telephone cords, power cords and network cables act as antennas, the RF they pick up is rectified in audio circuits, and the ham's voice or the clicking of Morse code comes out of the speaker. This is called fundamental overload, because the device is responding to the transmitter's intended signal. Much less often the transmitter itself is sending out harmonics or other spurious emissions, which is always the ham's responsibility to fix. Separately, the ARRL's Clean Signal Initiative works with equipment makers on benchmarks and test procedures for cleaner transmitters and amplifiers, aimed mainly at key clicks, splatter and transmitted noise that interfere with other stations on the band.

Cures at the station come first:

Stop common-mode current. A feed line carrying RF on the outside of its shield radiates into the house. A common-mode choke or balun at the antenna feed point, and another where the coax enters the building, cure a surprising share of RFI complaints.
Move the antenna away from the house. Indoor antennas and wires near the house wiring couple strongly into everything nearby.
Bond the station equipment together as described under Grounding and lightning protection. A long ground wire from an upstairs shack can make RF problems worse, not better.

At the affected device, clamp-on ferrite cores on its speaker, power and network cables, with the cable looped through several times, block the RF before it reaches the electronics. Plug-in RF filters are sold for telephones. Approach a neighbor's problem as a favor, not a legal argument.

The FCC generally treats a device that cannot reject a clean, legal signal as a problem with the device, not the station, but Part 97 does provide for restricted operating hours when a station causes general interference to broadcast reception on well-designed receivers.

Under federal law, RFI is regulated by the FCC alone. In 1982 Congress amended the Communications Act (Public Law 97-259) to give the FCC exclusive jurisdiction over RFI, and in a 2003 order an FCC bureau found that local zoning provisions which tried to regulate interference were preempted. In practice this means a city or county cannot impose its own interference rules on a ham station; only the FCC can enforce rules on interference, though it expects the people involved to try to solve the problem first.

Pacemakers and implanted medical devices

Hams with pacemakers or implantable cardioverter-defibrillators (ICDs), and their families, often ask whether a station is safe to be near. The answer depends on the particular device, so the first step is to ask the physician and the device's manufacturer; the ARRL's pacemaker page says there is no substitute for expert advice from a doctor and links to the manufacturers' own guidance. Published guidance gives a starting point. The American Heart Association says CB and amateur radios pose little or no risk to pacemakers but can affect how an ICD works, and it lists minimum distances from an ICD that grow with transmitter power. Medtronic advises keeping the antenna of a radio of 3 watts or less at least 6 inches from an implanted heart device, 12 inches at 3 to 15 watts, and 2 feet at 15 to 30 watts. The distances are measured from the antenna, not the radio. These tables stop at 30 watts. They do not cover a typical 100-watt HF or 50-watt mobile station, or an amplifier, so for those the specific advice of the device's manufacturer and the physician is needed. See also RF exposure.

When noise interferes with you

Noise from electrical devices has risen steadily, and in many suburbs it now limits HF reception. Common sources include:

Switching power supplies in phone chargers, laptop adapters and televisions, which produce a buzzing hash or signals repeating at regular intervals, typically 10 to 200 kHz apart and often drifting.
LED bulbs and their drivers, especially cheap ones and those on dimmers.
Solar power inverters and panel optimizers. Noise that appears at sunrise and fades at dusk points straight at a solar installation.
Variable-speed motors in heat pumps, pool pumps and appliances; powerline networking adapters; electric fence chargers; and grow lights.
Power lines. Arcing from loose hardware or a failing insulator on a utility pole makes a harsh buzz, in North America usually with a 120 Hz character, that often changes with the weather and can cover several bands.

Most of these are unlicensed devices under Part 15 of the FCC rules. Part 15 requires that they not cause harmful interference, and that if one does, its operator stop using it until the problem is corrected. Electric utilities have the same obligation for noise from their lines.

Locating a noise source

Begin by ruling out the station: if the noise disappears when the antenna is disconnected, it is arriving from outside. Then listen on a battery-powered receiver while switching off the home's main breaker. If the noise stops, the source is inside the house; restore the breakers one at a time, then unplug devices on the circuit that brings the noise back. Remember that a battery backup unit or a device on its own battery keeps running when the power is off.

Do the breaker test before knocking on a neighbor's door. A surprising number of noise sources turn out to be in the ham's own house.

If the noise is still there, it is outside. A portable AM broadcast radio tuned to a quiet spot near the top of the band makes a simple direction finder, since its internal antenna is directional, and the noise grows louder as you walk toward the source. Near the source, switching to a receiver on a higher frequency, such as the VHF aircraft band, narrows the search to a single house or pole, because noise at higher frequencies doesn't travel as far along the wiring. Never climb or touch utility poles or their hardware. Report power-line noise to the utility with the pole's location and the times the noise is present. How to track down RFI walks through the search in more detail.

Getting help

The ARRL publishes extensive RFI information on its website and in its book on the subject, and its technical staff advises members on difficult cases, including power-line noise that a utility has not resolved. The FCC enforces Part 15 and Part 97, but it generally expects the people involved to work the problem out first, with formal enforcement as a last step. Good records, with dates, times, frequencies and recordings, help in every case.

External links