Some authors have demonstrated recently the capability of microwave imaging of subsurface defects and material characteristics by scanning a suitably excited open-ended rectangular waveguide over a multi-layered composite [1,2]. This near-field approach of microwave NDE is in some aspects similar to the conventional eddy-current based techniques for defect detection and imaging in metals [3]. Of course, in non-metallic materials, it is the discontinuities in the dielectric property, instead of the electrical conductivity, that gives rise to the defect signal. Bahr [4,5] has reported various techniques for the detection, discrimination, and processing of microwave signals, which are also applicable in the lower frequency regime typical of eddy current testing.
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