I have been told that the current loading coil at Rugby now consists of
an assembly made up of 50mm diameter copper pipe with a slider to permit
rapid tuning changes. The coil is about 3m high and a quick calculation
means that for 2.5mH (the antennas probably haven't changed much since
those days) there will be something like 40 turns and be 3.7m length -
which sounds about right based on hearsay. I'm trying to negotiate a
trip there with the group who remotely monitor that transmission as a
health check, but need a valid enough sounding reason to get past the
gatehouse. It will have to be on a Tuesday of course, wouldn't like to
be near that shed when the Tx is running !
Studying the layout of the masts on a map (a 2km long site with the wire
in a figure of 8 pattern, plus multiple vertical feeds, say 8km in
total) then do a quick calculation of wire capacitance based on 5pF/m,
the capacitance comes out in the region of 40nF - which just happens to
be the value to resonate with 2.5mH at 16kHz. Nice to see the rule of
thumb calculations working out for big systems like this :-)
Andy G4JNT
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