Re: variable coils


From: Michael Silva (
Date: Sun Jun 26 1994 - 14:57:09 EDT

I've been having trouble with my system and I don't know if this made
it to the list. I think it's intriguing enough that I'm sending it
again to be sure. Please excuse the repetition.

>Well now, what would happen if you mounted a space (air) would
>coil so that you could compress or stretch the turns?????
Another old technique (an awful lot of this stuff was known by the '20s)
is to use two coils in series (or parallel), one fixed and one
rotatable. The rotatable one can either be inside or next to and in
line with the fixed one, and as it's rotated the coupling goes from
maximum (coils aligned) to minimum (coils at 90 degrees), corresponding
to maximum and minimum inductance. Mechanically this is much more
stable and repeatable than turn scrunching. I don't know how the
inductance will vary as the coil is rotated, but I suspect it starts
dropping gradually and falls off quickly at the end. For our purposes,
we might want to limit the rotation to a few degrees or tens of
degrees. The advantage of this method over a tuning slug is that the
required movement is a rotation (easy) rather than a linear

BTW, if you rotated the coil a full 180 degrees the coils would fully
oppose, giving an even lower inductance. Same principle as a
non-inductive wirewound resistor.

Mike, KK6GM

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