**warning - this is clearly crazy**
** just for fun, so pls bear with me! **
** calls for sanity -- /dev/null :-D **

Hello everyone!

I am considering building a broad tuning range, high impedance HF
tank circuit that could be used for several _old-fashioned_ things,
like a regen receiver or an analog RF generator, with a pair of
twists:
- PTO TUNING, as seen in many professional and ham radios, but with
very limited tuning range and custom-made slow motion drives.
- a generic MICROMETRIC SCREW for tuning slow motion drive was, as I
saw in one of Jiri
Vackar's patents.

PTO HF TUNING
I envision using a moving magnetic core either for main tuning or for
band setting. A designer of interesting hobby radios by the name of
(Sir) Douglas Hall obtained extremely wide tuning ratios with PT coils
and extremely small fine-tuning variables, in the order of 10pF,
without band-switching on HF, unattainable with variable capacitors.
He also claimed better Q over the whole range. But his PT drive was
really rough, only good enough for approximate band setting. It should
be easy to do better.

MICROMETRIC SCREWS
Standard mechanical micrometers used for depth or thickness
measurement contain a screw that moves a steel rod over a 25mm / 1
inch range. They typically have a resolution of 0.01mm using the
venier scale. This implies repositioning to 1 part over 2500 over the
full range. Other features are extremely smooth motion, very small
backlash, and low price. I also have a thermostat that I could use to
heat the whole mechanical arrangement and help stability....

QUESTION
Assuming that my favorite HF range is 3-15 MHz and that core travel
will be 25mm or less, what core size and mix you propose for best Q
and stability over a broad tuning range?

MY CURRENT CANDIDATE
Amidon cyl. core for HF PTOs R61-050-180
1.8" length x 1/2" diameter, material 61.
Excess length is necessary to keep linearity throughout the range.
I wish diameter was larger, closer to a roughly square length/diameter
ratio, but that's what's available.

SECOND BEST (probably WAAAY SECOND)
A cylindrical RFI core. I can count many many reasons not to use one,
but I wonder if I am grasping at straws.

Any comments?

Thank you in advance!

N1JPR