Recently pulbished, experimental research on microwave NDE of an internal defect has followed either one of two approaches, each with its own adantages and drawbacks [1]. In the first (generally referred to as the far-field approach), a broad, microwave beam is launched from an antenna some distance away from the material (typically, a foot or more). The scattered signal sensed by either the same or another antenna is related to some characteristics of the defect. In the second approach, the transmitting antenna is replaced by an open-ended coaxial line or wave guide. The electromagnetic field generated in the "near-field" of this aperture is modified, when a piece of material with an internal defect is placed adjecent to it. As this "near-field" probe is mechanically scanned over the material with a defect, the resulting changes in the admittance, standing wave ratio, or the amplitude and phase of the reflected signal in the waveguide are related to the characteristics of the defect. This paper describes work using the "far-field" approach.
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