Optical measurement of millimeter-wave antenna for imaging concealed cracks on concrete surface

We have developed a handy near-field imaging system (Crack Scan) to nondestructive inspect decrepit concrete structures. In Crack Scan, 77-GHz band tapered slot antennas (TSAs) are arrayed to simultaneously detect MMWs scattered from surface cracks on the concrete covered with paint, wallpaper, and repair material. An optical electric-field sensor based on electro-optic effect characterizes the TSA to improve the spatial resolution of Crack Scan. Antenna measurements with the optical electric-field sensor demonstrated that our TSA with substrate cutting and corrugation provides high directivity. A measured near-field distribution indicates that directivity and polarization is degraded by an antenna electrode formed on each face of a substrate. Those results present useful guidelines to design the high directive antenna of Crack scan.