Crack Detection and Corrosion Mapping Using Loaded-Aperture Microwave Probe

Employing circular aperture probes for microwave imaging in the near-field has been proven to render high resolution images of a variety of samples under test (SUT) when compared to the conventionally utilized rectangular aperture probes. In this paper, it is proposed to further enhance the near-field imaging resolution and sensitivity by loading the circular aperture (operating in the K-band frequency range) with a resonant iris. It is shown that the probe exhibits a confined and localized near-field distribution close to the iris opening, thereby improving its resolution and sensitivity. The design and analysis of the probe presented herein are aided by 3D electromagnetic simulations and validated experimentally. The spatial resolution of the probe is established through line scans of the probe over a pair of targets interspaced at known distances. To further demonstrate the utility of the probe in critical non-destructive testing (NDT) applications, a manufactured prototype is devised to detect cracks on metal structures and map corrosion areas hidden under paint layers. It is shown here that the proposed probe provides significant resolution and sensitivity improvement over the un-loaded circular aperture probe. Specifically, for crack detection, the proposed probe provides two-fold and more than three-fold improvement in the resolution and sensitivity, respectively. For corrosion mapping, the proposed probe yields 2 to 5 times higher image signal-to-noise ratio compared to the unloaded aperture probe.