On May 11, 4:51�am, Tim Shoppa wrote:
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.

I was referring specifically to the "PTO" (Permeability Tuned
Oscillator) that first saw wide use in Collins radios. That
tuning *IS* linear in frequency versus knob position. If you've
ever opened one to examine it, you will understand. Most of
the Collins PTOs also had a minor correction mechanism for
that tuning but, in essence, the winding pitch of the tuning
inductor varied according to some internal Collins manufacturing
rule.

I've also had my hands on many an R-391 and understand that
mighty mechanical monster still - even though it was four decades
ago - and would say that trying to find a correct winding pitch for
all those (broadband relative to tuning rate) inductors would have
made the production cost way too high. It wouldn't be
important because the actual Q of those variable inductors made
the front-end resonators broad enough so that "crude" cam
adjustments were okay for practical purposes.

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.

I began actually handling of lots of high-power HF transmitters
in the early 1950s, notably the ones made by the Lewyt Vacuum
Cleaner Company! :-) No real relationship of capacitance or
inductance curves to tuning...wayyyy too many variables involved
in antennas-lines-etc. to pin down any "necessary curves."

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!

High disagreement there. Having learned and owned a slide
rule in high school of the late 1940s (and understood logs
and elementary transcendentals), I switched to sceientific
calculators (forever, I think) as soon as they were on the
market, never looked back. I severely dislike non-linear tuning
as it has NO physical-sensor-body correlation with signal
bandwidths or carrier positions.

I see Joel's roller inductor question connected with my
experience with Press-Wireless 15 KW transmitters and
their dual copper tubing final amplifier inductors. QSYs
on those PW-15s required unwrenching the shorting links
then rearranging the shorting links, remounting them,
again with a wrench. Having helped make better sets of
shorting links and doing some intial tuning (to set preset
tables for number of links), the unshorted links were
generally in the inductance values predictable by old-style
equations of inductors. There was some effect of the
shorted turns which was arrived at during the preset tune-
up tests. Those were feeding rhombics directly and there
were no fancy tuning-matching circuits involved in the
typical 4 to 18 MHz RF region.

73, Len AF6AY