RADIOMETEO · IZ6CLV

QRM Troubleshooting

RADIOMETEO · RADIO LAB · QRM

Find the source of the noise

QRM troubleshooting: a guided path to work out where the noise in your receiver comes from, with profiles of the most common sources, two measurement tools and advice on reporting interference that does not come from your home.

QRM troubleshooting: guided path
iYour answers stay in your browser and are not sent anywhere. For the main-breaker test use a battery-powered receiver (a portable radio or a transceiver on battery).

Answer four questions. The result ranks the most likely source families and lists the tests to run: it is guidance, not a certain identification of a device.

1. What do you hear?
2. When does it happen?
3. With the main breaker of your home switched off, the noise…
4. Where is it strongest?

Answer at least the first question to see the result.

Common QRM sources
i“Mix 31” and “mix 43” are ferrite materials suited to HF; the choice and the number of turns depend on the frequency you need to suppress.

How to recognise them, where you hear them and what you can do. The remedies listed are those most widely used by radio amateurs: how well they work depends on the case.

Reception

Switch-mode power supplies and chargers

How to recognise it
Carriers repeating at regular intervals, often tens to hundreds of kHz apart, that may drift slowly; sometimes a “comb” of noise.
Where you hear it
Mainly low HF, but higher too.
Tests
  • With a battery-powered receiver unplug power supplies one at a time (phones, PCs, routers, TVs, LED strips).
  • Use the birdie analysis below to estimate the switching frequency.
Remedies
  • Clip-on ferrites suited to HF on output and mains cables, with several turns where possible.
  • Replace the noisiest supplies with quality or linear models.
Reception

LED lamps, drivers and dimmers

How to recognise it
Comb or broadband noise that appears when the lights are on, often in the evening.
Where you hear it
HF and VHF.
Tests
  • Switch lights off by group or zone at the consumer unit.
Remedies
  • Replace noisy lamps or drivers.
  • Ferrites on external driver cables; avoid dimmers not designed for LEDs.
Reception

Solar PV systems and optimisers

How to recognise it
Comb or broadband noise only with sunshine, strongest around midday and absent at night.
Where you hear it
Mainly HF.
Tests
  • Compare day, night and cloudy days.
  • Listen in AM with a portable radio, at a safe distance, near the inverter and panel cables.
Remedies
  • Ferrites on DC and AC cables near the inverter and filters: work for the installer.
  • Ask the manufacturer or installer for the product’s EMC compliance.
Reception

Powerline adapters (PLC)

How to recognise it
Strong broadband HF noise, sometimes with “gaps” on the amateur bands; periodic ticking when no data is flowing.
Where you hear it
HF, roughly 2 to 30 MHz (some models higher).
Tests
  • Unplug the powerline adapters in your home; ask neighbours whether they use them.
Remedies
  • Replace them with Ethernet cable or Wi-Fi.
Reception

VDSL and telephone lines

How to recognise it
Flat “stepped” noise or dense carriers over part of HF, caused by data on the telephone pair.
Where you hear it
HF up to about 17 or 35 MHz, depending on the VDSL2 profile.
Tests
  • Switch the modem off: if the noise remains it may come from the building’s or neighbours’ line.
Remedies
  • Ferrites on the telephone cable and a good twisted-pair cable; report to the provider if the noise comes from the line.
Reception

Motor inverters and charging

How to recognise it
Noise that follows the cycles of heat pumps, inverter air conditioners, washing machines, EV wallboxes or e-bikes on charge.
Where you hear it
HF and low VHF.
Tests
  • Log the times and compare them with when appliances run, including neighbours’ ones.
Remedies
  • Ferrites and mains filters fitted by qualified personnel; report the problem to the manufacturer or installer.
Reception

Power lines and arcing

How to recognise it
Rhythmic 100 Hz rasp (twice the 50 Hz mains), stronger in dry or windy weather, often audible on medium-wave AM too.
Where you hear it
Low HF, medium wave, up to low VHF.
Tests
  • Follow the noise along the line with a portable AM radio: it grows near the arcing point.
Remedies
  • Contact the electricity distributor: faulty insulators and clamps can only be repaired by them.
Reception

Receiver overload

How to recognise it
Strong broadcast signals (medium wave, FM, nearby transmitters) appearing on frequencies where they should not be.
Where you hear it
Any band, especially near powerful stations.
Tests
  • Switch in 10 dB of attenuation: if the unwanted signal drops by much more than 10 dB it is generated in the receiver (intermodulation); if it drops by 10 dB like the others it is a real signal.
Remedies
  • Turn the preamp off, use the attenuator, band-pass filters or medium-wave/FM band-stop filters.
Reception

Noise from your own station

How to recognise it
Noise generated by computers, monitors, the radio’s power supply, USB, audio interfaces or routers; currents on the coax braid carry it to the antenna.
Where you hear it
All bands.
Tests
  • Replace the antenna with a dummy load: if the noise disappears it arrives via the antenna.
  • Run the radio on battery and switch off the PC and peripherals.
Remedies
  • A common-mode choke on the coax near where it enters the house.
  • Ferrites on USB, audio and power cables.
Transmission

Interference caused when transmitting

How to recognise it
While transmitting, nearby devices (TV, audio systems, door phones, alarms) pick up RF, or RF gets back into the station: distorted audio, a microphone or keys that “bite”.
Where you hear it
Often only on some bands, where cables resonate.
Tests
  • Try low power and different bands.
  • Transmit into a dummy load: the problem should disappear.
Remedies
  • A 1:1 choke at the antenna feed point and moving the antenna away from house wiring.
  • Ferrites on the affected device’s cables; a low-pass filter if harmonics fall in other bands.
Natural

Atmospheric noise (QRN)

How to recognise it
Irregular crashes from lightning, even far away, mainly on the low bands, in summer and in the evening. It is not QRM and cannot be removed.
Where you hear it
Mainly 160–40 m.
Tests
  • Compare with thunderstorm activity in your area: the noise grows as storms approach.
Remedies
  • Receive-only antennas, DSP filters and noise blankers.

Measurement tools

Two local calculations to put numbers on the noise: the spacing of repeated carriers and a before/after comparison of levels.

Birdie analysis
iRead the frequencies in CW or USB with a narrow filter, centring the whistle. Separate values with spaces or semicolons; a comma is accepted as decimal separator (7,015).

Enter at least three repeated carriers read on your receiver. The tool estimates their spacing, which usually matches the switching frequency of the source.

The result will appear here.

S readings before and after
iIARU Region 1 scale: S9 = −73 dBm below 30 MHz and −93 dBm above 30 MHz, 6 dB per S-unit. Many real S-meters do not follow this scale exactly: use the result as a comparison, not as an absolute measurement.

Converts S-meter readings into dBm and shows how much the noise dropped after a fix (ferrite, filter, device switched off).

The result will appear here.

If the noise does not come from your home

A practical path to document the interference and involve those who can fix it.

  1. Document it: date, time, frequencies, S level, an audio recording or an SDR spectrum image.
  2. Locate it: walk around your home with a portable radio in AM (medium wave or HF); the noise grows as you get closer to the source.
  3. Talk to your neighbour or building manager: bring the data you collected and suggest a simple test, such as switching a device off for a few minutes.
  4. For power lines and substations contact your local electricity distributor; do not approach live installations.
  5. If the interference continues, in Italy radio emission control is the responsibility of MIMIT and its regional inspectorates (Ispettorati territoriali); see the official page for remit and contacts. Elsewhere, contact your national spectrum regulator.
MIMIT — radio emission control (Italian)mimit.gov.it MIMIT — regional inspectorates (Italian)mimit.gov.it
Safety

Do not open mains-powered equipment and do not work on electrical or photovoltaic installations: such work is for qualified personnel. Clip-on ferrites and cables can be handled without opening anything.

See also: A web receiver far from home tells you at once whether the noise is only yours; analyzers and SDRs help you measure it. WebSDR receivers · SDR lab · Antennas and measurements · Radio lab tools

Sources: ARRL — RFI, ARRL New England Division — RFI guide, IARU Region 1 — EMC C7, IARU Region 1 — HF Manager’s Handbook (S-meter, §4.1.3), MIMIT. Sources consulted on 16 October 2026. Links open on the publisher’s website.

Articles on QRM and interference

RadioMeteo articles about noise, interference and EMC appear here automatically when they are published.