Guys, I wrote this and a friend posted it on his website at
www.qsl.net/n4tn/noise.html but that site is sloooooooooooow these days. It
explains specifics of unique problems and solutions and is intended to show
how an open mind and some thought can help find sources of noise. I call
your attention particularly to Section 1, about "bell-shaped" insulators.
Please tell me of any similar experiences you've had.
Regards,
Mike -N4NT-
__________________________________________________
POWER LINE (AND OTHER) INTERFERENCE PROBLEMS EXPERIENCED BY N4NT
1. The first power line interference problem I ever saw cured was found by
a ham named Clyde Goforth WA4NTZ who worked for the Knoxville Utilities
Board. It was his job to locate and remedy these problems.
I lived in an apartment complex and was experiencing strong interference on
80 and 40 meters which usually began in the morning and lasted until
nightfall and often later. Rain cured the problem temporarily.
Clyde came out, looked around, and told me he saw the problem right off. He
termed it "slack-span interference." I thought he was joking as he pointed
out the "drop" from the main line to the pole near the apartment building.
The line was not pulled taut, but drooped between the insulators at its
ends. Each end of the line was supported by a bell-shaped insulator. The
line connected at its ends to the open ends of the bells. The other end of
each bell had a loop which was connected through an eye-bolt which passed
through the pole.
Clyde explained that where there is enough tension on this mechanical
connection between the bell's loop and the eyebolt, their metal surfaces
keep each other clean and make good electrical contact. Where there is not
enough tension on the line, the corrosion builds up and causes very small
arcs because of voltages induced or coupled into the hardware. The radio
frequency energy created by these arcs then passes through the bell just as
if it were a capacitor dielectric, and the whole line radiates. In my case,
removing the slack from the line by pulling it taut cured the problem, just
as Clyde predicted it would.
Since that time I have seen over five similar instances of this type
interference and all were cured the same way. This problem is most common on
residential drops because pulling the wires taut enough to keep the metal
parts clean puts a force on the pole which may cause it to lean if that
force is not opposed by a guy wire. By leaving slack in the line, the
company can avoid putting a guy wire in someone's lawn, and they often do.
Often you can spot these easily by just looking, but it is far better to
localize the particular pole or drop which is the source of the
interference. We have had the best success using a VHF single-sideband
receiver with an S-meter. On HF, the interference travels along the lines
and can seem just as strong quite some distance from the source as right
under it. There are also "nulls" between strong signal readings as one
travels under the lines. This can be deceptive. (Some hams have gone quite
bald scratching their heads in utter confusion trying to locate a source
using an AM broadcast radio.) On VHF, on the other hand, the signal is
rapidly attenuated as the distance from the source increases. UHF might be
even better, but we never tried that. An aircraft band receiver with an
S-meter might work but we never tried that, either.
Localizing the interference has kept us from sending the power company
employees on "wild goose chases," and thus kept our relationship with them
good.
_________________
2. When I first moved to Johnson City, I had very high levels of power line
noise. The power company traced it to a lightning arrestor strip atop a pole
transformer. The strip had come loose or had bent and was making contact
with whatever it was supposed to arc to. They were glad to find that one,
said it could have been dangerous.
_________________
3. About 300 yards from my home there was a 33kV transmission line
supported by wooden poles. The interference it radiated was intermittent,
but was generally S-9 or higher when present. As time went on, it became
almost continuous around the clock. A neighboring Ham (Rick VanLandingham
KF4FL) used his 2 meter SSB receiver to help me localize it to one pole. Our
power company had previously tried to find the source, wasting many man
hours. After Rick localized it to one pole (which I should have done in the
first place), we had the power company try again. They could not find the
problem. Late one cold, clear night when the interference was bad I walked
out to look at the pole and saw, up near the top, a small spark that looked
like a distant star. As I moved around, the spark would disappear and then
reappear as I returned to my original position. I saw a soda can on the
ground and used it to mark my vantage point. Then I got Rick, my camera, a
tripod, and a flashlight and we went back to the pole. I set up the camera
and opened its shutter. While the shutter was open, I painted the pole with
light from the flashlight so it would show up in the photograph. Because the
shutter was open for over a minute, the spark showed up clearly in the
photo. The pole and its hardware were clearly visible, too. The problem was
caused by a diagonal steel support, which supported the wooden cross member
of the pole, coming in contact with the ground wire which ran down the pole.
Apparently enough voltage was being induced or coupled into that support to
create the arc. The ground wire connected with the top cable of the three
wire system and served as the antenna for the interference source. Armed
with the photo, the power company made quick work of remedying the problem.
________________
4. One section of Johnson City had very strong interference over an area
covering several square miles. The lower the frequency, the further out the
interference extended. Again using Rick's 2- meter SSB receiver's S-meter,
we localized the source as a main power station which served much of Johnson
City.
We went back at night and this time were carrying camera, etc. We observed
many little sparks like distant stars above transformer housings, but it was
too dark to see the problem. We set up the camera some distance from the
station with a telephoto lens (to give good perspective) and opened the
shutter. I walked over to the station's perimeter fence and set off a camera
flash to illuminate the station's hardware in our photo.
The power company officials were amazed with the results. The photos clearly
showed spring clips mounted atop long insulators holding a bus bar. At each
point where the bar passed through the clips, an arc showed in the
photograph. They called TVA for advice and someone applied a conducting
grease to those mechanical joints and cured the problem. I believe they
re-grease those connections on a regular basis now.
_______________
5. One spring and summer there began to be an interference which made a
clicking sound, stronger on lower frequencies, over S-9 on 80 meters. My
dim recollection is that those clicks came about every two or three seconds.
The source was a neighbor's electric fence in "weed burning" mode. Walking
the perimeter of the field, we found many weeds/vines/saplings, etc. grown
up and touching the fence wire. Fixing that one was easy -- we told the
owner and he bushhogged the whole thing. We didn't have to lift a finger.
The owner was happy because the fence might not have kept his animals
confined when grounded out like that.
_______________
6. As I remember, this sixth instance of interference was strongest on 15
meters. It was not uniform in strength as I tuned across the band. I could
tune to the raggedy edge of it near the low end of the band and below that I
did not notice it. The interference was strong, S-9 or stronger, and changed
sound as I tuned through it, unlike typical power line type interference. My
noise blanker was completely ineffective against it. My recollection is that
as I tuned about seven to ten kHz the pattern of sound would begin to
repeat. It sounded much like power line interference, but had a slight
difference. The low end of the interference would drift up in frequency. The
band of interference was at least 250 kHz wide.
Using my beam antenna, I lined up an azimuth for the interference which ran
across two houses in the neighborhood. Beyond them in that line was farm
land. A friend, Al LeFevere KM4TR, came with a VLF receiver which was
directional because of its loopstick antenna. Walking on the street between
the two houses showed clearly which one contained the source. We went to the
door, explained to the man our mission and asked for his assistance. While
we listened to the VLF receiver, he turned off his circuit breakers, one by
one. At last, one of them stopped the noise. He took us to the room which
was served by that breaker, but we could not find the source of the noise.
We unplugged a power cube, unhooked his cordless telephone, etc. The last
device was an innocuous looking small lamp. He touched it, the lamp came on
and the receiver went wild.
It was a "touch lamp." Merely touching any metal part of the lamp would turn
it from off to dim to bright. We told him that was surely the culprit. He
said, "You know, that thing has a 'short' in it. It comes on and goes off by
itself sometimes." I didn't tell him, but suspect a kilowatt of RF from next
door beamed at his "touch lamp" was the 'short.' We volunteered to fix the
lamp by removing the touch circuit and installing a switch on the lamp's
base. He loved the idea and turned the lamp over to us. We did a nice job
with good hardware and returned it to him. He thanked us profusely, and we
did likewise to him. A win-win situation, I'd say.
__________________
xxxx
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