[RFI] Line noise vs Frequency

[Hare, Ed, W1RFI]

Most of the time, sparking/arcing noise is louder at lower frequencies.  This happens for two reasons -- the noise is stronger at lower frequencies and the long power lines that the noise gets coupled onto are pretty much broadband radiators.  I have heard reports of noise that is stronger on higher frequencies, though.  This seems to occur when the radiating conductors are smaller than long power lines.  If there were induced currents in the metal crossarms of a structure, for example, and a gap where sparking could take place, the resultant noise would broadly peak on the resontant frequency of those crossarms.

As is almost always the case, thouh, the question is not "what is it?," as much as it is "where is it?"  If we were to guess it was the very phenomenon I described above, that would not help you find the  pole it is originating from.

If youc can, rotate a beam at your QTH to get a direction, then head out in that direction.  You can either use a VHF or UHF beam when you get close, or use a receiver with an S meter and when you can hear the noise, put a short antenna the same distance from the ground wires present on different structures in the general area. It will be strong on the pole where the noise is being generated or on the ground for the step-down transformer feeding a building where the noise is being generated.  I have managed to isolate noise on 3.5 MHz by using a step attenuator to bring the noise just above the receiver noise and was able to ID it to a building on 3.5 MHz entirely by ear.  I did have correlation between the noise at the victim and at the source because I was able to hear the standing wave all along the line from the ham's QTH to the source.

And, a word of caution.  Don't contact the ground wires or guy wires on any structure.  Something is wrong on the pole and if that ground is defective in some way, there is a remote risk that it could be partially live, so be safe, not sorry.

Ed Hare, W1RFI
________________________________
From: RFI <rfi-bounces+w1rfi=arrl.org at contesting.com> on behalf of Pete Smith N4ZR <pete.n4zr at gmail.com>
Sent: Sunday, August 8, 2021 8:24 AM
To: RFI List <rfi at contesting.com>
Subject: [RFI] Line noise vs Frequency

A (probably) dumb question:  I have a severe line noise to my west.  For
some reason it is manageable on 20, much worse on 15 and crippling on
10.  These subjective descriptions are confirmed on my S-meter. Why
would this be happening, and does it offer any helpful clues to finding
the source?

--
73, Pete N4ZR
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