Off-Axis Primary Inductance

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Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>" <youngs-at-konnections-dot-net>

Hi Coilers,

For my next TC, I am thinking of having a fixed number of turns in a flat
spiral primary, and some additional off-axis variable inductance in series
with the primary.  The variable inductance will be used to fine tune the
primary resonance as the coil is operating.  I am thinking the off-axis
inductor will be two smaller spirals parallel to each other, with the
spacing between them variable to change the mutual inductance.

Can any of you help with the following question?

Let's say the primary is of inductance L.  Let's say I start decreasing the
primary inductance a few percent at a time, and compensate for it by adding
off-axis inductance to maintain the same L.  About what percentage of the
total inductance can be off-axis before coil performance noticeably
degrades?  Anyone have some practical experience with this?

I seem to remember a few years ago someone described an experiment of
removing one secondary from a large twin coil, so the remaining coil had
half it's primary inductance off-axis.  I believe he said the performance
was still very good.

Thanks,
--Steve Young