John,
Power line arc noise can happen anywhere, even in a semi-rural area like
your neighborhood (I used to live in Acton MA). There are many web
resources for learning how to identify and track down power line arc
sources. Here are some things I've learned by hard experience.
A good, though not infallible, indication of a power line arc source for
your noise is to check the bands during or shortly after a rain storm. Most
arc sources are quenched temporarily by rain if they're exposed to the
elements. When I first began tracking down noise sources at my QTH several
years ago, I found that keeping a log of baseline noise levels (using the
S-meter indication with the K3 in AM mode, widest filter you have) on all
bands versus time of day and weather conditions was helpful in establishing
any correlation with precipitation or neighbors' activities. I would also
add comments about the qualitative audio signature of the noise ("buzz",
"raspy", "intermittent buzz" - you get the idea).
Simple preliminary source location techniques (once you've eliminated
anything in your own home or yard) include using the AM radio in your car on
a careful drive around your neighborhood to find candidate sources. This is
not particularly selective - you'll hear other interferers in addition to
true arc sources, and you'll want to tune to an unoccupied AM channel - I
use channels high in the AM broadcast band.
My preferred method once I've identified some candidate arc source sites is
to use an inexpensive handheld VHF/UHF scanner with a rubber-duck antenna
and a switchable attenuator. You don't need a fancy lab-grade attenuator -
I find that a home-made 6 dB or 20 dB fixed resistor attenuator (50 ohm,
shielded box) is sufficient. I tune the scanner to an open channel in the
aircraft band (around 135 MHz; it automatically switches to AM detection in
that band, which is necessary to do direction-finding based on signal
strength - FM detection won't work), and walk around the suspect site. Turn
off the squelch completely and set the audio gain at a fixed, comfortable
listening level (I use open-ear headphones for safety as I walk on roads).
I hold the rubber-duck antenna in the horizontal orientation and sweep it in
an arc to either side of my body. The rubber-duck has a pattern null off
the ends, and the reduction in audible noise is very evident if you're close
to the arc source. You can resolve the 180 degree direction ambiguity of
the null by using your body as an additional shielding attenuator - the
difference in audio levels is your clue as to which null direction is the
true one. You will find that you'll need to insert one of the fixed
attenuators between the rubber-duck and the scanner antenna port if you're
very close to the arc source - otherwise it will be difficult to perceive a
null. I've used this technique successfully to locate multiple arc sources
in my own neighborhood over the past few years. The one drawback is that one
cannot hear the arc noise at VHF if one is more than a couple hundred yards
from the arc source unless you use a small handheld Yagi antenna. It's easy
to make a 3 element Yagi for VHF - there are several simple, inexpensive
designs out there to be found.
As Jim, K9YC will likely point out, you may also be hearing switch-mode
power converters that have a much different noise signature than arc noise.
If you can borrow a P3 panadapter from a nearby friend, it is an excellent
tool for distinguishing types of noise. There are many other RF noise
emitters - look at the archives of the Elecraft forum for some examples -
the list of offending devices is too long to include here.
One can, of course, purchase commercial DF'ing units that offer more
features. However, you may already have the basic DF'ing tools in hand, or
you may borrow them from a local ham until you find that you have a
recurring need for this set of tools.
It's likely that your local electric utility has a person trained in finding
arc noise sources, but they love it when you can identify the specific pole
on which you have found the likely source!
73,
Mike, K8CN
Durham NH
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