Solar inverter or motor-drive noise
Noise from rooftop solar inverters and from variable-speed motors in heat pumps, pumps and appliances.
What does it sound like?
A buzz, whine or rough 'comb' of tones that comes and goes with the equipment.
Who uses it?
Not a station: equipment in your home or nearby.
Where and when can I hear it?
Across shortwave; solar noise follows the daylight hours.
The science
Solar panels themselves are essentially large diodes and make no radio noise; the noise comes from the electronics that convert their direct current to household alternating current. The ARRL reports inverters switching at fundamental frequencies of about 18 to 60 kHz and panel-level optimizers at 39 to 200 kHz, with harmonics (whole-number multiples) reaching through shortwave into low VHF. Variable-speed motor drives use similar pulse-width switching, and a Honda patent describes how harmonics of a 17 kHz motor drive landing near an AM station's frequency are heard as noise on a car radio.
Why it matters
Because the converters run on sunlight, solar noise normally follows the sun, which is a useful clue when you are hunting it; the ARRL notes optimizers can still make some noise when they are not delivering power, as long as sunlight reaches them. The ARRL advises working through the system's owner, who can open a case with the installer, and being ready with details such as what times of day the noise occurs, which bands it affects and how far the array is from your antenna. It reports that installers and major manufacturers have been cooperative in resolving cases.
Lore
The same patent's cure is neat: it sets the motor drive's switching rate to an exact multiple of the 9 kHz AM channel spacing used in its example, so every harmonic lands exactly on a channel frequency. In the patent's example, a harmonic at 595 kHz beside a station at 594 kHz would otherwise be heard as a 1 kHz tone.
Easily confused with
- Switching power supply / LED noise: an ordinary power supply doesn't follow the sun