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.
Radiation is usually limited by the shielding of the metal enclosure.
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.
The harmonic content of an arcing transformer is relatively low due to
shielding and very non-resonant "antennas."
I've had to change only a very few pad mount transformers. Defects vary but
they are just as capable as overhead transformers of producing RFI.
I'd say run with your plan. When the pad mount is set, ask the crew to drive
one or two extra ground rods to help mitigate lightning surges which I suspect
are the leading cause of transformer arcs. If possible, be there when the pad
mount is made hot. Wait 2 minutes and then use a battery operated portable
AM/FM/SW receiver to listen from 1.7 MHz to 30 MHz to make sure the new
transformer isn't a dud from the start.
Consider a whole house surge protector just past the service entrance and
ground it well. I like Joslyn units but they are pricey. They do nothing for
RFI but help minimize surges (a more frequent and damaging issue.)
Hopefully the utility will install fresh lightning arresters on the overhead
lines supplying your home. Fresh ones should keep them from being a problem for
a few years depending on lightning frequency.
Good luck!
73,
Frank N. Haas KB4T
Utility RFI Investigator
Sent from the field by an RFI (Really Fine Individual) Investigator
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