In article , "Airy R.Bean"
writes:

I wonder if there is any mileage to be had from adapting
the numerous junk-mail CD's as the vanes of ATU
capacitors?

Good grief...now there are "Technical TROLLS" from the person
wanting amateurs to get into the vacuum valve fabrication. :-)

1. We already have a form that is circular (no waste) and
with an accurately-positioned central hole.

Goodie...now you can make a 360 degree variable capacitor
with an extemely low minimum-to-maximum capacitance
ratio.

2. There is a metallic film sandwiched between the plastic.

Not really. You can read all about it in free downloads from
the Philips people who developed the recording system.

If it were really conductive, you could measure it directly with
a simple Ohmmeter. [not exactly rocket science, old boy]

3. Is this film to thin to stand up to the RF currents that
will spread out across it when charging & discharging?

First you find out what you think is a "metallic film," then you
can conjecture all you want. Hint: What you see on plastic
is not necessarily "metal."

4. How would we make metallic contact to the film without
destroying the film at the point of contact?

Use metal-free solder? :-)

5. Does anybody know the type of metal making up the film, i.e.,
can we solder to it?

Do you have a soldering iron? Can you not use it?

Or are those FREE, no-cost-to-you CDs so valuable that you
cannot do a few-minutes-of-your-time experiment on your own?

6. Is the plastic of a suitable quality? Will it arc-through?

Do you have the smarts to use a power supply to rig up a test to
prove this for everyone? Or is everything you have tied up in the
proposed vacuum-valve production line you've talked about?

========

Those free CDs that many receive have some redeeming features:
They can be coasters for placing drinking vessels on; they can be
"frisbees" for use in playing with a dog; they can make decorative
wind chimes to hang on a porch of those who don't like the sound
of wind chimes. If Paul Bunyan were alive today he might like to
use them as Christmas tree decorations. Other than that and
their excellent use as a time-stable data storage medium,
those left-over CDs are rather useless.

It would be nice if folks in here would share ACTUAL data and
information, not propose a lot of fantasy ideas.

CD blanks are nicely circular and MIGHT make some kind of edge-
friction-drive dial but the center (or centre) hole is non-standard at
about 7/16 inch diameter (1.11 cm). Yes, a moderate machine
shop could make a shaft adapter for it...but that same machine shop
could make a much-superior edge-drive dial out of metal in only
slightly more time. That same machine shop could also turn out
capacitor plates for a variable, the shafts for same, end bearings,
etc. Several firms on both sides of the Pond have done that for
over half a century. [Jackson Brothers in the UK, James Millen
et al in the USA]

Rather than get all fired-up in pipedreaming on the keyboard, try
searching for a "dielectric capacitor" on the Internet. There are
several hobbyist sites which explain how they can be made using
thin polymer sheet plates rotating between two metal plates.
Maximum chapacity change ratio is limited to the dielectric
constant of the polymer (that's "plastic" to you non-chemical
types). Withstanding voltage can be figured from dimensions
and plate spacings using pencil and paper (crayon and cardboard
can be used but not recommended).

Presuming that "ATU" is an acronym for Automatic Tuning Unit,
how in the silly blue heck are you going to make it "automatic"
presuming someone else already solves "your" questions? The
servotuning system needs far more thinking attention than gazing
at used CDs pondering the whichness of the what about them.

Hint: There's already been some very nice hobbyist work done
in regard to ATUs with much on the servotuning system (at least
one made from scratch) plus the sensing-detection system to
get the servotuning going in the desired "automatic" direction.
Another Hint: The detection-sensing was solved in 1955 by a
"Yank" surnamed Bruene who was then working at Collins Radio.
[that's in the middle of the USA, in the state of Iowa]

An even better ATU uses a microcontroller chip plus a Bruene
Detector, and some latching relays to switch stepped inductors
and capacitors in/out in a two-element L-C network. Both L and
C step in the binary progression of 1-2-4-8-16-... sequence. That
sort of network and its micro are at the heart of at least three
commercial ATUs built and sold in the USA. It has the advantage
of tuning memory, can return to the same setting by actually
measuring the carrier's approximate operating frequency! In the
U.S. Army such an ATU in incorporated in the backpack HF
transceiver AN/PRC-104 (design by Hughes Aircraft Ground
Systems, in operational use for nearly two decades). On the
ARRL website you can find a reprint of a nice digital ATU
construction article that used to be available frree for download
but that may have been changed to members-only availability.
At the Stoner Goral Company (SGC) website is much interesting
information on their ready-built ATUs, installation and use on HF,
plus operating manuals (those were free downloads).

www.sgc.com

Founders Don Stoner and Pierre Goral (both hams but sadly,
both SK now) developed the SG-2020, a very small 20 W, full-
frequency-range-on-HF SSB transceiver, back-pack size almost
as small as the PRC-104 (also 20 W PEP).

The step-variable tuning system seems very strange to those
accustomed to big handwheel-operated variable capacitors, but
that is quite versatile in that you can use readily-available fixed
(and sealed) capacitors and toroidal inductors. Toroids are easy
enough to make, much much easier than trying to shape CDs
into a semblance of capacitor dielectric blades. Toroids can be
potted or well coated with varnish, enough to withstand outdoor
environments in snow country or the tropics. Relays are
generally expensive but Omron makes some nice relatively
inexpensive relays (including latching types for built-in memory)
for the higher withstanding voltages found with 100 W RF at
50 Ohms with up to 20:1 VSWR. All the big electronic
distributors carry Omron products.