In article ,
Jim wrote:

i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

For a parallel-plate capacitor, capacitance equals

0.0885 e[r] (N - 1) A / t

where A is the area of one plate (in square inches), N is the number
of plates, t is the thickness of the dielectric (separation between
plates, in an air-variable cap) measured in inches, and e[r] is the
dielectric constant of the dielectric (1.0 in the case of air).

A cap with two stator plates and one rotor plate would have twice the
capacitance of a cap with one stator and one rotor, all else being
equal and neglecting parasitic effects.


--
Dave Platt AE6EO
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