I solved a similar problem here about a year ago. The "propagation" was along
the ground wire and showed up at any pole that had a bare wire to ground. It
was most pronounced on three poles at an intersection and I checked each
numerous times with my handy "transistor beam" (little AM transistor radio).
One time (and one time only) one of the poles was a little louder than the
others. This was where I asked the crew from the utility (PG&E) to concentrate
their efforts. What they found was that one of the staples securing the solid,
bare copper ground wire to the pole had been hammered in a little too tight.
It cracked the wire UNDER the crown (not visible) and as long as there was a
little moisture present either the wood swelled enough to push the cracked ends
together or the damp wood was conductive enough to complete the circuit. When
it dried out it became a "dandy" spark gap transmitter.
Hope this helps you (or someone else). I'd like to hear any other suggestions.
I'm compiling a compendium of RFI problem/solutions on our local ham bbs but
so far I'm the only contributor. This is specifically where we hams are being
interfered with, not where we are doing the interfering (TVI).
73 - JC,firstname.lastname@example.org
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