Tom,
My response to Ford's post was purposely short. I did not want to write a
treatise on the subject.
To answer your question: NO. I was stating that, in my area of the country
with the types of distribution construction and the distribution voltages
(7.2/12.47 kV and 14.4/24.9 kV) that I have worked with for 34+ years, the
primary voltage lines themselves and the equipment connected to them are the
source of a small percentage of the RF noise problems.
Again, in my dry climate and with the type of construction I engineer,
construct, operate and maintain, the majority of the noise sources are
related to ungrounded hardware in the near field of the energized conductor.
The expanding and collapsing electromagnetic field of the AC line creates an
induced voltage in that piece of ungrounded hardware. If the piece of
ungrounded hardware is within arcing distance (determined by the amount of
induced voltage and the dielectric strength of the air gap) of a grounded
piece of equipment, that is all that is needed.
A common example of this is the staples holding the pole ground wire near
the top of the pole (in the near e/m field). As a wood pole ages, it
shrinks and wears away minutely. When it has shrunk enough to allow the
ground wire to be loose inside the staple, a small air gap is created. If
the gap is small enough for the induced voltage in the staple to arc to the
ground wire, BINGO.
Older two phase or three phase line construction in many instances used
metal crossarm braces. The brace is a comparably large piece of metal in
the near e/m field. The induced voltages can be quite high, therefore
capable of arcing across a larger gap (1/8-3/16"). Pole ground wires,
lightning arrester ground wires, equipment cases (transformers, capacitors,
etc.), and guy wires are common pole top hardware that is grounded and may
through poor installation practice, wear and tear, or replacement work come
in close proximity to the metal braces.
There a numerous and various different scenarios that can create the air gap
between ungrounded and grounded hardware on the pole top. That is why
experience and expecially a knowledge of pole top construction can help
immensely in spotting the culprit once the source is narrowed to a single
pole. Tried and true methods of using an AM receiver with an S-meter are
still the common man's best way to locate an RF interference source.
The occurences of equipment failure where the noise source is internal to a
transformer, regulator, lightning arrester, capacitor or insulator are few
in comparison to the hardware scenario I have described above.
I have already received one private E-mail today taking me to task relative
to some of my statements. I stand by them with the qualifiers I expressed
in my original post. There are certainly an order of magnitude of other
potential noise sources in other types of construction and certainly in
other climates. I do not have to deal with salt air, industrial
smoke/fumes, extreme wetness, ice storms, tornados, huricanes, etc. Each of
those climes in conjunction with the engineering standards and construction
techniques prevalent in those areas will dictate a different type and ratio
of noise sources.
The thrust of my short post was to express my professional opinion regarding
Ford's post relative to the possibility of measuring current in a wooden
pole to determine which one was the source of the RF interference. I
apologize to the reflectees if I mislead anyone with my brief explanation of
why that was NOT a viable location method.
Back in my hole. 73 de Milt, N5IA
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