I was surprised by one of the things we found here this past summer.
One of my stronger noise sources was being heard on all bands 1.8 to
50 MHz. It may have extended well beyond that but I don't have
higher bands at my station. It was about -85 dBm at 50 MHz with 7
element yagi (10 dBd). I tracked it to a particular pole three
quarters of a mile away. Conduction was definitely involved. I
initially had some trials pinpointing the source but UHF eventually
left no doubt. The power company thought it was a lighting arrestor.
Fortunately the day they worked on this one the noise investigator
came out with the line crew. Replacing the lightning arrestor and
tightening hardware did nothing, so he directed them to go get a new
pole transformer. Problem solved. If I had to make a wild,
uneducated guess, I'd guess sparking on the high voltage lead inside
the transformer. Conducted noise was on the 13.2 kV lines.
I was surprised at this level of RFI from a source *apparently*
inside a pole transformer. I was even more surprised they replaced
it without more "prompting" by me. I like it when the investigator
comes out with the line crew. It isn't standard practice, but seems
like it should be. Far more was accomplished that day than all the
other days combined.
Paul
On 01/10/2013 10:47 AM, Frank N. Haas KB4T wrote:
RFI from an overhead transformer propagates by both radiation and
conduction. In my experience conduction is the bigger problem and
is often difficult to pinpoint.
In my exprience, transformer RFI is pretty rare. Ham antennas
fairly close to a transformer-based source can easily pickup such
RFI due to being close to the radiating field produced within the
enclosure or RFI conducted by the attached wires and radiating
from those wires. Yet the entire field of influence extends only
a few hundred feet and is often heard only up to 8 to 10 MHz
(usually because the RFI signal is weak compared to an arcing
lightning arrester or bolt/nut combo out in the clear.
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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