Evanescent-Mode Waveguide Resonator for High-Sensitive Nondestructive Testing Application

This article presents a novel semi-open resonator design based on a pair of evanescent waveguides (EWs) and a dielectric material under test (MUT) for high-sensitivity material structural health monitoring. Compared to conventional microwave resonant cavities, the electric field energy is primarily concentrated within the dielectric material at the resonant frequency, which significantly enhances the interaction between the electromagnetic field and the dielectric. Minor variations in dielectric properties caused by local defects will induce measurable shifts in the resonant frequency. Another distinctive feature of the designed resonant cavity is its semi-open architecture, where the dielectric material is not placed inside a rectangular waveguide but is positioned between two mirror-symmetric open-ended waveguides. This design enables the resonator to achieve scanning detection for larger-area MUTs. To mitigate electromagnetic leakage and enhance the quality factor of the resonator, a parallel plate waveguide (PPW) is incorporated at the open ends of the evanescent waveguides. To demonstrate the detecting ability, an extensive investigation of the sensitivity to local defects inside the material is presented. Two different dielectric materials, including F4B and ceramic samples, are tested using a prototype device. The experimental results demonstrated frequency shifts of 11 MHz at 4.32 GHz and 10 MHz at 3.65 GHz for subwavelength defects with diameters and thicknesses less than 2 mm. Compared to conventional microwave-based sensors, our proposed method offers advantages in detecting minute defects within relatively thick materials at lower operating frequencies.