Reflective-Mode Permittivity Sensors Based on Distributed and Semi-Lumped Coupled Resonators

One-port reflective-mode phase-variation permittivity sensors based on weakly coupled resonators (either distributed or semi-lumped) are presented in this chapter. It is demonstrated that by weakly coupling the resonators, the split resonances that appear as consequence of coupling are very close one to each other, and the phase of the reflection coefficient experiences a significant variation (360° if losses are negligible or small) between such frequencies. The consequence is an enhancement in the sensitivity when the device is tuned in between the two resonance frequencies, as demonstrated in the chapter. It is shown that the sensitivity increases as the inverse square of the coupling coefficient, and unprecedented sensitivities are reported in the various examples included in the chapter, where inter-resonator coupling is very weak. The effects of losses in the material under test (MUT), either small or significant, are also studied in the chapter.