In article ple.org,
Michael Black wrote:

Sorting through the junk box (50 years + and growing!) I am amazed at
the number of ex-valve-consumer-radio 2-gang tuning condensers (not
capacitors in them thar days!) that I have in the junk box, which was the
germ of an idea for preselection without coil switching - ie, one twin gang
condenser giving two tuned circuits for each band in the HF range.

It was covered in "73", at least, two articles, the same author, in "73".
One was definitely in 1964, but I can't remember which. The first one
used one of those large air variables, the other used one of those mylar
insulated cariables as seen in small transistor radios.

A websearch says the small one was in "73" in December 1964, W6SFM was the
author of that and the earlier article. Both definitely have an
explanation of how it works, two coils with quite different inductance,
one coming into use while the other is mostly insignificant as the
variable capacitor changes value, and vice versa.

That sounds a bit like the "single coil" Z-match antenna tuner
design. It uses a split-stator capacitor, one part in series with the
entire coil primary, and the other part in series with a tap at the
halfway point (halfway in windings, not in inductance).

http://users.tpg.com.au/users/ldbutl...CoilZMatch.htm