Dielectric permittivity is the quantity that relates the electric displacement in a material to the applied field, written as a complex number whose real part describes stored energy and whose imaginary part the loss. Both parts depend on frequency and temperature, so a value quoted without them means little. At microwave frequencies the measurement is usually made by placing the specimen in a resonator or a transmission line and comparing the response with and without it; microwave dielectric ceramics are commonly characterised by two complementary methods, one based on the resonant frequency and quality factor, the other on reflection [1]. Thin sheets and liquids are more conveniently handled with an open-ended coaxial probe, whose fringing field senses only the near surface and which needs careful calibration to give absolute values [2]. High-permittivity materials are hard to measure in a closed cavity because the specimen perturbs it too strongly, and an extra-cavity arrangement avoids that [3]. Predicting the effective permittivity of a mixture from its constituents and their arrangement is a separate problem, addressed by mixing rules that take the inclusion shape into account [4]. Data-driven models fitted to measured spectra now serve the same purpose for heterogeneous materials, wood included [5].
- complex permittivity
- dielectric loss
- microwave measurement
- resonance method
- coaxial probe
- effective medium
- polarisation
Microwave and Dielectric Measurement Techniques·Electrical and Electronic Engineering·Engineering·Physical Sciences
References
- Xiao, F.; Chen, R.; Huang, Z.H.; Xiong, Z.X.; Two Methods for the Measurement of Complex Permittivity of Microwave Dielectric Ceramics. Key Engineering Materials 2007, 280-283, 61-64, 10.4028/www.scientific.net/kem.280-283.61.
- Wu, M.; Yao, X.; Zhang, L.; An improved coaxial probe technique for measuring microwave permittivity of thin dielectric materials. Measurement Science and Technology 2000, 11, 1617-1622, 10.1088/0957-0233/11/11/311.
- Ni, E.; Jiang, X.; Microwave measurement of the permittivity for high dielectric constant materials using an extra-cavity evanescent waveguide. Review of Scientific Instruments 2002, 73, 3997-4002, 10.1063/1.1505102.
- Drozdov, A.; deClaville Christiansen, J.; Modeling dielectric permittivity of polymer composites at microwave frequencies. Materials Research Bulletin 2020, 126, 110818, 10.1016/j.materresbull.2020.110818.
- Menguc, E.; Helhel, S.; Microwave measurements and data-driven modelling of complex dielectric permittivity in hardwood specimens. Measurement 2026, 272, 121116, 10.1016/j.measurement.2026.121116.