[RFI] Help with tracking power line noise

Jim Brown jim at audiosystemsgroup.com
Sun Feb 21 20:17:32 EST 2016


On Sun,2/21/2016 4:27 PM, David Winarsky wrote:
> Thanks, what has me puzzled is I have S7 noise on 10 meters and with the loop antenna, I believe I have it tracked to a utility drop for a house 2 doors down; however I don't pick anything up on 2 meters or 440.

There's another fundamental principle here that some are overlooking. 
The noise PRODUCED by power systems is nearly always the result of 
ARCING, so it is IMPULSE NOISE. This noise is, by its nature, quite 
broadband, although its spectrum is often shaped by the transmission 
medium. Impulse noise often extends from power frequencies to UHF, but 
the parts of the antennas that radiate it are generally most effective 
very close to the source. The lower frequency components are radiated by 
much longer wires, and are sometimes conducted along those wires as a 
transmission line, to lower frequencies are not nearly as effective for 
locating the source. When we use an AM RX at VHF and UHF, we are using 
that principle -- when the signal is strong at UHF, we're zeroing in on 
the source.

The above principle does NOT apply to most ELECTRONICALLY GENERATED 
NOISE, simply because it's NOT an impulse. Rather, it's a square wave, 
usually in the high audio spectrum, with a TON of harmonics produced by 
the rise time of the square wave. These sources are FAR less broadband 
-- they are typically harmonics of the square wave intentionally  
freuency-modulated by random noise to get around FCC regulations about 
power in a single frequency (in the digital world, we might describe 
that modulation as dither). If the noise source is a free-running 
oscillator or clock, it will usually drift (a lot) as it warms up, so we 
hear these noise sources as humps of noise as sidebands of a carrier 
that drift up or down the band. These drifting humps of noise are the 
signature of a SWITCH MODE POWER SUPPLY.

If the noise does NOT drift, they're likely a clock for a microprocessor.

We CANNOT chase ELECTRONIC SOURCES with a VHF or UHF FM receiver, 
because they are NOT impulses, and because they rarely have much spectra 
up there. Rather, we much chase them with an MF or HF RX that can tune 
to the part of the spectra where they are strong, ideally where they are 
bothering us.

CAPS added for emphasis and clarity. :)

There are experts lurking here who are professional noise chasers and 
far more experienced than I, so I'll defer to them on their methods. I'm 
simply trying to improve understanding.

73, Jim K9YC


More information about the RFI mailing list