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

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

Alas, on the wretched hour of Thu, 04 Jan 2007 09:56:29 -0800 K7ITM
thusly proclaimed:

Ceriel Nosforit wrote:

measure twice, snip once

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

Hello Tom. Thank you for your reply.

I'll separate the plates with spacer disks and bolts on a 6 mm threaded
steel rod, indeed through the pointy ends. 6 mm is IMO too thin on its
own, but the disks and bolts should add a lot of stability. The entire
thing will be propped up on plexiglass plates at the ends, and I'll have
one additional 6 mm threaded steel rod where I marked 60 degrees on my
schematic. If it still not stable I'll add something to the top and bottom
to keep the rods stretched straight. If that still isn't enough, I'll have
to build some sort of support for the mid-section.

The blades will separate from each other more than completely, if I want
to. I don't know the electrical implications of what would happen if I did
so. I guess at some point the curcuit would simply be broken... Maybe
permanently? Either way, infinite ratio from max to min.

I plan on either adding a counter-weight, a rubber band, or some mechanics
to keep the rotor from simply falling down. A counter-weight should give a
nice 'feel' to the dials, which is very important for any sense of
"quality".

If the parts of the ATU turn out reasonably pretty I'll make an enclosure
largely out of plexiglass for them. A cardboard box is however a lot more
likely. I expect the end result to look horrible and work marginally
better. XD

--
Nos

Ten Tec in USA offers a variable capacitor kit, max C is 500pf @ ~5kv.
Cost is about $80 USD. Takes about a hour to assemble. I use them in
a 1.5kw ATU and they work fine. Kit #1207.

If you get all the parts, cut them up, and assemble the whole thing,
homebrew, makes the $80 price tag seem reasonable. I could see you
spending more than half on just materials.

Good Luck,
Gary N4AST

"Ceriel Nosforit" wrote

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...

I'm doing a random wire "semi-automatic" tuner, and also trying to make
varicaps for it. I found a eBay seller from Germany, if I remember right,
who sells ready-made air-varicap plates. I bought few dosens of them for 20
euros. I think the seller ships worldwide. I'm sure there is other producers
too...

73 de Jukka, oh6mwq

Alas, on the wretched hour of Fri, 05 Jan 2007 15:32:13 +0000 Jukka
Pappinen thusly proclaimed:

"Ceriel Nosforit" wrote

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...

I'm doing a random wire "semi-automatic" tuner, and also trying to make
varicaps for it. I found a eBay seller from Germany, if I remember right,
who sells ready-made air-varicap plates. I bought few dosens of them for 20
euros. I think the seller ships worldwide. I'm sure there is other producers
too...

73 de Jukka, oh6mwq

That's a rather good deal. I paid 25€ for material costs and an additional
5€ to my neighbour who got them for me. My brother who teaches handicrafts
at a school is going to let me use the workshop to do the rest of the
work, so right now I'm looking at a cost of 33€ for materials, but I still
haven't bought the plexiglass.

How is your tuner going to be semi-automatic, if I may ask?


p.s I can see you're a fellow Finn aswell, so here is a rather useful link
I found earlier today:
http://www.tkk.fi/Misc/Electronics/f...liikkeita.html
I'm sure Vekoy is a familiar place for you already, but still...

--
Nos

Alas, on the wretched hour of Thu, 04 Jan 2007 15:19:59 -0800 jgboyles
thusly proclaimed:

Ten Tec in USA offers a variable capacitor kit, max C is 500pf @ ~5kv.
Cost is about $80 USD. Takes about a hour to assemble. I use them in
a 1.5kw ATU and they work fine. Kit #1207.

If you get all the parts, cut them up, and assemble the whole thing,
homebrew, makes the $80 price tag seem reasonable. I could see you
spending more than half on just materials.

Good Luck,
Gary N4AST

Those are not all that awful prices for the rest of their kits... The BFO
circuit looks especially tempting. I'll be sure to check it again when I
decide I'm going to build an actual radio. Thanks for the link!

--
Nos

How is your tuner going to be semi-automatic, if I may ask?
=========================
Suggest you consider a stepper motor from a dumped
printer/scanner/floppy drive or HD ,if necessary including a shaft
rotational reducer when steps are too big.
Google will provide you with suitable (simple) circuits to drive the
motor ,usually involving one or more opamps.
From semi-automatic the circuit can evolve towards a fully automatic
system with a SWR sensing circuit.

Frank KN6WH/GM0CSZ

"Ceriel Nosforit" kirjoitti viestissä:5Sznh.20298

How is your tuner going to be semi-automatic, if I may ask?

"Remote controlled tuner with few memories for spot frequencies" maybe
describes better what I'm going to do.

- Jukka, oh6mwq

Alas, on the wretched hour of Fri, 05 Jan 2007 23:52:01 +0100 Highland
Ham thusly proclaimed:

How is your tuner going to be semi-automatic, if I may ask?
=========================
Suggest you consider a stepper motor from a dumped
printer/scanner/floppy drive or HD ,if necessary including a shaft
rotational reducer when steps are too big.
Google will provide you with suitable (simple) circuits to drive the
motor ,usually involving one or more opamps.
From semi-automatic the circuit can evolve towards a fully automatic
system with a SWR sensing circuit.

Frank KN6WH/GM0CSZ

You know... I just happen to have two old floppy drives in my drawer.
Might be able to control them as they are through the standard cable if I
have the right software. Would just have to make another mechanical
modification to put the motion to use. A standard diskette might come in
handy for that.

Excellent idea! Thanks!

--
Nos