Transmission-Mode Permittivity Sensors Based on Coupled Resonators

Loading resonators on a two-port transmission line creates transmission zeros, where each transmission zero corresponds to a resonant mode. By placing an additional resonator with a higher quality (Q) factor close to the existing lower-Q resonator and ensuring that their resonant frequencies overlap, the strong coupling between them makes the original single transmission zero transform into a high-Q transmission peak with a pair of transmission zeros on both sides. The strongly coupled resonators induce this transmission mode. The high Q factor and sensitivity make the transmission mode suitable for two-port transmission-type sensing measurements. In this chapter, we explain the basic theory, design several transmission-mode sensors based on coupled resonators, and use these sensors to detect the permittivity of various samples.