> Yes you are right Tom. It gets taped from the antenna side
> as a normal tank
> coil would. No progressive shorting. So on 20 meters most
> of the coil is
> shorted. It seems to work very well on 160 and 80 meters
> however. 40 meters
> things start to fall off.
> Why do you suppose it still works well on 80 meters where
> nearly half the
> coil is shorted?
It doesn't surprise me it falls apart on the upper end with
a tapped toroid. The closed core is almost covered with a
big shorted winding on the upper end. Unloaded Q has to be
terrible.
I've seen many people get away with that on 160 and 80
because the turns ratio from used area to short area isn't
so large. Flux leakage is often higher also on lower
frequencies.
We get away with sorting turns in any coil or coil system
because of flux leakage. If there isn't considerable flux
leakage then the short would cause high circulating currents
that lower Q and heat the coil.
Think about the irony of this. People get all AR about
spacing a coil away from surrounding metal to maintain coil
Q, then they install it in a circuit that shorts the turns!
Who came up with that brainstorm? Shorting turns works, just
like a metal wall in the area of the air wound coil doesn't
destroy performance like people often predict, because the
air wound coil has a great deal of flux leakage. Without a
magnetic core the flux can easily move away from anything
generating a counter MMF.
Change that system in some way so loaded (operating) Q is
really high or so flux leakage across the winding is very
low and things fall apart. For example if you take a
conventional 160-40 meter tank inductor that works fine with
a progressively shorting switch and short a single turn in
the MIDDLE of the active area with very short connections
and the turn will often overheat and discolor. Short a
single turn at the end the same way and it is much less of a
problem. Short it with long leads and it is no problem at
all.
Insert a core and things change for the worse. Close the
core ends to form a closed magnetic path and things really
get bad.With that in mind why would anyone who thought the
problem through tap a toroid in a high Q resonant circuit ?
It would especially be bad on the upper end where
circulating currents in the tank are higher and flux leakage
lower.
Now of course shorting turns on a closed core isn't always a
bad idea. It works OK in a Weller Soldering gun, but a
soldering gun would be a poor tank inductor.
73 Tom
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