[Lowfer] Beacon "USA" off the air due to contamination

[Lyle Koehler]

Yes, it's possible to make a variable capacitor by moving a dielectric
material in and out of the space between the plates. Glass is not
necessarily a good material, though. In my battered copy of Reference Data
for Radio Engineers, several types of glass are listed with a dissipation
factor (1/Q) of 0.002 or higher at 1 MHz. And contrary to intuition, the
dissipation factor of a dielectric often becomes greater at lower
frequencies. Polystyrene would be one of the best materials (dissipation
factor less than 0.00007 at 1 MHz), and for Teflon and polyethylene the
factor is shown as less than 0.0002. For high-Q applications, avoid stuff
like Nylon, Vinyl and PVC. Dielectric constants for the better materials are
in the range of 2.0 to 2.6, so theoretically you could more than double the
capacitance of a parallel-plate capacitor by inserting a close-fitting piece
of plastic between the plates. The formula for capacitance of a
parallel-plate capacitor in English units is

C (pF) = 0.225 x (dielectric constant) x (N - 1) x A/t

where N is the number of plates, A is the area in square inches, and t is
the thickness in inches of the dielectric material separating the plates.
For air, the dielectric constant is 1.

The other factor to consider is the breakdown voltage. In order to achieve
high capacity in a relatively small area, you need to make t fairly small.
But the breakdown voltage in air at sea level is about 40,000 volts per
centimeter (about 100,000 volts per inch) between flat plates; considerably
less than that if sharp corners are involved.

Lyle, K0LR