Boosting Sensitivity of a Microwave Gas Sensor via Electrically Tunable Frequency.

In this study, a tunable microwave gas sensor (T-MGS) is proposed and demonstrated. By cascading a pair of varactor diodes at the terminal of the coupled line (CL) resonator and adjusting the direct current (DC) bias voltage, the capacitance of varactor diodes and the surface current distribution of the resonator can be effectively modified, thereby synchronously regulating the equivalent capacitance and inductance of the resonator. This enables significant enhancement of both the electric field (E-field) and magnetic field (H-field) inside the resonator, which effectively compensates for the loaded quality factor (QL) and unloaded quality factor (QU), and further improves the sensing performance of the sensor. To systematically demonstrate the material-agnostic nature of the proposed sensitization strategy, experiments were conducted employing a diverse panel of sensing materials (SMs) spanning the full conductivity range from insulating to highly conductive, and their performance was characterized against reducing NH3 and oxidizing NO2 analytes, representing two fundamentally distinct redox-based sensing mechanisms. Experimental results show that after tuning, the minimum limit of detection (LOD) of the NO2 sensor can reach 0.7 ppb, with a peak sensitivity of 9.779 dB/ppm (1.7 times higher); the minimum LOD of the NH3 sensor can reach 2.3 ppb, with a peak sensitivity of 1.279 dB/ppm (7.6 times higher). Validated in all experimental systems, this sensitization method can significantly improve the sensor sensitivity by 1.7-16 times and greatly reduce the LOD, showing great application potential in the field of high-performance microwave gas sensing.