Analytical Methods to Retrieve the Permittivity by Means of Coupled Resonators

Conventional single-resonator sensors perform dielectric properties measurements by means of the well-known perturbation technique, where the resonant frequency and the quality factor are used to extract the complex permittivity values. However, coupled resonators exhibit two resonance frequencies, one corresponding to the odd mode, and the other one to the even mode. This characteristic can be used to measure dielectric properties by correlating the coupling factor of the two resonances to the permittivity. In addition, this can also be used to perform anisotropic measurements, since the geometry of the electric field lines is different for odd and even modes. Therefore, by utilizing the cavity perturbation technique for the different modes, anisotropy can be extracted. In this chapter, coupled-resonator techniques will be addressed for isotropic and anisotropic measurement.