Microwave Subsurface Imaging of Composite Structures Using Complementary Split Ring Resonators

A 2-D microwave imaging procedure to detect subsurface defects inside composite structures comprising of dielectric and metallic layers using the complementary split ring resonator (CSRR)-based planar sensor is proposed. Initially, the CSRR units of both rectangular and circular cross-sections are used for imaging. As the sensor based on the circular CSRR has higher sensitivity compared with that of the rectangular CSRR of same cross-sectional dimensions, the circular CSRR sensor is primarily used for imaging procedures. The higher sensitivity of the circular CSRR sensor is attributed to its higher resonant frequency. The proposed scheme requires raster scanning of the test structure for recording three parameters, viz., the magnitude and phase of the transmission coefficient and the resonant frequency at each scan point. If the magnitude of the transmission parameter at any scan point exhibits multiple resonant minima, then it indicates the presence of metallic layer in the test structure. In such cases, the method based on the scattering parameter magnitude at the unloaded resonant frequency is adopted. However, if the magnitude of transmission parameter exhibits only a single minimum while scanning the test structure, then it would usually indicate the structure to be purely dielectric and the resonant frequency shift can also be estimated in these situations at each scan point. The frequency shift can then be employed to obtain the permittivity image of the test structure using numerical models. The overall applicability of the designed CSRR sensor for imaging purpose is demonstrated using both simulation and experimental results.