On Thu, 12 Aug 2004 19:34:28 -0500, "Dave VanHorn"
wrote:

I have yet to see a formula for the capacitance of a
rod in space.

Hi Dave,

That is not particularly hard if you have mathematical software like
Mathcad. You simply do it "by parts." That is reduce the cylinder to
a sheet 1mm tall and figure its surface area (pi · D · mm²). This
will be a constant value. Iterate that value over succeedingly remote
distances from an infinite conducting plane. Add up all the
iterations. (This is simply the integral of the capacitance formula
for dz where z varies from an initial separation to a value equal to
the height - 1mm.) For capacitors, capacitance is always described by
the smallest plate so this is an accurate first pass approximation.

For instance (forgive the mix of units) a 1" thick rod, 9.4M tall,
standing 10mm off of ground presents a 0Hz capacitance of 5pF. You
could move it closer to ground (1mm) and raise this to 7pF, or you
could lop off 8 meters and lower it to 3pF (not much variation).

However (and this is one of those points so ill-treated in this group)
you cannot treat this capacitance as an equivalent capacitor for RF.
The real component is much too big and this violates the first
principles of Kirchhoff's requirements that physical dimensions must
be extremely short with comparison to wavelength. The last time this
one got wrapped around the axle was over the issue of lumped
impedances.

The antenna may present any complex value of reactance to its
terminals, including capacitances that belie this simple first pass
analysis above. This, I doubt, is something that SPICE is
sophisticated enough to deal with, as it also expects all lumped
components and circuits to exist within very small dimensions relative
to the wavelengths being supported.

Time to break out that XXth century museum piece, the Smith Chart.

73's
Richard Clark, KB7QHC