Greg, one only needs to drive down the street while listening to a low
frequency power line noise to find the answer to your question on bandwidth.
The power lines are the unmatched antennas for radiating a noise source.
Being unmatched they have large standing waves.
At any given point along a power line you may be in a null or a peak in these
standing waves depending on the frequency you are listening to. One can easily
hear the peaks and nulls along an offending line.
An arcing source is indeed a broadband source but its radiated bandwidth can
also affected by whatever antenna like conducting objects it can inject RF
power into. If the energy was injected into a broadband antenna the noise would
be radiated strongest across the efficient wide bandwidth of the antenna. When
the noise is injected into an essentially unmatched power line, the noise will
likely be strongest at resonant frequencies on the line. Being unterminated
there are likely many distinct resonant frequencies.
All that being said, it is also possible for noise generators like
switching supplies to be quite broadband in nature. Most often they have many
harmonics and are modulated by sharp line frequency splikes. This produces
centers of noise with very wide sidebands at every 60 Hz. Often these sound
very much like an arcing power line. Other noise sources like digital circuits
are often heard at their clock frequencies and their harmonics. These are also
often modulated at low frequencies like AC line or the frame rate of your
computer monitor producing a noise that also sounds like a power line arc.
The only things that I have seen and I have spent quite a few hours in the
power companies interference finding truck for identifying different noises is
lots of Experience and a small oscilloscope to look at the actual audio
waveform of the noise. Sometimes you can identify different kinds of sources by
their waveform signature.
My current power line guy must be one of the best. He has over 30 years
experience in radio. The cab of his truck is full of receivers from DC to 1
GHz. He has 5 different outside snap on antennas and some he can rotate from
the cab. He carries several ultrasonic units and several hand held vhf-uhf
receivers/antennas. He also has a small oscilloscope for monitoring audio
waveforms. Some of his receivers have internal spectrum display as well.
The bottom line of all this from my perspective is that each offender has
to be characterized and compared to known signatures. There is no easy way. You
have to turn over all the stones in this game and keep notes. Even with all
this equipment we often fail and have to wait until another time. My
perserverence and his experience characterizing the interference has produced
great results however. One arc took us 2 years to find and many easy ones take
less than a day. The last offender we found used nearly all the equipment in
his truck and 2 days. Turned out to be an arc in a junction cabinet for
underground lines.
Good luck Lee K7TJR Oregon
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