Generalized Multimode SIW Cavity-Based Sensor for Retrieval of Complex Permittivity of Materials

In this paper, the generalized substrate integrated waveguide (SIW) cavity technique is presented for the retrieval of broadband complex permittivity of medium loss dielectrics using a number of higher order modes of the proposed cavity. The proposed SIW cavity design improves the coupling by implementing a novel feeding topology to achieve a better quality factor for the higher order TE$_{10n}$ modes as compared with the conventional SIW feeding arrangement. The conventional SIW cavity perturbation formula is modified to overcome various limitations of the SIW topology, such as the lower $Q$ -factor, lower sample to cavity volume ratio, and larger frequency shifts observed for the higher order modes. The applicability of the proposed technique is first numerically validated using independent data obtained with the help of full-wave electromagnetic solver, and then, the accuracy is compared with various cavity-based techniques available in the literature. Finally, the proposed SIW cavity sensor is fabricated on Rogers' RT5880 substrate, where a number of standard RF substrates are tested using the network analyzer. The proposed generalized scheme based on the improved SIW cavity provides reasonably accurate values of the complex permittivity of the medium-loss dielectric substrates at multiband of microwave frequency in the range of 10-20 GHz.