Resonant Coupled Line Driven Highly Selective Microwave Sensor Design

This letter presents an efficient sensor design based on a coupled microstrip transmission line integrated with a quarter-wavelength suspended open-ended stub for material characterization. The design offers very high fringing fields at the end of the stub line. Therefore, loading a dielectric material on the high-frequency field location directly modifies the capacitance offered by the resonator and results in high sensing performance. The proposed coupled-line structure is designed to operate at 2.45 GHz with a bandwidth (BW) of 9 MHz corresponding to a fractional BW (FBW) of 0.36%, resulting in high selectivity. The performance of the sensor is studied analytically and verified with full-wave simulations, followed by experimental validation of various materials. The proposed design shows an excellent sensitivity of 46 MHz/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}$</tex-math> </inline-formula> with a high-quality factor of 272.8.