Influence of feed line on DOA estimation with dipole array antenna for directional borehole radar

In this paper, we investigate the influence of the feed lines on the radar signals in the dipole array antenna fed by coaxial cables for directional borehole radar. In this investigation, we utilize a model of dipole antenna elements in circle near a conducting cylinder in a borehole. The criterion proposed in this paper is calculated, and quantifies the influence of the conducting cylinder. This criterion is tested with the data obtained in experiments in air, and we verified that the criterion represents the influence of the cylinder on the direction estimation. We made the directional borehole radar system with the dipole array antenna, which was fed by semi-rigid cables. With this system, we conducted field experiments in granite. We confirmed that we can estimate direction of arrival after processing the array signals with the band-pass filter, which was designed optimally with the criterion.