On Sat, 08 Jul 2006 10:38:18 -0400, chuck wrote:

My question is this:

Looking only at the two plates separated
by 10 cm, what is the impedance the
plates would present to a measuring
device in the hf spectrum?

Hi Chuck,

From their own page:
"the capacitance of its plates (1.8 pF)"

There are enough dimensional clues to compute this yourself, however.

Is there a
simple way to calculate (estimate) the
impedance?

Xc = 1/(2 · pi · f · C)

Is the charging circuit time
constant a simple function of the
resistivity of air?

No.

Ordinarily, a 10 megohm instrument would
load such a probe excessively because
the probe's impedance would be much
higher than 10 megohms.

The probe's resistance is what that is important, not its impedance
(which is to say the reactance is immaterial except as a scaling
factor as described). In that regard, the resistance is nearly
infinite for all practical purposes. A 10 Meg Ohm meter would take 10
ms to discharge in comparison to 0.1 µs to recharge. The circuit is a
clamping circuit (or peak detector) that automatically disconnects any
load from the probe. The actual load is the sag in the charge to the
capacitor. The ratio of charge/discharge insures that the volt meter
is quite transparent to the probe - unless you are after an accuracy
that borders on delusion.

73's
Richard Clark, KB7QHC