On the Measurement of Dielectric Losses and Surface Conductivity of Dielectrics in Parallel Plane Test Capacitors

Thin slabs of dielectric materials are often tested for their dielectric properties in plane parallel plate capacitors. When surface conductivity is present, as for instance in freshly split mica, losses not connected with the bulk of the material arise. The present paper deals with the general theory of such measurements. The system is reduced to an assembly of lumped elements superimposed on distributed transmission lines. The treatment includes the presence of possible airgaps underneath the plates of the test condenser. It is shown that such losses depend on the reciprocal square root of the frequency. Losses due to this effect cannot be eliminated by guard ring measurement and much of the published data on the losses in mica must be reexamined in the light of the present work. Similar considerations may apply in the case of other materials.