In this paper, we propose a coaxial-fed circular dipole array in a borehole (CFCAB) for directional borehole radar and present a design method for the CFCAB. In the CFCAB, a circular dipole array is arranged around a conducting cylinder. When a wave is incident on the antenna, we may divide the signals received at the antenna into two components. One is caused by the waves arriving at the antenna directly, and the other is due to the waves scattered at the conducting cylinder. We proposed a criterion to quantify the influence of the conducting cylinder on the radar measurement. We show results of experiments in air and in granite to examine the validity of the criterion. Making use of the criterion, we designed and built the directional borehole radar system with the CFCAB working at around 200 MHz. Conducting the field experiments in granite, we confirmed that the CFCAB can estimate the directions of arrival waves on the time-frequency plane, as we expected with the criterion.Index Terms-Circular array, coaxial cable, dipole antenna, directional borehole radar, Method of Moments (MoM).

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