Dual-Mode Microwave Sensor Based on BF-CSRR for Liquid Permittivity Measurement

This paper proposes a dual-mode microstrip sensor based on a bridge-type folded complementary split-ring resonator (BF-CSRR) structure. The sensor combines high sensitivity with low cost and is specifically designed for measuring the dielectric constant of liquid samples. Each ring of BF-CSRR features a bent and folded structure, with bridges added to optimize electric field confinement. This design concentrates the electric field about the rings and the bridge. Two acrylonitrile butadiene styrene (ABS) containers are placed at the electric field convergence point of the two rings. By injecting liquid samples and measuring the frequency shift, the dielectric constant of the liquid can be accurately determined. The sensor exhibits two distinct resonance points at 2.0 GHz and 4.62 GHz, with maximum frequencies shift of 1500 MHz and 810 MHz, demonstrating excellent performance. In the two frequency bands, the sensor achieves average sensitivities of 1.14% and 1.68%, respectively. The bridge-type folded design further enhances electric field concentration, improving sensitivity. With its low cost, simple fabrication, compact size, and high sensitivity, the sensor is suitable for a wide range of liquid analyses.