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Re: Secondary coil forms
> Original Poster: Bill Fuller <wfuller-at-home-dot-com>
> Does it make sense to place plastic spacers equally around the coil form
> to lift the wire off the PVC to reduce the losses? This would seem to
> have the added benefit of reducing the parasitic capacitance caused by
> the PVC dielectric and adjacent coil wires. I would think that a "pure"
> inductor (the secondary) and a "pure" capacitor (the top load cap) would
> give better performance than having some of the capacitance down in the
> coil.
The cylinder+radial spacer arrangement is a good one,
although more for mechanical and fiscal reasons. The
parasitic capacitance and losses due to the plastic
coilform are negligible (consider that the turn-to-turn
C's all add in series). The sheet C to ground dominates,
and has the loss tangent of air, effectively. The bulk
of the E-field volume by far is on the outside of the coil.
I prefer the cylinder+radial spacer arrangement because
it allows the use of a smaller central pipe ($ goes as R^2)
and the radials can be easily notched to locate the wire.
The finished coilform is considerably lighter as well.
--
-GL
www.lod-dot-org