On Jul 27, 2021, at 6:34 PM, Jim, N5IB <n5ib@juno.com> wrote:

With the weekend's experiments showing a continuously variable inductance is reasonably possible, it's now time to see how to make the differential capacitor.
Photo attached of a first "proof of concept" effort.
Geared together, the two air variables turn in opposite directions, so when one is at max, the other is at min, and vice-versa.
The smaller gear that drives the pair has half the teeth, so a full turn of the "knob" results in 180 degrees at the caps.

Quite a bit of mechanical details to work out - not my strongest suit - but it looks like the thing can be done, and fit in a box certainly not larger than a Bayou Jumper.

BTW - I did locate the article that shows a variable inductor made with a moving iron powder core:
Discussed in Hayward's Solid State Design p166, referencing a Feb 1976 QST article by K1KLO

Jim, N5IB





<Capacitors and Gear Train.jpg>