Guided tapered slot antenna for near field millimeter-wave imaging

The deterioration of concrete structures built long years ago has been a serious social problem. One of the assessment indexes for the durability of the concrete structures is surface crack, which allows water to easily penetrate into concrete and rust reinforcing bars. An acceptable width of the crack, which differs by country, is usually 0.3 mm in dry air and less than 0.2 mm in humid air. Although visual inspection is the most widely used method for non-destructive testing, it cannot be used to cracks covered e.g., by wallpaper, tile, paint, and repair material. Using a near-field millimeter-wave, NF MMW, imaging technique we proposed [1] provides the location of the invisible cracks by seeing through covering material. However, it is too blurry to recognize cracks with a width of under 0.2 mm wide because of superposition between surface scattering from the crack and volume scattering from aggregates in the concrete. An NF MMW imaging with image reconstruction techniques such as a holographic technique [2] can focus on surface cracks away from 0.1-mm deeper aggregates, that is, increase crack signal and decrease the other ones. To use this technique, a 77-GHz-band monostatic-radar module with a constant-width antipodal tapered slot antenna (TSA) and in-phase and quadrature detection has been developed and achieved good phase sensitivity corresponding to a resolution of 0.01 mm in concrete [3]. However, a TSA designed based on far-field calculation illuminates a wide area in the near-field region of around a wavelength. It is not suitable for imaging a narrow crack with a small signal which is almost the same variation level as that of the other signal, that is noise, scattered from aggregates in the concrete. We propose a guided TSA locally illuminating to the near-field region to improve crack signal to noise ratio of the received signal.