Seung-Yeup Hyun

3-D Thin-Wire FDTD Approach for Resistively Loaded Cylindrical Antennas Fed by Coaxial Lines

For the efficient finite-difference time-domain (FDTD) analysis of electrically thin and resistively loaded cylindrical antennas, the 2-D cylindrical thin-wire approach with circular symmetry is extended to the 3-D Cartesian FDTD with non-cubic cells for asymmetric cases. The axial geometry of the antenna is represented as a set of piecewise-linear lumped resistors. And the near fields around the antenna and the coaxial feed aperture are approximated to the quasi-static fields with the cylindrical behavior. From the cylindrical-to-Cartesian coordinate transformation of the quasi-static fields and the contour-path integration along FDTD unit cells in the vicinity of the antenna and its feed, the 3-D Cartesian FDTD equations are derived. These equations may correspond to a full coarse-grid FDTD approach with the equivalent corrections. For some numerical examples, the proposed approach provides comparable accuracy to the reference data with fine-grid resolution. Effects of the cell size and the resistive loading profile are investigated numerically.

Efficient model of an open-ended coaxial-line probe for measuring complex permittivity

A virtually conical cable model of an open-ended coaxial- line probe for converting its measured reflection coefficients into the complex permittivity of a contacted material is presented here. Both reflection coefficients of air and pure water are calculated by employing the FDTD method, and the phase difference between the calculated and the measured reflection coefficients of pure water is used as a calibration factor of the probe. The virtually conical cable model renders the conversion of the complex permittivity of dry sand more accurate and faster than the integral equation model to the aperture admittance.

3-D Thin-Wire FDTD Analysis of Coaxial Probe Fed in Asymmetric Microwave Components

For the coaxial probe fed in asymmetric microwave components, an improved scheme on the 3-D thin-wire (TW) finite-difference time-domain (FDTD) is implemented without additional grid refinements and/or auxiliary update terms. The fine geometrical discontinuity such as the conductive arm, feed aperture, and finite end of the coaxial probe is approximated to the quasi-static models. The near fields in the vicinity of the probe end are also theoretically calculated from the uniformly charged disk model. It is shown that the spatial dependency of the near fields around the finite end of its probe agrees well with the direct solutions of the fine-grid (FG) FDTD simulation. The dominant functions of the near-field behaviors in the vicinity of the probe are easily incorporated into the correction factors of the coefficients for the 3-D Cartesian FDTD update equations. For the choice of the cell size in the proposed TW FDTD, the input admittances of a coaxial monopole probe in air are calculated and compared with the FG FDTD and the measured data. To evaluate the effects of the asymmetric geometry in the vicinity of the coaxial probe, coaxial-probe fed waveguide launchers are numerically analyzed as a function of the excitation frequency, the eccentric position, and the axial height of the coaxial probe. In comparison with the standard TW FDTD, the proposed TW FDTD provides a very close agreement with the reference data.

Thin-Wire Model Using Subcellular Extensions in the Finite-Difference Time-Domain Analysis of Thin and Lossy Insulated Cylindrical Structures in Lossy Media

Recently, a subcellular thin-wire model for the finite-difference time-domain (FDTD) simulation of resistively coated cylinders with lossless insulating and surrounding media was presented. In this paper, it is shown that this model can be extended to lossy cases. The material discontinuity between lossy insulating and surrounding media is corrected as the time-domain boundary condition. The convolution term of the boundary condition is solved by employing a recursive technique. Applying the contour-path integration to the FDTD unit cells around the wire, one may find the coarse-grid-based equation with the correction term and factors for the material discontinuity and the quasi-static field behavior around the wire. In the 2-D cylindrical coordinates with rotational symmetry, the validity of the proposed model is confirmed by an impedance analysis of insulated and resistive antennas according to the electrical properties of insulating and surrounding media, as well as the choice of cell size.

Accurate FDTD simulation of a GPR system

An accurate simulator of a GPR system is implemented by using the FDTD method. To calculate numerically the receiving responses for an actual GPR system, the simulation space contains the equivalent models of transmitting and receiving antennas, antenna feeds, dispersive ground, and buried target. Several GPR simulators were presented in literature, but all of them required additional signal processing procedures or an assumption of non-dispersive ground medium. Recently, some improved techniques of antenna modeling and the PML absorbing boundary condition for dispersive or layered media were extended by several investigators. In this paper, therefore, the conventional simulations are improved by introducing the extended techniques. An actual measurements of a GPR system with fabricated antennas is accomplished above a PVC tank filled with dry sand in our laboratory. The validity of our simulation is assured by comparing the FDTD results with the measurement data of the received voltages for a GPR in the same situation. Our simulator may be used as an effective tool in optimal design as well as performance improvement of a GPR system.

Coarse-Grid FDTD Approach for Capacitively Loaded Cylindrical Tube Antennas

A coarse-grid finite-difference time-domain (FDTD) approach for cylindrical tube antennas loaded with capacitive gaps is presented by using equivalent corrections. The loaded section of the antenna is equivalently transformed to the thin wire with the lumped capacitors. The equivalent capacitance of the loaded gap is calculated from the parallel connection of the uniform-field capacitance and the fringing-field capacitance over its gap. The near-field behaviors around the loaded and the unloaded sections of the antenna are approximated to the quasi-static fields with the radial dependency. From contour-path integration, those considerations are represented as the equivalent corrections of the FDTD update equations. This approach is only applied within one cell from the antenna axis. Thus, there are no requirements such as additional grid refinements for the fine geometry of its gap. Numerical results of the proposed approach are verified by comparing with the reference data of the capacitively loaded cylindrical tube monopole antenna driven by a coaxial line.