Maximum Sample Volume for Permittivity Measurements by Cavity Perturbation Technique

The maximum sample volume for accurate permittivity measurements of dielectric materials having various geometries (rod/bar, strip/disk, and sphere) by cavity perturbation technique has been investigated by determining the maximum volume ratio of sample to cavity (V_s/V_c)max based on analysis of the measurement theory. It is demonstrated that (V_s/V_c)max of a dielectric rod/bar with the height equal to that of resonant cavity relies exclusively on the relative dielectric constant, whereas (V_s/V_c)max of a dielectric strip/disk or sphere depends on both the relative dielectric constant and the dielectric loss factor. There is a relatively weak permittivity dependence of (V_s/V_c)max for dielectric property measurements of dielectric strips/disks compared with rods/bars or spheres. The maximum sample volume used in the measurements for different sample geometries follows the order: . Comparison between (V_s/V_c)max of low-loss Al₂O₃ and high-loss SiC reveals that low-loss materials can have a larger sample volume than high-loss materials for measurement. High-loss materials may require a strip/disk geometry to meet the measurement requirements. The variation in (Vs/Vc)max of Al₂O₃ having different geometries in a broad temperature range up to ~ 1400^°C shows that (V_s/V_c)max of the sample decreases with increasing temperature and the change in (V_s/V_c)max should be considered during the high-temperature permittivity measurements.