Power line noise
| Power line noise | |
|---|---|
| Also called | Power-line interference |
| Main cause | Arcing across small gaps in loose, broken or poorly spaced utility hardware |
| Sound | Harsh buzz or frying noise, with a strong 120 Hz character on 60 Hz lines |
| Frequencies | Broadband; strongest at low frequencies, reaching VHF and UHF near the source |
| Rules and help | |
| FCC classification | Incidental radiator under Part 15 |
| Key rules | 47 CFR 15.5, 15.13 and 15.15 |
| Who fixes it | The utility, for noise from its own equipment |
| Help for hams | ARRL RFI Desk and the ARRL–FCC cooperative agreement |
Power line noise is radio interference produced by electric utility lines and hardware, most often by tiny electrical arcs. It sounds like a harsh buzz across the receiver and can cover the lower HF bands for miles. It is one of the most stubborn forms of radio frequency interference, because the source is high on a pole and only the utility can work on it. Most utilities fix it once told, and there is a well-worn path to follow when they don't.
Causes
Nearly all power-line noise from utility equipment comes from arcing across a small gap. Voltage builds across the gap until the air breaks down, a spark jumps, the arc goes out, and the cycle starts again. Typical gaps are found in loose or broken hardware, cracked insulators, and parts spaced too closely, such as a ground wire lying near a staple.[1] Each spark is a burst of broadband radio energy, which the line radiates like a long antenna.
Corona, the partial breakdown of air around high-voltage conductors, is a different effect. It rarely bothers hams, since its low-current noise does not travel far.[1]
Not all noise that comes over the power lines is the utility's. Light dimmers, doorbell transformers and furnace controls in homes are frequent culprits, and their noise can travel along the wiring. The owner of a noisy device, not the utility, is responsible for fixing it.[1]
How it sounds and behaves
Alternating current at 60 Hz reaches a voltage peak twice in each cycle, so arcing comes in bursts 120 times a second. On an AM receiver that gives the familiar raspy buzz, and on an oscilloscope the bursts repeat about every 8.3 milliseconds.[1] True power-line noise is continuous across a band, is strongest at the lowest frequencies and fades out at some upper frequency.[1] Noise that repeats in regular steps as you tune is probably something else.
The highest frequency at which the noise can be heard is a clue to distance. Noise that is still loud at VHF or UHF is coming from somewhere close, while noise that dies out near the AM broadcast band may be miles away.[1]
Weather changes it. Rain and humidity often quiet an arcing gap, wind moves the hardware and makes the noise come and go, and heat and cold open and close gaps as metal expands and contracts.[1] Noise that tracks the weather almost always has an outdoor source.
Locating it
The first job is to prove the noise is not inside the house, by switching off the main breaker while listening on a battery-powered radio.[1] Outside, a portable AM radio finds the general area. The ARRL recommends switching to VHF or UHF for the final search, with a directional antenna such as a handheld Yagi and an attenuator, because HF can show false peaks at many points along a line.[1] Hams should never climb, strike or shake poles, touch guy wires, or guess at what part has failed; asking for the wrong hardware to be replaced often makes things worse.[1]
Getting the utility to fix it
Report the noise through the utility's normal complaint process, so that it is logged, and record every ticket number, name and date.[1] Describe when the noise is present, and offer to let the utility's investigator listen at your station.[1]
If the utility does not respond after a reasonable time, which the ARRL puts at about 60 days, the ARRL RFI Desk can take up the case under its cooperative agreement with the FCC.[1] The ARRL writes to the utility's chief executive, describing the rules and offering technical help. If that fails, the FCC can send the utility an advisory letter, and then a second one.[1] Unresolved cases can go to an FCC field investigation and formal enforcement.[2] Cases can take many months. ARRL membership is not required.[1]
The FCC's role
Under Part 15 of the FCC rules, power lines are an incidental radiator: a device that produces radio energy while operating, though it is not designed to.[3] Section 15.5 requires that such devices not cause harmful interference and that an operator stop using one, until it is fixed, once the FCC gives notice that it is doing so.[4] Section 15.13 tells makers of incidental radiators to use good engineering practice to limit interference,[5] and Section 15.15 notes that meeting the emission limits does not guarantee interference will never occur.[6] There is no fixed field-strength limit for power-line noise; the test is whether it causes harmful interference.[1]
Formal action is rare but does happen. In 2006 the FCC issued a citation to Lakeland Electric in Florida over noise that had troubled a local ham for more than a decade,[7] and in July 2007 it cited two utilities serving Lubbock, Texas, Lubbock Power & Light and Xcel Energy, in a long-running case.[8]
See also
- Radio frequency interference
- How to track down RFI
- Broadband over power line
- Federal Communications Commission
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Power-Line Noise FAQ, ARRL.
- ↑ Track and Solve Electrical Interference, ARRL.
- ↑ 47 CFR 15.3, Legal Information Institute.
- ↑ 47 CFR 15.5, eCFR.
- ↑ 47 CFR 15.13, Legal Information Institute.
- ↑ 47 CFR 15.15, Legal Information Institute.
- ↑ FCC Cites Florida Utility for Interference to Radio Amateur, ARRL, June 2006.
- ↑ FCC Issues Two Citations in Longstanding Power Line Noise Case, ARRL, 2007.