Switching power supply / LED noise
Noise from phone chargers, LED bulbs, power adapters and other modern electronics.
What does it sound like?
A whine, buzz or rough carrier that may drift, repeating every few tens of kilohertz as you tune.
Who uses it?
Not a station: devices in your home or nearby.
Where and when can I hear it?
Anywhere on shortwave; often a 'picket fence' of signals at regular spacing across the dial.
The science
A switching power supply converts wall power by chopping current on and off many thousands of times a second, and every sharp switch edge radiates radio energy. The ARRL notes that this switching rate is usually somewhere between 10 and 200 kHz, so the noise repeats at that spacing across the dial, producing the picket-fence pattern, and is usually strongest at lower frequencies. The switching frequency is not tightly controlled, so the peaks drift, shifting as the line voltage dips and surges, and in AM mode you can almost always hear a 120 Hz hum on the noise.
Why it matters
These supplies are used in nearly everything: phone chargers, TVs, LED bulbs and grow lights. In 2015 the ARRL's laboratory manager called them a more common noise source for hams than power lines, pointing mainly at small plug-in wall adapters. The ARRL's advice is to start at home: turn off every breaker while listening on a battery-powered radio, and if the noise disappears, it is yours.
Lore
The ARRL's lab manager writes that he has lost count of hams who searched the neighborhood for a noise, only to find something like a battery charger forgotten in their own garage outlet.
Easily confused with
- Steady carrier: a real carrier doesn't repeat every few tens of kHz