On May 9, 2:19 pm, AF6AY wrote:
In the few cases of linear-frequency-tuning, notably in Collins
PTO units and the "rack" assemblies in the R-390 family, the
windings were deliberately spaced to handle the powdered-
iron core position effect on inductance. I suspect that Collins
did a lot of cut-and-try to achieve the correct spacing changes
on those; very little quantitative information on it is available
in text or on the web. :-)

Note that the RF section slugs do not follow a linear-with-frequency
position; the while the shafts move linearly with frequency, the cams
introduce a necessary nonlinearity in the slug's linear position that
is necessary in the overall design.

And to get back to the OP's case of antenna tuners/pi matching
networks etc. it is not necessarily desirable to have the tuner's (or
pi-network's) inductances and capacitances vary linearly with knob
position. You actually want the curve of knob position vs value to be
logarithmic (look at the Hammarlund and Millen and National ads from
the 40's through the 60's to see all the various nonlinear variable
capacitor curves that are desirable in various uses) to make tuning
less critical on the high bands and more useful on the low bands.

Folks who grew up with pocket calculators and digital multimeters
might assume that everything should be linear. Those of us who learned
with slide rules know that in the real world, logarithmic is more
useful!

Tim.