Update: Home-built air-gap variable capacitor
 

"Lars Janqqvist" wrote in message
...
Frightening the yaks, -=jd=- just had to say:

It wasn't nearly as difficult as I had anticipated.
I finished up with 14 rotors and 15 stators.

The meter says 7pF with the plates unmeshed, and 314pF with
the plates fully meshed.

The trickiest part was patiently (and subtly) straightening
the plates after assembly. Cumulative time spent building it
was probably 4 hours. After building the first one, you
realize that the plates don't have to be perfectly cut - they
just have to rotate all the way around without contacting
anything. Just as the Cajuns know that "looks don't make it
taste good", capacitor plates that aren't cut exquisitely neat
don't mean it won't... um... "capacitate"...

-=jd=-

re uneven plates: The linearity will suffer but unless you're
calibrating it to something (tuning dial, antenna load) that
shouldn't matter.

I don't see anything linear in F vs C for a resonant circuit. In fact, in
order to make it anywhere close to linear, one has to go to extremes with
the shape of the plates.

Dale W4OP

Dale Parfitt wrote:

I don't see anything linear in F vs C for a resonant circuit. In fact, in
order to make it anywhere close to linear, one has to go to extremes with
the shape of the plates.


That's most likely why they resemble the profile of a snail's
shell...a logarithmic spiral of sorts.



mike

Frightening the yaks, Dale Parfitt just had to say:

re uneven plates: The linearity will suffer but unless you're
calibrating it to something (tuning dial, antenna load) that
shouldn't matter.

I don't see anything linear in F vs C for a resonant circuit. In fact, in
order to make it anywhere close to linear, one has to go to extremes with
the shape of the plates.

My bad. Not linear in the sense of d-pF/d-theta = K. But more
the idea of no sharp changes for slight adjustment of the rotors.
Continuously differentiable, I should have said.

But if a linear variable cap *was* desired, the plates shouldn't
be all that hard to fabricate.



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"Lars Janqqvist" wrote in message
...
Frightening the yaks, Dale Parfitt just had to say:

re uneven plates: The linearity will suffer but unless you're
calibrating it to something (tuning dial, antenna load) that
shouldn't matter.

I don't see anything linear in F vs C for a resonant circuit. In fact, in
order to make it anywhere close to linear, one has to go to extremes with
the shape of the plates.

My bad. Not linear in the sense of d-pF/d-theta = K. But more
the idea of no sharp changes for slight adjustment of the rotors.
Continuously differentiable, I should have said.

But if a linear variable cap *was* desired, the plates shouldn't
be all that hard to fabricate.

You are correct Lars. I seem to recall this in some early tube sets. I was
able to make the VFO in a hi end homebrew RX I am building quite linear by
making certain the variable cap comprises a relatively small portion of the
total tank C- and this using the typical 1/2 round plates. All sorts of
games can be played by using combinations of series and parallel capacities
along with the variable. An Excel spreadsheet gives quick insight.
Also nice to see a bit of differential calculus on the group!
Dale W4OP

"-=jd=-" wrote in message
...
On Thu 11 Nov 2004 02:43:51a, Lars Janqqvist
wrote in message
:

Frightening the yaks, Dale Parfitt just had to say:

re uneven plates: The linearity will suffer but unless
you're calibrating it to something (tuning dial, antenna
load) that shouldn't matter.

I don't see anything linear in F vs C for a resonant
circuit. In fact, in order to make it anywhere close to
linear, one has to go to extremes with the shape of the
plates.

My bad. Not linear in the sense of d-pF/d-theta = K. But
more the idea of no sharp changes for slight adjustment of
the rotors. Continuously differentiable, I should have
said.

But if a linear variable cap *was* desired, the plates
shouldn't be all that hard to fabricate.


Aluminum roof flashing - it cuts easily with scissors. True,
it's fragile outside of the capacitor, but once it's in there,
it should be fine. This first set, I cut with painstaking
care. The rotors have a 3" radius and the stators are just
rectangular. I'm thinking of making a larger one (more
plates). I'm only planning to use it in a receive only antenna
project. Now, I'm not sure how "perfect" it needs to be; but
it it's like any other SWL antenna projects, they have been
*extremely* tolerant of non-precision... Thank goodness...

Also don't forget variable coaxial style capacitors- although large values
can be a pain, this can be an easy to build approach.

Dale W4OP