Karl,
Glad to meetcha!
I just uploaded KI6GD's Capacitor Calculator program which you will
find in the Files section.
This program takes the plate area, spacing and number of plates then
calculates the capacitance and voltage rating. It does not take into
account any dielectric other than air (dielectric constant = 1) but
you can "fudge" that in if you know the constant for the material.
Just multiply the capacitance as given by the calculator by the
dielectric constant to get a ballpark figure. Other dielectrics will
also affect the voltage rating, of course.
It would be interesting to find a source for curves defining Straight
Line Capacitance, Straight Line Frequency and Straight Line
Wavelength plate areas. I have looked for them but have not been able
to find them. I used to have some of these years and years ago but
they have long since vanished.
73,
'Bear' NH7SR
--- In loopantennas@..., "Karl" <portlypaddler@y...>
wrote:
Hi All! New to the group, but not to radio. When I was a kid (very
long ago), we used to make our own variable capacitors. Basically,
we'd take a square of wood, varnish it well, and mount a square or
disc of aluminum in the center, and attach a lead to it. This
became
the stator. A piece of celluloid (later plastic) was glued over
this
stator plate, and a similar piece of aluminum with a lead attached
and a piece of phenolic rod glued in it's center, was placed on top
to become the rotor. Both leads were attached to the board by
fanstock clips. By manipulating (sliding) the rotor plate across
the
stator plate, you could go from full mesh to wide open. A friend of
mine and fellow Boy Scout even took india ink and marked the
receive
locations of his favorite stations on the celluloid, making
essentially, a rudimentary dial. Now, why am I bringing this all up
(other than to wallow in nostalgia)? It seems to me that variable
capacitors are becoming hard to find these days, and when you can
find them they're pricy, and usually have to be combined with other
items to make a "minimum purchase". Not good for a geezer like me,
living on a fixed income. Does anybody have any experience
homebrewing VCs? If so, maybe you could share some data or formulae
for scaling the plates. Well, talk with you all again soon, Karl
