On 1/27/2020 12:38 PM, N4ZR wrote:
But at least as I understand it, that's just as true for arcing
powerline equipment as for any connected RFI generator.
Yes, wiring nearest any kind of source does most of the radiating, for
several reasons. First, RF noise is generally attenuated by transformers
in the distribution path. Second, the highest frequency components of
noise are most attenuated as the noise current flows along the wiring as
a common mode or differential mode signal. This is true of both noise
generated by arcing (power line noise) and by electronic circuitry.
Impulse noise has harmonics well into the UHF spectrum, so the above
mechanisms are why, when we get close to the source, we listen at the
highest possible frequency FOR IMPULSE NOISE. But electronic noise
sources are NOT impulse noise, their energy is distributed across the
spectrum in more or less unpredictable ways that depend on the nature of
the waveforms of the noise and the circuitry that generates AND the
circuitry that couples them to whatever wiring radiates it.
Electronic noise is mostly radiated as a common mode current on ANY
wiring connected to the source, both on the power line side and low
voltage side (CATV wiring, telephone wiring, control wiring, audio and
video wiring, wiring to computers, etc.) This is why well-designed
common mode chokes placed on source wiring work to suppress it.
From here to Pete's problem -- that fact that he's not hearing the
noise on a VHF receiver COULD mean that the receiver and/or antenna
aren't good enough, or that he's not close enough to the source, or that
it's not impulse noise.
Using direction antennas like dedicated loops or the loopsticks built
into haldheld radios like the Kenwood TH-F6A, Tecsun receivers, and even
AM broadcast radios are a great way to start looking for the source AT A
DISTANCE, when you have no idea where the interference is coming from,
but 1) you've got to look for it on frequencies where you hear it and 2)
you have to be able separate out or identify WHICH of many noise sources
may be around you.
All of the above is why I continue to pound on the advice to first
identify what TYPE of noise you're chasing by studying it's spectrum and
waterfall. If it's impulse noise, it will show as uniformly spaced
HORIZONTAL lines in a fast waterfall, corresponding to the arcing on
60/120 Hz peaks that produces it. If it's electronic noise, it will show
up as either consistently spaced carriers that don't drift
(microprocessor clocks, etc) or humps of noise that drift in frequency,
also at consistently spacing. AND -- when you're hearing multiple
electronic sources, the spacing between carriers and humps of noise will
appear to be irregular, because the rectangular waves producing them
almost certainly running at different fundamental frequencies.
73, Jim K9YC
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