Resonator- and Filter-Induced Slow Waves for High-Sensitivity RF Interferometer Operations

Resonators and filters have engineered radiofrequency (RF) spectrums and dispersions, which induce slow waves for increased local RF field intensities and prolonged field propagations. When used for sensors, the interactions between RF waves and material-under-test (MUT) can be significantly strengthened due to enhanced RF fields and longer interaction time when compared with uniform transmission lines. In this paper, a microstrip resonator (MR) and a coupled line (CL) filter are designed and incorporated into an interferometer. The obtained results are compared with that of a coplanar waveguidebased interferometer. Both simulated and measured results show that the MR and CL can significantly enhance RF interferometer sensitivity, e.g., by >5 times of frequency shift and up to 40-dB more of transmission coefficient change ~2 GHz. Further work is needed to fully understand the physical processes and obtain quantitative MUT permittivity properties from the measured scattering parameters.