Nondestructive Microwave Self-Oscillating Sensor Based on Complementary Spiral Resonator for Defect Evaluation of Metallic Mesh

The metallic mesh is a typical transparent conductive film commonly used in the field of transparent electromagnetic shielding. The nondestructive evaluation (NDE) of such films has attracted considerable interest from researchers due to its applicability to the quality assessment and maintenance of the films. In this study, we developed a microwave feedback-type oscillator with an integrated complementary spiral resonator (CSR) for the NDE of metallic meshes. The optimized CSR structure with bandpass characteristics can transform two typical defects of metallic meshes—cracks and increase in sheet resistance into a downshift in resonant frequency and decrease in quality factor. Next, the CSR was integrated into a feedback oscillator with an additional loop filter to achieve self-sustained measurement and self-switching effect for cracks. Both small-signal and steady-state analyses were conducted to predict the variation in the output power and the static drain current under different defect conditions. The experimental results show that the developed sensor can detect cracks with 0.4 mm width with self-switching effect and can detect a 3.6% increase in sheet resistance with a variable measurement range. The sensor has a low profile and power consumption and requires only a microwave power detector or dc ammeter as the readout device. The proposed sensor can not only be applied to detect defects in metallic meshes but also a good candidate for the NDE of other conductive films.