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.

- wire drive, like in old radios
PRO can reuse hardware like in old radio, if scale is linear-motion
the tuning indicator is directly attached to ferrite plunger
CON may suffer backlash, hardware disappearing, very short scale.

I only saw very rough solutions, such as gravity pulled ferrite in
vertical coil, or using brass screw directly as core for small band
coverage. No idea if there's some promising and available type of
surplus drive.

Any ideas? Any plans anywhere for DIY? Kits???

(again, I _did_ see use of a small brass screw w/o ferrite!).

R & TIA

Filippo N1JPR

Hey OM:

Brass is the old way to lower inductance. I like to use bismuth the
best metal for lowering inductance of a coil. There's no way that a
low power signal can cut thru bismuth.
And you can increase range by using tapered cone shaped slugs, and
centripetal casting is a lead pipe cinch with bismuth that melts at
271 degrees C.

I got a hold of sum 10 gram bismuth cylindrical slugs for like a buck
a piece. Really sweet for PTO and RF front ends. Depending on the size
of coil you have to match the right N150 to N750 capacitor, to
compensate for inductive temp drift. Or stick it in a small temp
controlled oven.
Or the new age method use AFC or PLL to catch the drift.

73 OM
de n8zu

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.

Thank you all around!

I need to make an update.

If one keeps the slug travel requirement to 1" (which isn't bad at
all, even for a broad-range PTO) and is willing to use an "outside"
micrometer (for thickness/diameter) it's a different game: micrometric
screws become really cheap! Bear in mind that repositioning trumps
absolute precision, so cheap isn't perforce bad. You can get a 0-1"
micrometer for well under $10, i.e. generally cheaper than depth
micrometers and in the ballpark for axial slow motion drives with
scales.

You can't directly drill holes in the base for front panel mounting,
as you can do in depth gauges, but the arch that positions the
reference surface opposite to the screw can be cut / drilled and used
for securing the device internally. Many models have screw-fastened
plastic grips and may not need drilling. Also, by securing it well
inside the front panel, the protrusion of the screw can be reduced.

On ebay, this search produces drossfree results:
outside micrometer -electronic -digital -laser -stage -depth -internal
-bore -torque -caliper -lea* -marking

I'll experiment with supergluing a bit of ferrite to a micrometric
screw tip with a bit of acrylic dowel in between.


On Jul 12, 8:46*pm, spamhog wrote:

- 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.