C. L. Zekios

DC and Imaginary Spurious Modes Suppression for Both Unbounded and Lossy Structures

A generalization of the tree-cotree technique for the removal of imaginary and dc spurious modes in finite-element-based eigenanalysis of 3-D lossy unbounded structures is introduced. Five frequently encountered types of polynomial eigenvalue problems are tackled including: 1) closed structures with finite metals conductivity losses; 2) closed structures with material losses due to migrating charge carriers; 3) open-radiating structures using the absorbing boundary conditions of both first- and second-order; 4) open-radiating structures with finite conductivity metallic objects; and 5) any combination of the aforementioned cases. The resulting polynomial eigenvalue problems are linearized utilizing both the companion and the symmetric approaches. The different linearization techniques are being compared for their efficiency and robustness.

Eigenfunction expansion for the analysis of closed cavities

A hybridization of the finite element formulation (FEM) and an eigenfunction expansion for the eigenanalysis of electrically large cavities including small objects — perturbations is proposed. These small objects are enclosed into correspondingly small fictitious boxes and the field inside them is formulated using FEM. For the large cavity field (outside the objects) an expansion into an infinite number of TE and TM modes (eigenfunctions) of the empty cavity is adopted. Equivalent electric and magnetic current densities are considered on each fictitious box surface through Loves equivalence principle. These are in turn expanded into infinite sums of all possible TE and TM modes, more generally eigenfunctions, of the corresponding box (general field solutions in a 3-D domain). In turn the field continuity conditions on these fictitious surfaces are enforced through the equivalent currents by strictly following the Dirichlet-to-Newmann mathematical formalism. The proposed approach could be considered as a domain-decomposition technique. Finally, the whole procedure yields an eigenvalue problem for the loaded cavity resonant frequencies, which involves only the FEM mesh for the small fictitious boxes. This is in turn solved using the Arnoldi algorithm to yield both resonant frequencies and the modal field distributions. The method is applied for the analysis of numerous loaded cavities and particularly to reverberation chambers.

Finite Element Based Eigenanalysis for the Study of Electrically Large Lossy Cavities and Reverberation Chambers

An eigenanalysis-based technique is presented for the study and design of large complicated closed cavities and particularly Reverberation Chambers, including conductor and dielectric material losses. Two different numerical approaches are exploited. First, a straightforward approach is adopted where the finite walls conductivity is incorporated into the Finite Element Method (FEM) formulation through the Leontovich Impedance boundary conditions. The resulting eigenproblem is linearized through an eigenvalue transformation and solved using the Arnoldi algorithm. To address the excessive computational requirements of this approach and to achieve a fine mesh ensuring convergence, a novel approach is adopted. Within this, a linear eigenvalue problem is formulated and solved assuming all metallic structures as perfect electric conductors (PEC). In turn, the resulting eigenfunctions are post- processed within the Leontovich boundary condition for the calculation of the metals finite conductivity losses. Mode stirrer design guidelines are setup based on the eigenfunction characteristics. Both numerical eigenanalysis techniques are validated against an analytical solution for the empty cavity and a reverberation chamber simulated by a commercial FEM simulator. A series of classical mode stirrers are studied to verify the design guidelines, and an improved mode stirrer is developed.

A three dimensional finite element eigenanalysis of reverberation chambers

A study of reverberation chambers based on a three dimensional finite element eigenanalysis is elaborated. For this purpose the electric field vector wave equation is transformed into its weak form using the Galerkin procedure and discretized with the aid of tetrahedral edge elements. The resulting system of equations is then formulated into a generalized eigenproblem for a complex frequency (ω). When losses are ignored this eigenproblem is linear but is highly singular resulting to spurious solutions, a characteristic common to all numerical methods when analyzing lossless cavities. Including wall and material losses the singularity is reduced but the eigenproblem becomes nonlinear. Herein, the lossless case is considered first with an attempt to reject spurious solutions. Artificial air losses are then introduced to handle the singularity and the effects of these losses are checked against analytical solutions for a simple cavity. The rigorous non — linear eigenproblem including the realistic conducting walls and materials losses is currently under consideration.

Reducing transmission complexity in MIMO-WCDMA networks employing Principal Component Analysis

The goal of this paper is to investigate the performance of MIMO-WCDMA networks, where Principal Component Analysis (PCA) is employed at the reception. Multipath propagation is exploited, as the individual received signals can be seen as different instances of the same physical phenomenon (i.e. transmission and reception of WCDMA sequences). In this context, the received data are first transformed using an orthogonal representation. Afterwards, the constructed covariance matrix is used in order to reduce the overall complexity of a proposed transmission strategy for signal transmission in MIMO-WCDMA networks in diversity combining transmission mode. As results indicate, for a 2×2 MIMO orientation (i.e. two transmit and receive antennas) and six multipath components, the complexity of the proposed algorithm can be reduced up to 15%/60% for SNR equal to 0/5 dB, respectively.

Eigen-analysis of composite radiating structures with emphasis on characteristic modes: A review

Eigenanalysis of canonically shaped structures served as an indispensable tool the conception and establishment of a plethora of electromagnetic structures from filters to cavities and antennas. However, the recent evolution of radio systems and wireless communications ask for the design of multifunctional rf front-ends in compact form and conformally integrated on mobile devices. The eigen-analysis features could again support this evolution but could only be performed numerically. It is toward this ambitions aims that our research effort is directed during the last years. This article presents an overview of the related work, which is based on finite difference, finite element, mode matching and moment method. The applications elaborated include closed as well as open-radiating structures loaded with inhomogeneous and/or anisotropic media. Particular effort is devoted to external eigenmodes and characteristic modes, especially for antenna design.

Printed antennas studies based on equivalent magnetic current characteristic modes

In the past 10 years due to their attractive features, the characteristic modes have been applied widely for the wideband and Multiple Input Multiple Output (MIMO) antennas design. In these works the radiator was a metallic structure located in air. Fabres in [M. C. Fabres, “Systematic Design of antennas using the theory of characteristic modes.” Universidad Politecnica de Valencia, Valencia, Ph.D. dissertation, pp 118–120, February 2007] applied the classical theory of characteristic modes to study parallel plates separated by air. However, utilizing the typical characteristic modes analysis the resulting electric eigencurrents do not give clear information about the structure radiation.

Eigenanalysis of photonic structures based on finite element with efficient suppression of spurious modes

The establishment of finite element based eigenanalysis as a numerical tool for the indepth understanding and revealing the characteristics of Terahertz and photonic structures, constitute the scope of this work. A tree-cotree splitting formulation method is used for the removal of imaginary and dc spurious modes. The Ritz vectors are appropriately restricted during the linearization technique of the polynomial eigenproblem. The capabilities of the method are demonstrated presenting certain microdisk and microrings whispering gallery modes eigenvalues and eigenvectors.

Exploiting characteristic mode theory for the design of a chipless RFID tag

The work at hand explores the chipless RFID coding enhancement via the employment of the characteristic mode theory. The presented results are comparable in performance with some of the state of the art ones giving confidence that much better coding density can be achieved.

A novel leaky mode and mode coupling effects occurring in finite substrate microstrip lines and patch antennas

Mode coupling phenomena on the finite substrate microstip lines are studied. The effect of the finite substrate is related to the general concept of mode coupling initially appeared on stub loaded leaky wave structures. The nonlinear nature of the analysis of open waveguides in general is addressed. Common misunderstandings on the use of HFSS™ regarding such an analysis are dicussed. Preliminary results based on a non-linear eigensolver [1], and their interpretation considering the effect of the finite substrate and the occurrence of a new propagating substrate mode will be presented. Finally, some comments are made on the consequences of this mode and its coupling to the first order leaky mode on the design of patch and leaky wave antennas.