[RFI] Electrical transfomer near shack, a problem?

[Frank N. Haas KB4T]

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