"-=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