Transmission lines loaded with pairs of magnetically coupled stepped impedance resonators (SIRs): Modeling and application to microwave sensors

This paper is focused on the analysis and modeling of transmission lines loaded with pairs of shunt-connected stepped impedance resonators (SIRs), and their application to differential sensors for dielectric characterization, and for diagnosis and quality control of material samples by comparison to a reference. It is demonstrated that by placing the SIR junctions in the same position of the line, the SIRs are magnetically coupled. Such coupling has significant influence on the sensitivity of the sensor, determined by the split in frequency caused by an asymmetric dielectric loading of the SIRs. The circuit model of the structure, including magnetic coupling between SIRs, is proposed and validated through electromagnetic simulations and measurements. Finally, the principle of sensing is experimentally validated by a proof-of-concept demonstrator.