I've been following this discussion with interest. I spent the
summer tracking down more than 20 sources of power line noise. As an
offshoot of that I've made it my mission to help clean up RFI in my
little corner of the world. I've been slowly drifting away from
DXing and this has turned out to be my new area of interest.
Since this topic is perhaps of interest to topbanders, and since my
experiences seem to vary somewhat from the typical reported here so
far, I thought I would take a moment to share some observations. I
should probably note I was dealing with a mix of 13.2 kV
distribution lines and 46 kV transmission lines. My hunt was made
more challenging by the fact I had multiple sources in a relatively
small area and could often hear more than one at a time even with
directional antennas and attenuation.
My tools this summer were 135 MHz AM receiver with 3 element yagi
and step attenuator; LF/MF/HF/VHF/UHF AM receiver with DF loops for
low bands and 7 element yagi for 445 MHz; ultrasonic receiver with dish.
Low frequencies, eg. AM BC or 160 meters were *occasionally* useful
in locating a general source area. Sometimes the area identified
turned out to be an area of *radiation* but the noise was
*generated* elsewhere. Often the relatively close proximity of
multiple sources made low frequency tracking useless.
VHF was always useful in finding a source area, 80% of the time
resolving it to a single pole. The sharp, deep null at exactly 90
degrees off axis of the yagi proved very useful for confirming a
source structure. Poor resolution/accuracy of signal strength
metering was perceived as a problem.
UHF was very helpful in a few areas where the noise was particularly
strong at VHF and/or signal strength so close over a span of several
poles that VHF could not pick the source pole with high confidence.
So far, experience indicates this is more likely to happen on the
transmission lines. They're a bear. Again, poor or no signal
strength metering (signal below AGC threshold) was perceived as a
problem.
The first ultrasonic unit tried was a waste, finding something at
only 10% of RF noisy poles. The second unit was able to hear
something from about 60% of the same 21 poles. The figures are
averages over more than 10 runs with each unit. The two were also
tested on a spark signal range under somewhat controlled conditions.
These things are definitely not created equal!
To date I have identified and had the power company fix almost
everything I have worked on. The remaining open case involves a
short section of a 46 kV transmission line which is extremely
perplexing due to the specific nature of the issue (details on
request). Just when I starting thinking I was getting good at this,
I came up against this one.
If I were doing this strictly for myself these tools would be more
than adequate. Since I'm not and I only have so many hours in a day
I have several upgrades on my wish list:
HF/VHF/UHF AM receiver with wider bandwidth better signal strength
metering
Log periodic dipole array covering ~100 to ~900 MHz for frequency
agility while maintaining some directional properties
Portable oscilloscope for observing noise signatures in the field
(I'm hoping it helps sort out overlapping source radiations)
Yet another upgrade of the ultrasonic unit.
Comments are welcome, even if it's to tell me I'm clueless! :)
73
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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Stew Perry Topband Distance Challenge coming on December 29th.
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