Power-line noise
Electrical noise from power lines and pole hardware, usually from loose or worn parts making tiny sparks.
What does it sound like?
A harsh, raspy buzz, often with a 120-times-a-second rhythm (100 in 50 Hz countries), or crackling.
Who uses it?
Not a station: electric utility equipment.
Where and when can I hear it?
Worst on the lower shortwave and AM bands; often worse in dry weather and near the faulty pole.
The science
Most power-line noise comes from a tiny spark. When a connector, insulator, lightning arrester or other piece of pole hardware works loose, cracks or corrodes, a small gap forms, and the voltage jumps across it near each peak of the alternating current. North American power peaks twice per 60 Hz cycle, so the sparks come in bursts 120 times a second, one about every 8 milliseconds, giving the harsh, raspy buzz. The noise is strongest at low frequencies and weakens as you tune higher; the higher it reaches, into VHF and UHF, the closer the source usually is. High-voltage lines can also produce corona discharge, but the ARRL says corona is rarely a radio problem.
Why it matters
Power-line noise often fades in rain or humid weather, because moisture fills the tiny gaps, and returns as the hardware dries. The ARRL's first test is to switch off your home's main breaker while listening on a battery-powered AM radio: if the noise stops, the source is inside the house. If not, carrying a portable AM radio around the area to find where the noise peaks can narrow it down, and the next step is a complaint to the utility; the ARRL suggests contacting its RFI Desk if the utility is still unresponsive after about 60 days. The ARRL also warns never to climb or bang on utility poles.
Lore
Hunting it down has become a specialty: noise trackers use directional VHF and UHF antennas to close in on a single pole and ultrasonic dishes to confirm which device on it is arcing. Earth blocks the noise well, so lines on the far side of a hill are unlikely suspects.
Easily confused with
- Electric fence ticks: a fence pops about once a second instead of buzzing
- TV or display buzz: TV buzz stops when the screen is switched off