A scaled-down experiment using cross-borehole pulse radar is performed in laboratory to extract a more precise decision rule on detecting an intrusive man-made tunnel in Korea. To minimize the total size, a scaled-down model of an air-filled tunnel in underground rock is replaced by a cylindrical ceramic rod in pure water. When the upper arm of sleeve dipole antenna of the scaled-down cross-borehole pulse radar is directly connected to coaxial cable, the measured B-scan data are contaminated by unwanted striped pattern. Such multi-path coupling between the sleeve antenna pair is generated by the glass tube-guided waves. To suppress the glass tube-guided waves, the coaxial cable is clad in ferrite cores. As the total length of the ferrite core loading is increased up to 150 mm, the unwanted coupling is suppressed gradually and the target signature can be recognized more clearly.