Hey everybody and Happy Holidays,
This is at the end of my awake hours but I'm going to attempt to do this and
make sense.
The power line systems constructed with wooden poles are built when the wood
is damp from treatment and because of that they are swelled. As time goes by
they dry out and the hardware loosens due to the wood shrinkage, similar to
the wood on a new deck. The hardware being loose may no longer have a good
electrical connection to the adjoining piece, which by the way isn't usually
a consideration at the time of construction. This hardware consists of
things like thru bolts in the pole to staples on the pole grounds to
bracket. Now, we can't forget about those energized lines, they also are a
factor. They radiate this thing called EMF. This EMF field induces a voltage
into the conductive material used for hardware and energizes it to a
typically very low voltage. This in itself isn't a problem until the
different pieces of hardware become separated by air, corrosion and other
types of insulators and the voltage starts to change. One piece of hardware
is energized at a slightly higher voltage than the adjoining piece and now
you have two energized objects very close to each other. By now you should
be getting a visual impression of something like a spark plug.
As the gap between the two breaks down from the stress of the difference in
potential voltage a discharge occurs and you now have an arc. This by itself
also wouldn't be so bad but the voltage, after the discharge, is induced
into the hardware again and you have the spark all over again. Guess what??
That's correct, every 120Hz it repeats itself until some kind sole climbs up
and shorts the gap.
The repetitive spark generates RF and this RF is generated as a bundle of
frequencies from below 1meg to upwards of 2gig at the source. That's right
all of them. As you travel away from the source the higher freqs are
naturally attenuated by the distance between the source and the receiving
antenna. The RF that is generated, by itself, wouldn't be much of a problem
if it were for having such an admirable antenna. The RF radiated away from
the source and gets induced back into the hardware, primary, neutral
conductors as well as Telco, and CableTV conductors. Oh lets not leave out
the street light circuits, guy wires, and last but certainly not least the
pole grounds.
Everything conductive on or near the source will become a radiator for the
arc.
Weather: Wet weather or even the slightest amount of humidity can short the
gap or even swell up the wood to close the gap and stop noise. Damp or rainy
weather can on rare occasions start a noise. This is when the moisture
closes a larger gap making it small enough for a spark to occur. These
things can happen at all voltage levels.
Fowl weather RFI:
This is the reference used when extra high voltage lines react to fowl
weather. Although years ago it was a more common occurrence, two types of
noise are generated by this and both are a rarely the cause of a complaint
to utilities. As the moisture forms on these lines it causes the surface to
become irregular and loose its generally smooth surface. At the places where
the drops of water are formed a plume will emit from that spot and continue
as long as the surface is protruding at a point along the line. Visualize
what would happen if you had thousands of these droplets along the line! The
corona radiating would be all over the place. The good thing about this is
the corona doesn't travel like the RF sources caused by gaps. Corona RF is
limited to usually less than 100 feet or so from the source. This is the
same type of scenario you might witness when walking under a 230kv or higher
transmission line and hear the audible characteristic of a corona discharge
from a single point source such as a rough surface or protruding piece of
hardware.
Now it gets a little worse. In the last scenario we had moisture on the
lines and a little noise problem, well in the rain we get an additional
problem. As the rain approaches the EHV Lines a gap is created just before
the water contacts the lines. This causes a gap, then a discharge and the
RF. Now you have the moisture condition on the line and corona. We aren't
done yet we still have the lower half of the line to examine. Now the water
rolls around the line and forms a drip on the line which creates another
corona source. So what do you think happens now? Most of you got it right.
As the water drips and falls away from the line you have another gap and
another RF source. Now, if you only have one drop of water the process ends.
If not these very low level RFI sources generate a noticeable noise level to
mostly hams that live within a few hundred feet of the lines.
I do about 500 RFI complaints a year and this scenario has only been an
issue once in over 5 years.
I hope this helps guys; it wasn't easy writing all this with Rudolph the Red
Nose Reindeer in the background.
Happy Holidays,
Michael C Martin
RFI Services
6469 Old Solomons Island RD
Tracys Landing, MD 20779
240-508-3760
mike@rfiservices.com
www.rfiservices.com
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