Effect of ferrite beads on reduction of resonance of thin conducting cylinder in a water-filled borehole

In this paper, we will utilize ferrite beads so that the coaxial-fed circular dipole array in a borehole (CFCAB) can work below 100 MHz. The CFCAB suffers from scattered wave from the centered conducting cylinder (CCC), and this disturbs estimation of direction of arrival. The commercial ferrite beads works as resistance typically above 100 MHz, and it does not consume the electromagnetic power below the frequency. However the MoM analysis showed that the ferrites on the CCC below 100 MHz works as an inductor rather than resistance, and this makes resonance frequency of the CCC lower. This leads to extension of the operating frequencies of the CFCAB. We conducted two field experiments to verify the effect of the ferrite. In these experiments, although the CFCAB without the ferrite beads failed in DOA estimation, the CFCAB with ferrites succeeded in estimation because of the effect of the ferrites.