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Steve White wrote:
> I currently have the bottom of my 9" x 30" toroid
> mounted 2.3" above the top of the secondary winding.
Yes the toroid is a great big shorted turn, but that doesn't
necessarily do any harm to performance.
The magnetic coupling to the shorted turn of the toroid will
raise the resonant frequency a little, perhaps half a percent
or so, and will introduce some loss due to dissipation in
the toroid resistance. Generally the 'power match' to that
loss resistance is poor and the effect on secondary Q will be
tiny - small enough to be hard to measure. I expect that any
loss incurred will be more than compensated by the ability to
run at higher power without risk of corona from the top turns.
The secondary current near the top of the coil will in any
case be relatively small and most of the magnetic coupling
to the toroid comes from the top end of the secondary.
A shorted turn near the base of the secondary (eg a closed
loop strike ring) might be more of a problem, higher secondary
current there, plus the primary field too, and perhaps a
more significant power match to the loss resistance.
The ground plane beneath the coil may also be a big shorted
turn (unless constructed of radials). Same effect here, a
small pull on the resonant frequency but probably not much
loss from the circulating ground plane currents.
I suppose if the toroid is not getting hot after a run,
you don't have significant loss from the induced current.
--
Paul Nicholson
--
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