Measurement of dielectric strength at high frequencies and high temperatures

A re-entrant resonant cavity was constructed for measuring the breakdown strength of dielectric samples at 630 mc/sec. The cavity was tunable by means of a choke plunger to accomodate various thicknesses and permittivities of dielectric. It was also pressure tight to allow pressurizing if necessary to avoid arcing or gas breakdown around the sample. This was an unnecessary precaution, however, since the shape of the electrodes and sample was effective in preventing such phenomena. The sample could be heated either with an electric hot plate applied to the end of the cavity or by inserting the end of the cavity in an electric furnace. No tests at high temperature were actually conducted however. The dielectric samples were discs about 1-1/2 inches in diameter and one sixteenth of an inch in thickness; with a 9/16 inch diameter spherical concavity in the center, leaving a thin section on the centerline of the order of five or ten thousandths of an inch. Hard glasslike substances can be prepared this way in optical laboratories by conventional techniques. Soft substances can be cut with a ball end milling cutter. Replaceable stainless steel balls obtained from bearing manufacturers were used for the spherical electrode; the flat electrode being a sheet of shim stock between the flat side of the dielectric sample and the cavity end plate. For measuring the cavity parameters a micrometer screw was provided in conjunction with a flat-ended rod so that accurately known airgaps could be obtained. For testing dielectrics a spring loaded rod with snap fingers for gripping the spherical electrode was employed.