Showing papers by "George A. Kyriacou published in 2005"

Insulated Cylindrical Antenna in a Cold Magnetoplasma

Abstract: A study is made of the characteristics of a perfectly conducting cylindrical antenna insulated from the surrounding cold collisionless magnetoplasma by an isotropic coaxial cylindrical sheath for the case where the antenna is aligned with an external magnetic field and is excited by means of a delta-function voltage generator. A rigorous representation is obtained for the current distribution on an infinitely long antenna. It is shown that in the whistler frequency range, the current distribution of a sufficiently thin antenna is determined mainly by the eigenmode whose guided propagation is found to be supported along the antenna. Based on the results obtained for an infinitely long antenna, a generalized transmission-line theory is developed for determining the current distribution and the input impedance of an insulated antenna of finite length located in a resonant magnetoplasma. The influence of the sheath parameters on the antenna characteristics is analyzed.

Analysis of Circular Patch Antenna Tuned by a Ferrite Post

Abstract: A circular patch antenna tuned by a ferrite post is presented. The DC-biasing magnetic field is assumed perpendicular to the patch and the ferrite losses are taken into account. Both right- and left-hand circular polarizations are considered. The “perfect magnetic walls” approximation is employed and the resulting closed-form expressions can be used in the design of patch antennas. Also, this approximate but analytical study offers a clear physical insight with regard to the ferrite effects on the patch antenna's resonance characteristics. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 234–237, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20954

Eigenvalue Analysis of Curved Open Waveguides using a Finite Difference Frequency Domain Method Employing Orthogonal Curvilinear Coordinates

Abstract: Eigenvalue analysis of open curved geometries is performed by using a two dimensional (2-D) Finite Difference Frequency Domain (FDFD) eigenvalue method employing orthogonal curvilinear coordinates, in conjunction with a perfectly matched layer (PML) tensor. This method can be used to compute the dispersion characteristics of open curved structures such as open microstrip lines printed on curved substrates. Numerical results for the eigenvalues of several geometries are presented, and compared against already published results, so as to validate the accuracy of the method.

A Modified Perturbation Method for Three-Dimensional Time Harmonic Impedance Tomography

Abstract: A reconstruction algorithm for three dimensional time harmonic impedance imaging based on the Modified Perturbation Method (MPM), [1], is proposed. Both the object conductivity (σ) and permittivity (e) are reconstructed. The original Perturbation method developed for static problems was modified in order to apply in time harmonic problem in higher frequency. In this case complex permittivity and complex voltages are involved. So, major modifications have been made in order to achieve accepted results. The jacobian matrix is expressed in complex form and the modified perturbation reconstruction algorithm formulated accordingly. A number of successful reconstructions were carried out for different complex permittivity profiles, but all of them based on a computer phantom approach.

A Nonlinear Eigenvalue Hybrid FEM Formulation for Two Dimensional Open Waveguiding Structures

Abstract: A non linear hybrid finite element formulation, for the two dimensional eigenvalue analysis of open waveguides is proposed. The infinite solution domain of this kind of problems is divided into two region using a fictitious cylindrical surface-C. Inside the surface-C the finite element method is employed. Outside the surface-C the infinite domain is modeled through an infinite sum of cylindrical harmonics. The two solutions are coupled considering the continuity of the tangential field components along the surface-C. The overall procedure ends up to a nonlinear eigenvalue problem of the form A(β) � x = 0 where β is the propagation constant along the axis of the waveguide. For the solution of the nonlinear eigenvalue problem the Regula Falsi method is considered. The solution procedure is based on the initial values provided from a linear approximation of the problem. Finally, the validity of the method is verified by comparison with measurements presented in the bibliography. Introduction During the last years a particular research effort is directed towards the solution of the eigenvalue problem of arbitrary cross-section waveguiding structures in a unified and general way. All of the numerical techniques developed towards this direction try to convert the open-radiating problem to an equivalent closed one and in that way truncating the solution domain. This can be achieved either by making use of an artificial boundary transparent to the solution or by combining the Finite Element Method (FEM) with methods, such as the method of moments, capable of modeling the unbounded region. When the artificial boundary is considered one method to truncate the solution domain is to impose on it either the Absorbing Boundary Conditions (ABCs) or employ the Perfect Matching Layer (PML). An alternative method is to express the field in the unbounded region as an expansion of solution satisfying both the Maxwell equations and the radiation condition. However, for the two dimensional (2D) open waveguides, the performance of the already proposed techniques in the solution of the corresponding eigenvalue problem, is very poor. In particular, while PML is quite efficient in the estimation of the field distribution generated by a specific source, when this is used in the solution of eigenvalue problem leads to spurious (or corrupted) solutions. In the present work a hybrid finite element method capable of handling problems involving open arbitrary shaped waveguides is described. The problem at a first stage is approximated by means of a linear eigenvalue formulation. The formulation is derived by combining the finite element method and an approximate expansion in cylindrical harmonics (1). Namely, the radial wavenumber in the unbounded media is considered approxi- mately equal to that of free space. This is a reasonable approximation for the spectral region only around cut-off. The eigenvalues calculated using this approach were in good agreement with experimental results. However, aiming at a more generally valid method, the present effort considers the accurate radial wavenumber, which unfortunately (as it is already expected) yields a non-linear eigenvalue problem. The final nonlinear algebraic system is formulated employing a full electric field FEM formulation discretized by mixed edge/node triangular elements. The final nonlinear system is solved using the Regula Falsi technique (2) and employing the solution of the first linear approach as an initial guess.