Compact and Ultra-Low-Power Microwave Planar Sensor for Solid Relative Permittivity Measurement Based on Defected Microstrip Resonator

Traditional methods for measuring the dielectric constant of solid materials are often limited by bulky equipment, stringent sample preparation, and difficulty in on-site non-destructive testing. This study proposes a compact, low-cost, and planar microwave sensor based on a defected microstrip structure (DMS) for the non-destructive measurement of solid dielectric constants. The sensor is fabricated on a low-cost 1.6 mm thick FR4 substrate and features a miniaturized planar design. By placing the material under test above the DMS resonant region, the dielectric constant is determined via the shift in the resonant frequency. Simulation and experimental results demonstrate a large, normalized frequency shift and a high average sensitivity over the experimentally validated permittivity range, and full-wave simulations predict a monotonic resonant response over a considerably broader permittivity range. A second-order calibration curve is established for the quantitative extraction of unknown permittivities. Furthermore, the ultra-low-power characteristics of the sensor were explicitly evaluated. Experimental results indicate that the sensor maintains stable sensing performance at an ultra-low input power of −50 dBm (10 nW) and remains operable down to −70 dBm (100 pW). With its advantages of ultra-low power, compact size, low cost, planar structure, portability, and non-destructive capability, this sensor provides an effective solution for efficient solid material dielectric measurement.