High-phase-resolution 77-GHz-band radar module for near-field millimeter-wave imaging

Near-field millimeter-wave (MMW) imaging is a unique approach for inspecting cracks in covered concrete surfaces to assess the deterioration of concrete structures. We have developed a 77-GHz-band radar module with in-phase and quadrature detection which is superior to measuring both of the amplitude and phase of MMW in a near-field range. A monostatic radar system is hybrid integrated into the module with a constant-width antipodal tapered slot antenna with an antenna gain of 14.5 dBi which is connected to a microstrip-line coupler with an isolation of higher than 26 dB using a leakage cancellation technique. In distance measurements in free space, the phase resolution is less than 8 degrees corresponding to the distance resolution of smaller than 0.01 mm, which is high enough to distinguish MMW signals scattered by the cracks from multiple reflection of the concrete surfaces and inside aggregates.