An Integrated Microwave Sensor System for Defect Evaluation of Materials

As crucial materials for electromagnetic transmission, even minor defects in microwave dielectric substrates can impact or deteriorate the performance of designed microwave circuits. To address it, an active microwave defect detection system has been proposed. The system employs a 3 dB directional coupler paired with two identical half-mode substrate integrated waveguide (HMSIW) resonators, forming a symmetric-structured sensor to enhance sensitivity and distinguish minute substrate defects. One port of the sensor is loaded with defective materials under test (MUT), while the other port remains unloaded as a reference. The proposed sensor can transform sample defect information into variations in the magnitude and phase of the transmission coefficient. The sensor is integrated into an active gain/phase detector (GPD) to output two dc voltages, both of which can be used to predict the substrate defect conditions. System simulations and measurements are performed by detecting several commercial substrates, including FR4, RO5880, RO4350, and others. Experimental results show that the maximum sensitivity is 258.8 mV/ $\Delta \varepsilon _{\text {r}}$ ’ (where $\Delta \varepsilon _{\text {r}}$ ’ represents the varying relative permittivity). With the advantages of high sensitivity, low cost, and robustness against external environments, this work paves the way for portable defect detection and monitoring of nonmetal materials.