K.K. Mei

Time-domain finite difference approach to the calculation of the frequency-dependent characteristics of microstrip discontinuities

The frequency-dependent characteristics of the microstrip discontinuities have previously been analyzed using full-wave approaches. The time-domain finite-difference (TD-FD) method presented here is an independent approach and is relatively new in its application for obtaining the frequency-domain results for microwave components. The validity of the TD-FD method in modeling circuit components for MMIC CAD applications is established. >

A subgridding method for the time-domain finite-difference method to solve Maxwell's equations

A modification to the time-domain finite-difference method (TDFDM) that uses a variable step size is investigated. The entire computational volume is divided into a coarse grid with a large step size. A fine grid with a small step size is introduced only around discontinuities. The corresponding time increments are related to the spatial increments with the same ratio in order to minimize the numerical dispersion. The fields within the coarse and fine grids are found using the TDFDM, while an interpolation in space and time is utilized to calculate the tangential electric field on the coarse-fine grid boundary. This subgridding decreases the required computer memory and therefore expands the capability of the TDFDM. The technique is shown to be numerically stable and does not entail any extra numerical error. The method is applied to the calculation of waveguides and microstrips. >

On the integral equations of thin wire antennas

The feasibility of direct numerical calculations of antenna integral equations is investigated. It is shown that integral equation of Hallens type is the most adequate for such applications. The extension of Hallens integral equation to describe thin wire antennas of arbitrary geometry is accomplished, and results are presented for dipole, circular loops, and equiangular spiral antennas.

Calculations of the dispersive characteristics of microstrips by the time-domain finite difference method

A direct time-domain finite-difference method is used to recharacterize the microstrip. Maxwells equations are discretized both in time and space and a Gaussian pulse is used to excite the microstrip. The frequency-domain data are obtained from the Fourier transform of the calculated time-domain field values. Since this method is completely independent of all the above-mentioned investigations, the results can be considered as an impartial verification of the published results. The comparison of the time-domain results and those from the frequency-domain methods has shown the integrity of the time-domain computations. This method is very general and can be applied to model many other microwave components. >

Superabsorption-a method to improve absorbing boundary conditions (electromagnetic waves)

The authors propose a technique, which they call superabsorption, for improving absorbing boundary conditions in finite-difference time-domain methods. This method can be applied to every known absorbing boundary condition and greatly reduces the numerical error caused by the boundary reflection. The principle and analysis of the superabsorption method are presented. Numerical tests indicating the improvements obtained on many absorbing boundary conditions are reported. >

Uni-moment method of solving antenna and scattering problems

It has been shown by this investigator and numerous others [6], [7], [8] that exterior boundary value problems involving localized inhomogeneous media are most conveniently solved using finite difference or finite element techniques together with integral equations or harmonic expansions, which satisfy the radiation conditions. The methods result in large matrices that are partly full and partly sparse; and methods to solve them, such as iteration or banded matrix methods are not very satisfactory. The unimoment method alleviates the difficulties by decoupling exterior problems from the interior boundary value problems. This is done by solving the interior problem many times so that N linearly independent solutions are generated. The continuity conditions are then enforced by a linear combination of the N independent solutions, which may be done by solving much smaller matrices. Methods of generating solutions of the interior problems are discussed.

Scattering by perfectly-conducting rectangular cylinders

The problem of determining the fields scattered by a perfectly-conducting rectangular cylinder is reduced to the solution of an integral equation. This equation is then solved by digital computer methods. Data are given for surface currents, radiation patterns and scattering cross sections for both E - and H -incident waves.

Point-matched time domain finite element methods for electromagnetic radiation and scattering

Direct time domain computation of Maxwells differential equations will soon become a practical technique because of the availability of supercomputers. The principal methods used in time domain computations and the supporting theories are presented. The point-matched finite element method is chosen as the main feature of this presentation, which includes the discretization of equations, conforming mesh generation, dielectric and metallic interfaces, numerical stability and simulation of radiation conditions. Numerical results of scattering of Gaussian pulses are presented in time sequences.

Full-wave analysis of coplanar waveguide and slotline using the time-domain finite-difference method

The authors present a detailed full-wave analysis of a coplanar waveguide (CPW) and a slotline by the time-domain finite-difference (TD-FD) method. The transient waveforms propagating along the coplanar waveguide and slotline, which are excited by retarded Gaussian pulses, are found in the time domain. After the time-domain field distributions are obtained, frequency-domain parameters such as the effective dielectric constant and the complex characteristic impedance are calculated using Fourier transformations. The results agree well with available theoretical and experimental data over a wide frequency range. The validity of the quasi-TEM assumptions for CPW and slotline analyses is also checked by evaluating the ratios of the longitudinal and transverse field components directly. >

Low-frequency scattering by rectangular cylinders

An integral equation formulation is used to investigate potential problems associated with low-frequency scattering by both dielectric and perfectly conducting cylinders of rectangular cross section. Induced dipoles and scattering cross sections are obtained for 1) waves with \bar{E} or \bar{H} parallel to the axis, and 2) directions of propagation perpendicular and parallel to the broad side of the rectangle.