Veysel Demir

FDTD formulation for dispersive chiral media using the Z transform method

A finite-difference time-domain (FDTD) scattered-field formulation for dispersive chiral media is developed and presented in this paper. The FDTD formulation is based on the Z transform method and models the frequency-dependent dispersive nature of permittivity, permeability, and chirality as well. The permittivity and permeability are assumed to follow the Lorentz model while the chirality is assumed to follow the Condon model. The formulation is developed for three-dimensional electromagnetic applications. Results of this formulation are presented for the copolarization and cross-polarization of the reflected and transmitted waves from a chiral slab due to normal incidence of a plane wave and for the scattered field from a chiral sphere, a chiral cube, and a finite chiral cylinder. Validation is performed by comparing the results with those based on the exact solutions and those obtained from method of moments solutions.

PERMITTIVITY MEASUREMENT WITH A NON-STANDARD WAVEGUIDE BY USING TRL CALIBRATION AND FRACTIONAL LINEAR DATA FITTING

Modifications in the measurement of the complex permittivity are described, based on the transmission and reflection coefficients of a dielectric slab. The measurement uses TRL two- port calibration to bring the reference planes accurately to the sample surface. The complex permittivity as a function of frequency is computed by minimizing the difference between the measured and the ideal scattering parameters. An alternative procedure for determining the complex permittivity uses the fractional linear data fitting to a Q- circle of the virtual short-circuit and/or virtual open circuit data. In that case, the sample must be a multiple of one-quarter wavelength long within the measured range of frequencies. Comparison with results obtained by other traditional procedures is provided.

Plane wave scattering from three dimensional multiple objects using the iterative multiregion technique based on the FDFD method

An iterative approach using the finite difference frequency domain method is presented in this paper in order to solve the problem of scattering from large three-dimensional electromagnetic scatterers. The idea of iterative multiregion technique is introduced to divide one computational domain into smaller subregions and solve each subregion separately. Then the subregion solutions are combined iteratively to obtain a solution for the complete domain. As a result, a considerable reduction in the computation time and memory has been achieved.

Planar dipole antennas near the edge of an EBG ground plane for WLAN applications

The dipole antenna is a simple and fundamental radiator and meets requirements for an antenna structure with a low profile and compact configuration. However, it cannot radiate efficiently near a perfect electric conductor (PEC) ground plane due to the reverse image currents. The paper presents the idea of using an EBG ground plane to improve the radiation efficiency of a nearby planar dipole antenna. The validity of the idea has been demonstrated by both FDTD simulations and measured results. The proposed antenna structure has a good potential for wireless communications, such as WLAN, and radar systems.

The FDFD with the iterative multi-region technique for the scattering from multiple three dimensional objects

This paper presents a new technique based on an iterative approach using the finite difference frequency domain method to solve large electromagnetic scattering problems. It depends on dividing the computational domain into smaller sub-domains and solving each sub-domain separately. After performing a number of interactions between the sub-domains, the solution for the complete domain can be obtained.

Scattering from two dimensional problems using the iterative multi-region technique for large scale problems based on the FDFD method

The paper presents a new technique to solve large electromagnetic scattering problems, based on dividing the computational domain into smaller sub-domains and solving each sub-domain separately. Then, after performing a number of interactions between the sub-domains, the solution for the complete domain can be defined. An iterative approach using the FDFD method is presented to solve this class of problems that can be divided into separated sub-domains.

Dual-Grid Finite-Difference Frequency-Domain Method for Modeling Chiral Medium

A dual-grid finite-difference frequency-domain (DG-FDFD) method is introduced to solve for scattering of electromagnetic waves from bianisotropic objects. The formulations are based on a dual-grid scheme in which a traditional Yee grid and a transverse Yee grid are combined to achieve coupling of electric and magnetic fields that is imposed by the bianisotropy. Thus the underlying grid naturally supports the presented formulations. Introduction of a dual-grid scheme doubles the number of electromagnetic field components to be solved, which in turn implies increased time and memory of the computational resources for solution of the resulting matrix equation. As a remedy to this problem, an efficient iterative solution technique is presented that effectively reduces the solution time and memory. The presented formulations can solve problems including bianisotropic objects. The validity of the formulations is verified by calculating bistatic radar cross-sections of three-dimensional chiral objects. The results are compared with those obtained from analytical and other numerical solutions.

Efficient Analysis of Electromagnetic Scattering Problems using a Parallel-Multigrid Iterative Multi-Region Algorithm

This paper presents the use of multigrid (MG) technique to enhance the solution process of the FDFD method, hence speeding up the computational process of the iterative multi-region (IMR) algorithm. Furthermore, to achieve a high-performance computing, the presented IMR-MG technique becomes more efficient on a parallel platform, through the usage of multiprocessing, where each sub-region is solved on a separate processor. An example of the scattering from 2 two-dimensional objects using the IMR algorithm with multigrid technique in conjunction with parallel processing is presented to show the capabilities of the presented procedure to analyze large scattering problems with reasonable computer resources

Second-order adjoint sensitivities

The authors show in this chapter, two different approaches for efficiently estimating the second-order derivatives (Hessian matrix) of a given objective function. The cost of evaluating the Hessian using classical finite difference approach is O(n2) where n is the number of parameters. The first adjoint approach reduces the cost of estimating all components of the Hessian matrix to only 2n extra simulations. This approach is simple, and it uses the algorithms developed in previous chapters. A second approach for estimating the complete Hessian is also presented. This approach is more complex than the first approach and requires extra memory storage. This approach requires only n + 1 extra simulations per Hessian evaluation. It follows that the computational cost is approximately one half of the first adjoint approach. This saving comes at the cost of a more complex algorithm and more extensive storage.

Adjoint sensitivity analysis of anisotropic structures

In this chapter, we present an algorithm for adjoint sensitivity analysis of anisotropic materials. We show that using only one extra adjoint simulation, the sensitivities of the objective function with respect to all parameters are estimated. The material property tensors of the adjoint problem are the transpose of those of the original problem. The computational cost of the adjoint problem is the same as that of the original simulation. The derivation considered in this chapter addresses the nondispersive case. The considered tensors are assumed to be independent of time. The formulation presented in this chapter is adapted. The anisotropic and dispersive case is still a subject of research.