Investigation of wideband Coaxial-Fed Circular dipole Array antenna in a Borehole

In this paper, we study the design of a Coaxial-Fed Circular dipole Array in a Borehole (CFCAB) for operating frequencies between 20-250 MHz. The CFCAB is used for directional borehole radar. In the antenna, there is a long conducting cylinder near the array elements, and scattered waves at the cylinder interfere with received waves in the array antenna. Utilizing the interference criterion, which was previously proposed, we found that the antenna works in the frequency band, when a total length of the cylinder is 40 cm. In order to verify the design, we conducted field experiments in granite, and confirmed that the antenna with the cylinder can correctly estimate the direction of arrival.