On Jul 12, 4:46*pm, spamhog wrote:
I'm looking for ideas for sliding a ~2" ferrite core in and out of a
coil. *I've seen ways of tuning over a small range by means of a brass
screw used as core. I need smtg better than that, with repeatable
positioning and smooth movement.

Some ideas:

- micrometric screw:
use a preexisting screw taken from old depth gauge or bought
standalone
PRO calibrated, demoltiplicated, high resolution position readout
CON expensive if new, still need to invent a way to fasten ferrite
core to spindle, screw protudes.

- screw drive from scratch:
PRO can be taylored, can use knob with 360deg scale, turns determined
by screw pitch - say 0-100 over 360deg @ 20 turns/inch produces 8,000
divisions over 2" linear displacement (if with vernier reaches
80,000), may use knob with turns counter
CON quite complicated to design, may reguire machining and searching
for pieces.

Something that I thought always looked promising - but which I've
never used in a PTO - are the old floppy drives that used a Acme-style
gear rod driven by a stepper motor to slide the head back and forth
across the floppies. Most 8" floppy drives meet this description, but
only a few 5" floppies did (most 5" floppy drives use a steel band to
do the positioning).

The floppy head assembly had a set of nuts with some anti-backlash
springs. I would think that many of the parts would be quite useful
for a homebrew PTO drive.

Don't knock the "good enough" approach. The crude brass-rod-screw-and-
a-few-brass-nuts scheme used by KD1JV and others for a VFO is really
very usable, especially if you put a big 2" tuning knob on it, but for
portable use a smaller knob works without quite the smooth feeling.
It's not something that will let you cover a 0.5MHz band linearly, but
it will give you very nice coverage of for example a CW-subband.

Tim.