wrote in message
By placing the coax inside of a copper tubepipe you have in effect
created a triax
cable. Triax can (as in might or maybe) solve certain hard to control
RFI engress issues.

I think the ARRL Handbook suggests running coax into the building through a
3-4" diameter tube filled with steel wool as another way of suppressing
coupling along the outside of the shield from inside to outside and thence
to the antenna.

Coiling the co-ax with several turns at the building entrance also helps and
is the common technique at the base of the G5RV balanced feedline to choke
off coupling to the outside of the shield.

Tom