FDTD modeling and analysis of a broadband antenna suitable for oil-field imaging while drilling

Abstract

This paper develops a finite-difference time-domain (FDTD) approach suitable for modeling general antenna structures which include cylindrical dipoles. The medium averaging is a key factor of the method since the FDTD grid size is comparable to the dimensions of the antenna. It is essential in the FDTD calculation to account for the effective media seen by different electric field components. The input impedance calculated by the FDTD approach agrees well with the published results for a cylindrical dipole, although only one cell in the transverse direction is assigned as the metallic conductor. This FDTD approach is then used to model and analyze a novel broadband antenna which can be used in a borehole radar system for oil-field imaging while drilling. The antenna is essentially a horizontal cut on a metallic drill collar with a vertical cylindrical dipole placed across the cut and loaded at the ends. Based on the modeling results, we found that a proper resistive loading is the key in order to increase the bandwidth of the antenna and reduce ringing. When properly loaded, the antenna has a fairly flat input impedance ranging from 50 MHz to 600 MHz. The real part of the input impedance varies favorably around 75 Ω. In addition, the input impedance of the antenna is not very sensitive to the surrounding medium. The simulated antenna radiation of the electric field shows good azimuthal discrimination. For a lossless medium with a permittivity of 20ε 0 the front-back ratio climbs from around 15 dB at 100 MHz to about 45 dB at 600 MHz.