A near-field millimeter-wave (mm-wave) imaging system has been designed and built in the 94-GHz range for nondestructive inspection of nonmetallic (dielectric) materials. The imaging system consists of a transceiver block coupled to an antenna that scans the material to be imaged. Using a reflector plate placed behind the material, the system measures mm-wave reflected signals in a monostatic mode. A specially designed quadrature intermediate frequency (IF) mixer in the transceiver block enables measurement of in-phase and quadrature-phase components of the reflected signal with respect to transmitted signal. To minimize vibration-caused spurious reflections, the transceiver components consist of a Gunn-diode oscillator, isolator, directional coupler, quadrature IF mixer, and circulator fabricated in uniform blocks, which are then integrated and packaged into a compact unit (12.7 x 10.2 x 2.5 cm). This paper will present results of the mm-wave imaging system in examining a Kevlar-epoxy composite and high-density polyethylene that have known defects or anomalies. The results of using different antennas, including an open-ended waveguide, a corrugated conical horn, and a standard gain horn, will be discussed in relationship to imaging sensitivity and spatial resolution. Applicability of this system to on-line inspection of process materials will also be considered.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 807 - 810 Publisher:Ashgate Publishing Company
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