Hey guys,
Experience in listening is the best tool for identifying power line sources
verses other types including in-house arcing sources. A good scope helps as
well.
Here is some additional info that may help.
Arcing sources of all types, as previously mentioned by almost everyone, are
broad and that characteristic helps to identify itself. The fact is they
will radiate in the range from below 1 meg to above 2 gig. The frequency
range of an arcing noise source (noise) is determined by the frequency range
of the antenna being used and signal strength of the noise from the
receiving antennas position.
I really don't want to write a book but would like to clarify a few things.
If you're using an antenna tuned perfectly at 7 meg and the distance between
your antenna and the noise source is a distance of exact wavelengths for 7
meg, your signal for that noise will be peaked at 7 meg and will be at its
highest level at that frequency. If you move that 7 meg antenna at less than
1 wavelength increments the signal level will be lower than what it would be
if positioned at the peak of the wavelength. To make this simple, let's use
145 meg. We're parked on the street and receiving noise at 145 meg using a
1/4 wave antenna tuned for 145. We reduce our gain to a minimal signal level
and move the vehicle. We'll be able to see the signal level rise and fall
every 2 meters. If you find the vehicles physical position where you're
receiving the highest signal level, reduce your gain and then mark your tire
position on the street, then roll forward to the next peak and mark it you
can measure 2 meters between the marks. If you change the receiving
frequency the marks will change to the wave length of that frequency. This
is an example I use during my field classes. This same explanation is why
you'll receive a power line noise stronger on one antenna than received on a
different one of the same band or not. If you were to move the tower holding
the antenna you could increase or decrease the signal level of the noise. I
once had an RFI complainant in CA that I had explained this to. I received a
call from him at 11:00 pm explaining that he figured he had enough space in
his yard to move his tower out of the peak of the noise signal. I told him
he was a genius. However, What is the chance you'll be moving it to a
location that receives noise from other source(s)? Let's just fix the
original source.
While listening to your noise, for example at 7 meg, using AM mode, you
change your frequency and the sound of the noise changes with the different
frequency, it isn't power line noise. If the noise stops due to the small
change in frequency, it's not power line. You should receive your power line
noise throughout the receiving range of the antenna. You may see the noise
signal peak and fall with the antenna resonance but you shouldn't see it cut
of more rapidly than that. Using a spectrum analyzer (not a recommended
locating tool) you can see the antenna's bandwidth and strongest resonance
if receiving an arcing source. If it's a non arcing source you'll see the
narrow signal and possibly the harmonics. Power line noise doesn't create
harmonics.
Most inside arcing sources, when the audio is monitored using an
oscilloscope, appear more dense than power line sources and usually have a
timely and sharper start and stop signature.
It isn't likely you need to concern yourself with corona or flashover. These
terms are used allot and are almost never an amateur radio problem.
As for the hf spectrum, the power line noise will affect the range of your
antenna which is also affected by the strength of the signal.
A couple of good books on the subject are:
AC Power Interference handbook by Marv Loftness or anything written by him.
The ARRL RFI Handbook. Both available through the ARRL
I'm sorry for the lengthy input, you should have seen it before I read it
the second time.
Mike Martin
RFI Services
6469 Old Solomons Island Rd
Tracys Landing, MD 20779
rfiservices.com
240-508-3760
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