Ceriel Nosforit wrote:
Hi all!

I couldn't find a place that sells this type of variable capacitators, so
I decided to build my own, but I think I could use the feedback of people
who actually know what they're doing. Since I know none, I had to settle
for r.r.a.h. (Just kidding!! ;D )

I don't have a DX license yet. I'll probably get one last when I decide I
want to build a transmitter, but it'll be a good while until I know enough
to do that. An ATU seemed a simple enough curcuit to start with, and it
seems I'll be building every part of it from scratch. So, too late to cut
to the chase, what I'm planning is using

32 aluminum plates
200 x 50 x 2 mm each
cut diagonally in half
to make 64 plates total
40 cm^2 actual surface area each
spaced 3 mm apart.

This should make a cap weighting in at 743 pF with a voltage
tolerance of almost 9 kV according to the KI6GD Capacitator Calculator, if
I'm using it right. I'm going to build two of them. It should be... enough.

http://img413.imageshack.us/my.php?i...ict3833tj4.jpg
http://img138.imageshack.us/my.php?i...ict3834eu7.jpg

First pic is of the plates I'm going to cut, and second is my schematic.

As you can see I'm going for an unusual shape with the plates. This is
to minimize my work load. I'll just have to cut straight, drill a hole and
file down the corners to prevent corona discharge. Once the caps are
complete I'll just measure their values at different levels and make a
chart or scale.

So r.r.a.h, if you are still reading, what has this humble newbie already
screwed up? I know the sensitivity is going to be extremely high, but I
should be able to fix that with some simple mechanics. My old LEGO bricks
should do the trick, come to think of it...

Thank you very much for reading. Hope to see a reply, even if it's just
"Go for it!" or "You've got it all backwards".

--
Sincerely,
Nosforit

Well, this is basically a "go for it." I'm not quite clear on how you
are going to stack up the plates when you're done...what holds them in
position and so forth. One advantage of the "normal" semicircular
plates is that there's not such a long piece of metal hanging off the
rotor shaft as you will have, assuming the shaft runs through the
"pointy" end of the rotor pieces. Beware of what vibration does to
you, in other words. As thick as your plates are, I suppose that won't
be a problem.

The widest-range variable I have is built with circular plates, with a
tab off the edge of each through which the rotating shaft goes. There
are actually two rotors, which are coupled through equal-size gears on
each shaft, so one rotates clockwise as the other rotates
counterclockwise. The plates go from full mesh to full separation, and
give over 100:1 capacitance ratio from max to min. The shafts must be
insulated from each other, of course. There's some advantage to a
capacitor with a wide tuning range in an ATU, in getting the ATU to be
able to cover a wide range of matching impedances over a wide range of
frequencies. Anyway, I suppose you could do something similar with
your rectangular plates.

Cheers,
Tom