Title :
FDTD modeling and analysis of a broadband antenna suitable for oil-field imaging while drilling
Author :
Chen, Yong-Hua ; Coates, Richard T. ; Chew, Weng Cho
Author_Institution :
Schlumberger-Doll Res., Ridgefield, CT, USA
fDate :
2/1/2002 12:00:00 AM
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 20 ε0 the front-back ratio climbs from around 15 dB at 100 MHz to about 45 dB at 600 MHz
Keywords :
UHF antennas; VHF antennas; antenna radiation patterns; dipole antennas; electric impedance; finite difference time-domain analysis; geophysical equipment; geophysical prospecting; radar antennas; radar imaging; remote sensing by radar; 100 to 700 MHz; FDTD modeling; antenna radiation; antenna structures; azimuthal discrimination; bandwidth; borehole radar system; broadband antenna; cylindrical dipoles; electric field; finite-difference time-domain approach; front-back ratio; input impedance; lossless medium; medium averaging; metallic drill collar; oil-field drilling; oil-field imaging; resistive loading; ringing; Broadband antennas; Conductors; Dipole antennas; Finite difference methods; Image analysis; Impedance; Loaded antennas; Radar antennas; Radar imaging; Time domain analysis;
Journal_Title :
Geoscience and Remote Sensing, IEEE Transactions on