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@contesting.com> on behalf of Pete Smith
N4ZR <pete.n4zr@gmail.com>
Sent: Sunday, August 8, 2021 8:24 AM
To: RFI List <rfi@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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