Baek Ho Jung

An unconditionally stable scheme for the finite-difference time-domain method

In this work, we propose a numerical method to obtain an unconditionally stable solution for the finite-difference time-domain (FDTD) method for the TE/sub z/ case. This new method does not utilize the customary explicit leapfrog time scheme of the conventional FDTD method. Instead we solve the time-domain Maxwells equations by expressing the transient behaviors in terms of weighted Laguerre polynomials. By using these orthonormal basis functions for the temporal variation, the time derivatives can be handled analytically, which results in an implicit relation. In this way, the time variable is eliminated from the computations. By introducing the Galerkin temporal testing procedure, the marching-on in time method is replaced by a recursive relation between the different orders of the weighted Laguerre polynomials if the input waveform is of arbitrary shape. Since the weighted Laguerre polynomials converge to zero as time progresses, the electric and magnetic fields when expanded in a series of weighted Laguerre polynomials also converge to zero. The other novelty of this approach is that, through the use of the entire domain-weighted Laguerre polynomials for the expansion of the temporal variation of the fields, the spatial and the temporal variables can be separated.

Solution of time domain electric field Integral equation using the Laguerre polynomials

In this paper, we propose a numerical method to obtain a solution for the time domain electric field integral equation (TD-EFIE) for arbitrary shaped conducting structures. This method does not utilize the customary marching-on in time (MOT) solution method often used to solve a hyperbolic partial differential equation. Instead we solve the wave equation by expressing the transient behaviors in terms of Laguerre polynomials. By using these causal orthonormal basis functions for the temporal variation, the time derivatives can be handled analytically. In order to solve the wave equation, we introduce two separate testing procedures, a spatial and temporal testing. By introducing first the Galerkin temporal testing procedure, the MOT procedure is replaced by a recursive relation between the different orders of the weighted Laguerre polynomials. The other novelty of this approach is that through the use of the entire domain Laguerre polynomials for the expansion of the temporal variation of the current, the spatial and the temporal variables can be separated and the temporal variables can be integrated out. For convenience, we use the Hertz vector as the unknown variable instead of the electric current density. To verify our method, we compare the results of a TD-EFIE and inverse Fourier transform of a frequency domain EFIE.

A stable solution of time domain electric field Integral equation for thin-wire antennas using the Laguerre polynomials

In this paper, a numerical method to obtain an unconditionally stable solution of the time domain electric field integral equation for arbitrary conducting thin wires is presented. The time-domain electric field integral equation (TD-EFIE) technique has been employed to analyze electromagnetic scattering and radiation problems from thin wire structures. However, the most popular method to solve the TD-EFIE is typically the marching-on in time (MOT) method, which sometimes may suffer from its late-time instability. Instead, we solve the time-domain integral equation by expressing the transient behaviors in terms of weighted Laguerre polynomials. By using these orthonormal basis functions for the temporal variation, the time derivatives can be handled analytically and stable results can be obtained even for late-time. Furthermore, the excitation source in most scattering and radiation analysis of electromagnetic systems is typically done using a Gaussian shaped pulse. In this paper, both a Gaussian pulse and other waveshapes like a rectangular pulse or a ramp like function have been used as excitations for the scattering and radiation of thin-wire antennas with and without junctions. The time-domain results are compared with the inverse discrete Fourier transform (IDFT) of a frequency domain analysis.

Solving time domain electric field Integral equation without the time variable

An improved testing procedure using the marching-on-in-order method to solve the time-domain electric field integral equation (TD-EFIE) for conducting objects using the Laguerre polynomials is presented. Exact temporal testing is performed before the spatial testing, therefore the retarded terms composed of the spatial and the temporal variables can be analytically separated. The uniqueness of this testing procedure is that the time variable can be analytically integrated out and the accuracy can be improved. This paper is then an improvement over the earlier marching-on-in-order method. In addition, this methodology is quite different from the conventional marching-on-in-time algorithm as the present method leads to a set of final equations which need to be numerically solved containing only the spatial variables. Therefore, there is no requirement to have a Courant stability condition in this procedure. How the singular integrals are treated is also discussed. Several examples are simulated both for radiation and scattering problem. The results are compared with the inverse discrete Fourier transform of the frequency domain data and they agree well.

Transient electromagnetic scattering from dielectric objects using the electric field Integral equation with Laguerre polynomials as temporal basis functions

In this paper, we propose a time-domain electric field integral equation (TD-EFIE) formulation for analyzing the transient electromagnetic response from three-dimensional (3-D) dielectric bodies. The solution method in this paper is based on the Galerkins method that involves separate spatial and temporal testing procedures. Triangular patch basis functions are used for spatial expansion and testing functions for arbitrarily shaped 3-D dielectric structures. The time-domain unknown coefficients of the equivalent electric and magnetic currents are approximated using a set of orthonormal basis function that is derived from the Laguerre functions. These basis functions are also used as the temporal testing functions. Use of the Laguerre polynomials as expansion functions for the transient portion of response enables one not only to handle the time derivative terms in the integral equation in an analytic fashion but also completely separates the space and the time variables. Thus, the time variable along with the Courant condition can be eliminated in a Galerkin formulation using this procedure. We also propose an alternative formulation using a different expansion of the magnetic current. The total computational cost for this new method is similar to that of an implicit marching-on in time (MOT)-EFIE scheme, even though at each step this procedure requires more computations. Numerical results involving equivalent currents and far fields computed by the two proposed methods are presented and compared.

Conditions for generation of stable and accurate hybrid TD-FD MoM solutions

Broadband characterization of any electromagnetic (EM) data (e.g., surface currents, radiation pattern, and network parameters) can be carried out using partial information in the time domain (TD) and the frequency domain (FD). In this hybrid TD-FD approach, one generates the early time response using a TD code at a spatial location and uses a FD code to generate the low-frequency response at the same place. Then, the partial complementary information in both the TD and FD is fit by a set of orthogonal functions and its Fourier transform having the same expansion coefficients. Three different types of functions, namely, Hermite, Bessel-Chebyshev, and Laguerre, have been used for extrapolation. Once the expansion coefficients for these functions are known, the response can be extrapolated either for late times or high frequencies using the initial partial information. The objective of this paper is to explore the conditions under which this hybrid approach yields a stable and accurate solution. We investigate bounds for both the number of orthogonal functions needed to carry out the extrapolation and the scale factors needed to accurately fit the data in time and in frequency. Numerical examples have been presented to illustrate the efficacy of these bounds. It is important to point out that, in this hybrid approach of extrapolation, we are not creating new information but processing the available information in an intelligent fashion

Solution of time domain PMCHW formulation for transient electromagnetic scattering from arbitrarily shaped 3-D dielectric objects

In this paper, we analyze the transient electromagnetic response from three-dimensional (3-D) dielectric bodies using a time domain PMCHW (Poggio, Miller, Chang, Harrington, Wu) integral equation. The solution method in this paper is based on the Galerkins method that involves separate spatial and temporal testing procedures. Triangular patch basis functions are used for spatial expansion and testing functions for arbitrarily shaped 3-D dielectric structures. The time domain unknown coefficients of the equivalent electric and magnetic currents are approximated by a set of orthonormal basis functions that are derived from the Laguerre functions. These basis functions are also used as the temporal testing. Use of the Laguerre polynomials as expansion functions characterizing the time variable enables one to handle the time derivative terms in the integral equation and decouples the space-time continuum in an analytic fashion. We also propose an alternative formulation using a differential form of time domain PMCHW equation with a different expansion for the equivalent currents. Numerical results computed by the two proposed methods are presented and compared.

Time domain efie and mfie formulations for analysis of transient electromagnetic scattering from 3-D dielectric objects

In this paper, we investigate various methods for solving a time-domain electric field integral equation (TD-EFIE) and a time- domain magnetic field integral equation (TD-MFIE) for analyzing the transient electromagnetic response from three-dimensional (3-D) dielectric bodies. The solution method in this paper is based on the method of moments (MoM) that involves separate spatial and temporal testing procedures. Triangular patch basis functions are used for spatial expansion and testing functions for arbitrarily shaped 3-D dielectric structures. The time-domain unknown coefficients of the equivalent electric and magnetic currents are approximated using a set of orthogonal basis functions that is derived from the Laguerre functions. These basis functions are also used as the temporal testing. Numerical results involving equivalent currents and far fields computed by the proposed TD-EFIE and TD-MFIE formulations are presented and compared.

Use of discrete Laguerre sequences to extrapolate wide-band response from early-time and low-frequency data

Extrapolation of wide-band response using early-time and low-frequency data has been accomplished by the use of the orthogonal polynomials, such as Laguerre polynomials, Hermite polynomials, and Bessel-Chebyshev functions. It is a good approach to reduce the computational loads and obtain stable results for computation intensive electromagnetic analysis. However, all the orthonormal basis functions that have been used are all continuous or analog functions, which means we have to sample the polynomials both in time and frequency domains before we can use them to carry out the extrapolation. The process of sampling will introduce some errors, especially for high degrees or small scaling factors and, hence, may destroy the orthogonality between the polynomials of various degrees in a discrete sense. In this paper, we introduce the discrete Laguerre functions, which are directly derived using the Z transform and, thus, are exactly orthonormal in a discrete sense. The discrete Laguerre polynomials are fundamentally different from its continuous counterparts, except asymptotically when the sampling interval approaches zero. The other advantage of using these discrete orthonormal functions is that they do not give rise to the Gibbs phenomenon unlike its continuous counterpart. Using it in the extrapolation, the range or convergence can be extended both for the scaling factor and order of expansion, and at the same time, the quality of performance can be improved. Since the error of extrapolation is sensitive to the scaling factor, an efficient way to estimate the error as a function of the scaling factor is explained and its feasibility for any problem is validated by numerical examples of antennas.

Transient scattering from three-dimensional conducting bodies by using magnetic field integral equation

In this paper, we present a time-domain magnetic field integral equation (TD-MFIE) to obtain the transient scattering response from three-dimensional closed conducting bodies. The formulation is described for explicit and implicit solutions. This approach results in accurate and comparably stable transient responses from conducting scatterers. Detailed mathematical steps are included, and several numerical results are presented and compared with results from a time-domain electric field integral equation (TD-EFIE) and the inverse Fourier transform solution of the frequency domain results.