Microwave Permittivity Measurements Using Disk Cavity Specimens

A method is described for microwave permittivity measurements of anisotropic sheet dielectrics, especially of thin copper-clad laminates. The accuracy is calculating the permittivity is not dependent on measurement of specimen thickness. All exposed dielectric surfaces of a precisely machined disk (the edge only of disks of double copper-clad laminates) are preferably electroplated with copper to provide a resonant cavity completely filled with dielectric. By means of a special coupling fixture and smal openings provided in the conductive coating, the TMn10no resonances are excited and identified. The frequencies of these modes are independent of disk thickness. Permittivity is determined by dividing a constant calculated for each mode by the square of the resonant frequency. Constants and other data for the first 17 modes of a 1.000-in (2.540-cm)-diameter cavity are given together with some experimental results.