XXVIII.Capacitance at ultra-high frequencies

Summary An approximate formula is derived from the Maxwell equations for the input admittance of a circular parallel-plate condenser as a function of frequency. A simple equivalent circuit is shown which represents well the input admittance down to centimetre waves. Restrictions on the dimensions of the plates and on the properties of the dielectric medium between them are considered, and reference is made to the measurement of dielectric constants by the condenser method. An approximate expression is derived for predetermining the input reactance of a multiple-plate condenser from its static characteristics. Computed results for a General Radio Precision condenser and for a specially designed tuning unit are compared with the corresponding experimental ones measured at several ultra-high frequencies.