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doc. Ing. Lukáš Jelínek, Ph.D.

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A Generalized Friis Transmission Formula for Multiport Antenna Arrays

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., prof. Ing. Miloslav Čapek, Ph.D., Clavelier, E., Warnick, K., Broyde, F.
  • Publikace: Proceedings of IEEE International Symposium on Antennas and Propagation and INC/USNC‐URSI Radio Science Meeting (AP-S/INC-USNC-URSI) 2024. NEW YORK: The Institute of Electrical and Electronics Engineers, 2024. p. 1839-1840. ISSN 1522-3965.
  • Rok: 2024
  • DOI: 10.1109/AP-S/INC-USNC-URSI52054.2024.10686160
  • Odkaz: https://doi.org/10.1109/AP-S/INC-USNC-URSI52054.2024.10686160
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A generalized Friis transmission formula for multi-port antenna arrays, based on the concept of unnamed power gain, is presented. The examples included demonstrate two key properties of the generalized formula: “symmetry under link direction reversal” and “vanishing sensitivity to terminations for small coupling”

An Upper Bound for the Envelope Correlation Coefficient of Antenna Clusters

  • DOI: 10.23919/EuCAP60739.2024.10501312
  • Odkaz: https://doi.org/10.23919/EuCAP60739.2024.10501312
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A method for finding the upper bound on the envelope correlation coefficient for an arbitrary number of antenna clusters is introduced. In the described case, the envelope correlation coefficient depends on applied feeding. The theory shows a matrix description of the problem, which overcomes the need for repeated evaluations of far-field integration for different feeding vectors. This work further presents a formula for finding the highest possible value of the envelope correlation coefficient when the feeding of one antenna cluster is fixed and only the feeding weights of the second cluster can be manipulated. The proposed theory is shown in a simple example of six parallel dipoles divided into three antenna clusters.

Characteristic Modes and Nonreciprocity

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Wingren, N., Schab, K., Gustafsson, M., Sjoberg, D., prof. Ing. Miloslav Čapek, Ph.D., Lundgren, J.
  • Publikace: Proceedings of IEEE International Symposium on Antennas and Propagation and INC/USNC‐URSI Radio Science Meeting (AP-S/INC-USNC-URSI) 2024. NEW YORK: The Institute of Electrical and Electronics Engineers, 2024. p. 269-270. ISSN 1522-3965.
  • Rok: 2024

Characteristic Modes of Nonreciprocal Systems

  • DOI: 10.1109/LAWP.2024.3453211
  • Odkaz: https://doi.org/10.1109/LAWP.2024.3453211
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The scattering formulation of characteristic mode decomposition is utilized to extend modal analysis to lossless scatterers breaking time-reversal symmetry. This enables characteristic modes analysis on devices containing gyrotropic or moving media. The resulting nonreciprocity introduces features not observed in reciprocal scenarios, such as asymmetric phase progression in characteristic far fields. These new phenomena are carefully discussed using examples of varying complexity. Indicators of nonreciprocity based on modal data are also introduced.

Design Recommendations for Minimal Antenna Mutual Coupling Using Current Optimization

  • DOI: 10.23919/EuCAP60739.2024.10501107
  • Odkaz: https://doi.org/10.23919/EuCAP60739.2024.10501107
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This work addresses a first attempt at using current density optimization algorithms to gain insight into the relation between maximum achievable gain in a given direction and the mutual coupling to another antenna nearby. The methodology is presented on the example of the coupling between two patch antennas placed above an infinite ground plane. The optimized antenna gain is shown to have a strong relation to the geometry of the problem. The obtained insight is then formulated into design recommendations that can help in an actual design.

Generalized Friis Transmission Formula Using Active Antenna Available Power and Unnamed Power Gain

  • DOI: 10.1109/TAP.2024.3427400
  • Odkaz: https://doi.org/10.1109/TAP.2024.3427400
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    We use the concept of active antenna available power to derive a generalization of the Friis transmission formula for multiport antenna systems. With beamformer weights chosen such that the array patterns are the same when transmitting and receiving, the active antenna available power at the receiving antenna divided by the input power at the transmitter is symmetric under link direction reversal in the near field as well as the far field. These results generalize the Friis transmission formula to beamformed multiport antenna systems in an arbitrary reciprocal propagation environment.

Maximum Radiation Efficiency of Arbitrarily Shaped Implantable Antennas

  • DOI: 10.1109/TAP.2024.3365860
  • Odkaz: https://doi.org/10.1109/TAP.2024.3365860
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Performance limitations for implanted antennas, taking radiation efficiency as the metric, are presented. The performance limitations use a convex optimization procedure with the current density inside the implant acting as its degree of freedom. The knowledge of the limitations provides useful information in design procedures and physical insights. Ohmic losses in the antenna and surrounding tissue are considered and quantitatively compared. The interaction of all parts of the system is taken into account in a full-wave manner via the hybrid computation method. The optimization framework is thoroughly tested on a realistic implanted antenna design that is treated both experimentally and as a model in a commercial electromagnetic (EM) solver. Good agreement is reported. To demonstrate the feasibility of developed performance limitations, they are compared to the performance of a loop and a dipole antenna showing the importance of various loss mechanisms during the design process. The tradeoff between tissue loss and antenna ohmic loss indicates critical points at which the optimal solution drastically changes and the chosen topology for a specific design should be changed.

Preliminary Study on Gain Maximization via Density-Based Topology Optimization

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Density formulation of topology optimization is utilized within method-of-moments formalism to optimize antenna gain. The structure of the problem's solution space is outlined and compared to a solution space formed by Q-factor minimization. Dependence of antenna gain on ohmic losses includes additional complexity to the optimization problem and may negatively influence the progress of the optimization in early stages. The influence of the utilized interpolation scheme is also investigated. Optimized designs from the current state of the formulation are also presented.

Reply to Comments on “Transducer and Radiation Efficiency Figures of a Multiport Antenna Array”

  • DOI: 10.1109/TAP.2023.3336373
  • Odkaz: https://doi.org/10.1109/TAP.2023.3336373
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this reply, we clarify the assumptions under which the upper bound on the total active reflection coefficient (TARC) corresponds to the transducer efficiency figure. We point out that this equality is valid in certain cases only and is usually not represented in practice. The reasons for this statement are centered on the difference between available and input power, and the assumption of dealing with a lossless multiport antenna.

The Role of Arrays in Pulsed Radiation

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The radiation of high-power narrow pulses is addressed with an emphasis on multiport structures, especially antenna arrays, which might significantly improve the pulse quality. Using modern computational techniques such as electric field integral equation, method of moments, and convex optimization, the radiated pulse intensity is maximized, with the excitation signal being the degree of freedom. The methodology is demonstrated in linear arrays of dipole and bowtie antennas. Significant improvement over single-port radiators is reported.

The Upper Bound on Antenna Gain and Its Feasibility as a Sum of Characteristic Gains

  • DOI: 10.1109/TAP.2023.3323763
  • Odkaz: https://doi.org/10.1109/TAP.2023.3323763
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The upper bound on antenna gain is expressed as a sum of lossy characteristic modes, specifically, as a sum of characteristic far fields squared. The procedure combines the favorable properties of Harrington’s classical approach to maximum directivity and current-density-based approaches. The upper bound is valid for any antenna or array designed in a given design region for which optimal performance is determined. The decomposition into modes makes it possible to study the degrees of freedom of an obstacle, classify its radiation into normal or super-directive currents, and determine their compatibility with a given excitation. The bound considers an arbitrary shape of the design region and specific material distribution. The cost in Q -factor and radiation efficiency is studied. The extra constraint of a self-resonance current is imposed for an electrically small antenna. The examples verify the developed theory, demonstrate the procedure’s utility, and provide helpful insight to antenna designers. The feasibility of the optimal gain is studied in detail on an example of endfire arrays using the aforementioned decomposition of optimal current density into lossy characteristic modes.

Trade-Off Between Optimal Efficiency and Envelope Correlation Coefficient of MIMO Antenna Clusters

  • DOI: 10.1109/TAP.2023.3342939
  • Odkaz: https://doi.org/10.1109/TAP.2023.3342939
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The presented article proposes a theory for the optimization of multiple-input-multiple-output antenna performance by assessing feeding coefficients. Antenna clusters with multiple feeding ports are utilized, which brings additional degrees of freedom and improves the performance. This work considers fixed shape and matching networks. The method is based on quadratic programming and maximizes total efficiency constrained by channel correlation and channel power distribution. The formulation provided in the article enables establishing trade-offs between all mentioned metrics. Evaluating the performance in this manner provides comprehensive information about the chosen geometry and port placement. Selected examples demonstrate how efficiency and channel correlation can be both in agreement but also in significant conflict. The effect of frequency dispersion on feeding is also investigated.

Characteristic Mode Decomposition of Polarizability Tensors

  • Autoři: Escobar, A., Baena, J., doc. Ing. Lukáš Jelínek, Ph.D.,
  • Publikace: Proceedings of IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting AP-S/URSI 2023. NEW YORK: The Institute of Electrical and Electronics Engineers, 2023. p. 839-840. ISBN 978-1-6654-4228-2.
  • Rok: 2023
  • DOI: 10.1109/USNC-URSI52151.2023.10238304
  • Odkaz: https://doi.org/10.1109/USNC-URSI52151.2023.10238304
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This abstract presents a method for evaluating four polarizability tensors using characteristic mode decomposition and scattering description based on the method of moments formulation of field integral equations. The method's precision is confirmed by comparison with polarizabilities obtained from the six-waves method.

Characteristic Mode Decomposition Using the Scattering Dyadic in Arbitrary Full-Wave Solvers

  • DOI: 10.1109/TAP.2022.3213945
  • Odkaz: https://doi.org/10.1109/TAP.2022.3213945
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Characteristic modes are formulated using the scattering dyadic, which maps incident plane waves to scattered far-fields generated by an object of arbitrary material composition. Numerical construction of the scattering dyadic using arbitrary full-wave electromagnetic solvers is demonstrated in examples involving a variety of dielectric and magnetic materials. Wrapper functions for computing characteristic modes in method-of-moments, finite-difference time domain, and finite-element solvers are provided as Supplementary Material.

Characteristic Modes of Frequency-Selective Surfaces and Metasurfaces from S-parameter Data

  • DOI: 10.1109/TAP.2023.3324991
  • Odkaz: https://doi.org/10.1109/TAP.2023.3324991
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Characteristic modes of arbitrary two-dimensional periodic systems are analyzed using scattering parameter data. This approach bypasses the need for periodic integral equations and allows for characteristic modes to be computed from generic simulation or measurement data. Example calculations demonstrate the efficacy of the method through comparison against a periodic method of moments formulation for a simple, single-layer conducting unit cell. The effect of vertical structure and electrical size on the number of modes is studied and its discrete nature is verified with example calculations. A multiband polarization-selective surface and a beamsteering metasurface are presented as additional examples.

Density-Based Topology Optimization in Method of Moments: Q-factor Minimization

  • DOI: 10.1109/TAP.2023.3321373
  • Odkaz: https://doi.org/10.1109/TAP.2023.3321373
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Classical gradient-based density topology optimization is adapted for method-of-moments numerical modeling to design a conductor-based system attaining the minimal antenna Q-factor evaluated via an energy stored operator. Standard topology optimization features are discussed, e.g., interpolation scheme and density and projection filtering. The performance of the proposed technique is demonstrated in a few examples in terms of the realized Q-factor values and necessary computational time to obtain a design. The optimized designs are compared to the fundamental bound and well-known empirical structures. The presented framework can provide a completely novel design, as presented in the second example.

Fundamental Bound on Maximum Antenna Gain as a Sum of Characteristic Modes

  • DOI: 10.23919/EuCAP57121.2023.10133575
  • Odkaz: https://doi.org/10.23919/EuCAP57121.2023.10133575
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Characteristic mode theory for lossy obstacles is employed to show that the fundamental bound on maximal antenna gain can be composed of a simple sum of characteristic antenna gains. This connects traditional work on bounds proposed by Harrington and modern current-density-based approaches utilizing convex optimization. The developed theory helps to understand what the true performance bound of a design region is. The paper is accompanied by an example demonstrating the working principles and verifying the developed theory.

Iterative Calculation of Characteristic Modes Using Arbitrary Full-Wave Solvers

  • DOI: 10.1109/LAWP.2022.3225706
  • Odkaz: https://doi.org/10.1109/LAWP.2022.3225706
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An iterative algorithm is adopted to construct approximate representations of matrices describing the scattering properties of arbitrary objects. The method is based on the implicit evaluation of scattering responses from iteratively generated excitations. The method does not require explicit knowledge of any system matrices (e.g., stiffness or impedance matrices) and is well-suited for use with matrix-free and iterative full-wave solvers, such as FDTD (Finite-difference time-domain method), FEM (Finite element method), and MLFMA (Multilevel Fast Multipole Algorithm). The proposed method allows for significant speed-up compared to the direct construction of a full transition matrix or scattering dyadic. The method is applied to the characteristic mode decomposition of arbitrarily shaped obstacles of arbitrary material distribution. Examples demonstrating the speed-up and complexity of the algorithm are studied with several commercial software packages.

Maximum Peak Radiation Intensity

  • DOI: 10.1109/ICECOM58258.2023.10367960
  • Odkaz: https://doi.org/10.1109/ICECOM58258.2023.10367960
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents a method for maximizing radiated power flux in a specified direction and timeframe while accounting for the radiation mechanism and impedance matching in a full-wave manner. The proposed approach leverages the formulation of field integral equations using the method of moments and employs convex optimization techniques. By considering the interplay between radiated power and impedance matching, the process enables the attainment of optimal power flux. Through rigorous analysis and simulation, this study unveils insights into the optimal performance of pulse-radiating antennas.

Maximum Radiation Efficiency of an Implantable Antenna: The Role of High-Order Modes

  • DOI: 10.23919/EuCAP57121.2023.10133397
  • Odkaz: https://doi.org/10.23919/EuCAP57121.2023.10133397
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A combination of two numerical techniques of computational electromagnetics, namely, method of moments and vector spherical wave expansion, is used to show performance limitations on the radiation efficiency of implantable antennas and to efficiently resolve computation difficulties imposed by the interaction of an electrically small radiator with its host body. The results computed for ideal and realistic radiation sources prove the significantly limited performance of implantable antennas. The role of substructure characteristic modes decomposition in the formulation of this fundamental limit is explained.

Minimization of Channel Correlation Between Antenna Clusters

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An existing method for maximization of the total efficiency of antenna clusters is modified so that the correlation between the channels can be suppressed while maintaining good balance of powers in different channels. This is achieved by setting additional constraints based on matrix description which control the system radiated power by means of weighted feeding coefficients. The developed theory is demonstrated on the example of four parallel dipoles forming two antenna clusters.

Optimal Inverse Design Based on Memetic Algorithms - Part I: Theory and Implementation

  • DOI: 10.1109/TAP.2023.3308587
  • Odkaz: https://doi.org/10.1109/TAP.2023.3308587
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A memetic framework for optimal inverse design is proposed by combining a local gradient-based procedure and a robust global scheme. The procedure is based on method-of-moment (MoM) matrices and does not demand full inversion of a system matrix. Fundamental bounds are evaluated for all optimized metrics in the same manner, providing natural stopping criteria and quality measures for realized devices. Compared to density-based topology optimization, the proposed routine does not require filtering or thresholding. Compared to commonly used heuristics, the technique is significantly faster, still preserving a high level of versatility and robustness. This is a two-part article in which the first part is devoted to the theoretical background and properties, and the second part applies the method to examples of varying complexity.

Optimal Inverse Design Based on Memetic Algorithms - Part II: Examples and Properties

  • DOI: 10.1109/TAP.2023.3308583
  • Odkaz: https://doi.org/10.1109/TAP.2023.3308583
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Optimal inverse design, including topology optimization and evaluation of fundamental bounds on performance, which was introduced in Part 1, is applied to various antenna design problems. A memetic scheme for topology optimization combines local and global techniques to accelerate convergence and maintain robustness. Method-of-moments (MoMs) matrices are used to evaluate objective functions and allow determination of fundamental bounds on performance. By applying the Shermann–Morrison–Woodbury identity, the repetitively performed structural update is inversion-free yet full-wave. The technique can easily be combined with additional features often required in practice, e.g., only a part of the structure is controllable, or evaluation of an objective function is required in a subdomain only. The memetic framework supports multifrequency and multiport optimization and offers many other advantages, such as an actual shape being known at every moment of the optimization. The performance of the method is assessed, including its convergence and computational cost.

Quality Factor Minimization of Electrically Small Antennas by Density Topology Optimization

  • DOI: 10.23919/EuCAP57121.2023.10133497
  • Odkaz: https://doi.org/10.23919/EuCAP57121.2023.10133497
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Density-based deterministic topology optimization is formulated for method-of-moments numerical technique, and the corresponding stored energy operator is exploited to evaluate and optimize antenna Q-factor. The settings are briefly discussed, including material interpolation function, density filters, and projection filters. The proposed technique is used to improve fractional bandwidth, i.e., minimize the quality factor of a current density excited on a spherical shell. The results are compared with known fundamental bounds and with realized spherical helices.

Transducer and Radiation Efficiency Figures of a Multiport Antenna Array

  • DOI: 10.1109/TAP.2023.3264834
  • Odkaz: https://doi.org/10.1109/TAP.2023.3264834
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Optimal performance of antennas used in MIMO systems is addressed in this paper with optimality being expressed in terms of the power radiated which is subject to realistic, yet unknown, excitation and fixed, however arbitrarily complicated, matching. It is shown that if excitation is not specified, the optimality of a MIMO radiating system has to be understood differently as compared to the case of a specified excitation. Consequently, two important figures of merits -- transducer and radiation efficiency figures -- are adopted to measure the quality of the MIMO radiating systems. The communication is accompanied by examples illustrating the theoretical concepts.

Upper Bound on Instantaneous Power Flux

  • DOI: 10.1109/USNC-URSI52151.2023.10237969
  • Odkaz: https://doi.org/10.1109/USNC-URSI52151.2023.10237969
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This abstract presents a method for maximization of the radiation in a given direction and a given time. The radiation mechanism and the impedance matching are taken into account in a full-wave manner. The approach is based on the method of moments formulation of field integral equations and convex optimization.

Upper Bound on Transducer and Radiation Efficiencies of a Multiport Antenna Arrays

  • DOI: 10.1109/USNC-URSI52151.2023.10238259
  • Odkaz: https://doi.org/10.1109/USNC-URSI52151.2023.10238259
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The optimal performance of MIMO antennas is studied via the tools of convex optimization, utilizing two different concepts of power entering radiating systems: available and incident power. Two figures of merit – radiation and transducer figures – are defined as Rayleigh quotients over network system matrices and used to study several cases involving dipole antennas of various arrangements and balun transformers. It is concluded that the design strategy significantly differs depending on the amount of information known about the MIMO system and its excitation.

Characteristic Mode Decomposition of Scattering Dyadic

  • Autoři: prof. Ing. Miloslav Čapek, Ph.D., Lundgren, J., Gustafsson, M., doc. Ing. Lukáš Jelínek, Ph.D., Schab, K.
  • Publikace: Proceedings of IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting AP-S/URSI 2022. NEW YORK: The Institute of Electrical and Electronics Engineers, 2022. p. 1-2. ISBN 978-1-6654-9658-2.
  • Rok: 2022
  • DOI: 10.1109/AP-S/USNC-URSI47032.2022.9886601
  • Odkaz: https://doi.org/10.1109/AP-S/USNC-URSI47032.2022.9886601
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This abstract describes the decomposition of a matrix representing a scattering dyadic into characteristic modes. Scattering dyadic, as compared to conventionally used impedance matrices, are independent of numerical method used to compute them and the same characteristic mode formulation can be used for decomposition of composite and inhomogeneous materials. The utilization of scattering dyadic makes it possible to synthesize characteristic modes which are orthogonal over prescribed portions of the far-field sphere, or ex-post decomposition of measured data. The theory is demonstrated on a simple example and its features are discussed.

Cloaking Synthesis Based on Exact Re-analysis

  • Autoři: prof. Ing. Miloslav Čapek, Ph.D., doc. Ing. Lukáš Jelínek, Ph.D., Gustafsson, M., Schab, K.
  • Publikace: Proceedings of IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting AP-S/URSI 2022. NEW YORK: The Institute of Electrical and Electronics Engineers, 2022. p. 1-2. ISBN 978-1-6654-9658-2.
  • Rok: 2022
  • DOI: 10.1109/AP-S/USNC-URSI47032.2022.9886332
  • Odkaz: https://doi.org/10.1109/AP-S/USNC-URSI47032.2022.9886332
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A novel synthesizing procedure is introduced and employed to extract locally-optimal cloaking devices. Their performance is compared with fundamental bounds found using a convex optimization approach. The optimization method is based on a method-of-moments paradigm utilizing rank-1 updates of the structure iteratively performed based on a greedy algorithm. This approach produces locally optimal shapes and its incorporation into a global search strategy is described and applied. As compared to classical topology optimization schemes, no post-processing is required. Two canonical problems are solved and presented.

Computation of Fundamental Bounds for Antennas

  • DOI: 10.23919/EuCAP53622.2022.9769248
  • Odkaz: https://doi.org/10.23919/EuCAP53622.2022.9769248
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Fundamental bounds play an important role in antenna design. Using method of moments and electric field integral equation, this paper shows a formulation of fundamental bounds on antenna metrics based on optimal current density. This methodology is applied to two representative and challenging examples. The first example examines Yagi-Uda antenna and compares it with performance limits on Q-factor, radiation efficiency, and directivity. The second example shows how to determine fundamental bounds when a designer has far-field constraints. In their entirety, the examples demonstrate variability and generality of this treatment and also recall the existence of an open-source computational package, which can be used for evaluation of fundamental bounds on various metrics including their mutual trade-offs.

Gradient-Based Topology Optimization in Method of Moments with Black & White Material Elements

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A topology optimization technique based on exact reanalysis is proposed within method-of-moments formalism. The optimization is formulated over a fixed discretization grid by performing general block structural modification. The procedure is based on an inversion-free evaluation of topological sensitivities, constituting a gradient-based local step that is iteratively restarted by the genetic algorithm. The proposed method sacrifices structural resolution at the expense of lower computational time and direct manufacturability. The method's validity and effectiveness are demonstrated in two examples.

Method of Moments and T-matrix Hybrid

  • DOI: 10.1109/TAP.2021.3138265
  • Odkaz: https://doi.org/10.1109/TAP.2021.3138265
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Hybrid computational schemes combining the advantages of a method of moments formulation of a field integral equation and T-matrix method are developed in this paper. The hybrid methods are particularly efficient when describing the interaction of electrically small complex objects and electrically large objects of canonical shapes such as spherical multi-layered bodies where the T-matrix method is reduced to the Mie series making the method an interesting alternative in the design of implantable antennas or exposure evaluations. Method performance is tested on a spherical multi-layer model of the human head. Along with the hybrid method, an evaluation of the transition matrix of an arbitrarily shaped object is presented and the characteristic mode decomposition is performed, exhibiting fourfold numerical precision as compared to conventional approaches.

Performance bounds of magnetic traps for neutral particles

  • DOI: 10.1103/PhysRevA.106.053110
  • Odkaz: https://doi.org/10.1103/PhysRevA.106.053110
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Knowledge of the fundamental limitations on a magnetic trap for neutral particles is of paramount interest to designers as it allows for the rapid assessment of the feasibility of specific trap requirements or the quality of a given design. In this paper, performance limitations are defined for convexity of magnetic trapping potential and bias field using a local approximation in the trapping center. As an example, the fundamental bounds are computed for current supporting regions in the form of a spherical shell, a cylindrical region, and a box. A Pareto-optimal set considering both objectives is found and compared with known designs of the baseball trap and Ioffe-Pritchard trap. The comparison reveals a significant gap in the performance of classical trap designs from fundamental limitations. This indicates a possibility of improved trap designs and modern techniques of shape synthesis are applied in order to prove their existence. The topologically optimized traps perform almost two times better as compared to conventional designs. Last, but not least, the developed framework might serve as a prototype for the formulation of fundamental limitations on plasma confinement in a wider sense.

Shape Regularization and Acceleration of Topology Optimization via Point Group Symmetries

  • DOI: 10.23919/EuCAP53622.2022.9769019
  • Odkaz: https://doi.org/10.23919/EuCAP53622.2022.9769019
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The existent technique for shape optimization based on exact reanalysis of method-of-moments models is extended by symmetry operators. Their application is twofold: to prescribe a given symmetry and accelerate the optimization by reducing the number of unknowns, or to penalize unsymmetrical shapes, constraining thus the regularity and simplifying potential manufacturing.

Shape Representation in a Fixed Discretization Grid for Accelerated Topology Optimization

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The topology optimization based on iterative changes of discretization is introduced. The optimal solution on coarse discretization is translated and enhanced on dense discretization. The similarity matrix used for solution translation is defined. Two examples of Q-factor minimization are shown to depict improvements in evaluation spee

Thickness Evaluation of Hollow Nonmagnetic Cylinders Utilizing a Motional Eddy Current

  • DOI: 10.1016/j.measurement.2021.110463
  • Odkaz: https://doi.org/10.1016/j.measurement.2021.110463
  • Pracoviště: Katedra elektromagnetického pole, Katedra měření
  • Anotace:
    This paper presents a way to calculate the shell thickness of nonmagnetic hollow cylinders for nondestructive applications. Aluminum cylinders with a solid structure and with a hollow structure are considered. The motion component of the induced eddy currents in a conductive cylinder is utilized to evaluate the shell thickness of hollow conductive cylinders at various frequencies and at variable speeds. One axisymmetric excitation coil and two axisymmetric pickup coils with antiserial connection are used. An analytical method using an axisymmetric computational model is developed for a parametric analysis of solid and hollow cylinder structures and shell thickness calculations, in which Fourier series are utilized. A 2D axisymmetric finite element method is also performed for a comparison with the results of the analytical method. The measurements at variable speeds and at various frequencies are presented with various hollow aluminum cylinders. The high linearity of the induced voltage versus the speed curve makes it possible to calculate the shell thickness of nonmagnetic hollow cylinders at different speeds.

Transition Matrix in Characteristic Modes Theory

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Gustafsson, M., Schab, K., prof. Ing. Miloslav Čapek, Ph.D., Moreno Pérez, E.
  • Publikace: Proceedings of IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting AP-S/URSI 2022. NEW YORK: The Institute of Electrical and Electronics Engineers, 2022. p. 291-292. ISBN 978-1-6654-9658-2.
  • Rok: 2022
  • DOI: 10.1109/AP-S/USNC-URSI47032.2022.9887318
  • Odkaz: https://doi.org/10.1109/AP-S/USNC-URSI47032.2022.9887318
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This abstract presents the versatility and computational effectiveness of characteristic mode decomposition based on the transition matrix. The described treatment is valid for all lossless and linear material distributions and is independent of the numerical method used to resolve a particular scattering scenario.

Unified Theory of Characteristic Modes - Part I: Fundamentals

  • DOI: 10.1109/TAP.2022.3211338
  • Odkaz: https://doi.org/10.1109/TAP.2022.3211338
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A unification of characteristic mode decomposition for all method-of-moment (MoM) formulations of the field integral equations describing free-space scattering is derived. The work is based on an algebraic link between the impedance and transition matrices, the latter of which was used in early definitions of characteristic modes and is uniquely defined for all the scattering scenarios. This also makes it possible to extend the known application domain of characteristic mode decomposition to any other frequency-domain solver capable of generating the transition matrices, such as finite difference or finite element methods. The formulation of characteristic modes using a transition matrix allows for the decomposition of induced currents and scattered fields from arbitrarily shaped objects, providing high numerical dynamics and increased stability, removing the issue of spurious modes, and offering good control of convergence. This first part of a two-part article introduces the entire theory, extensively discusses its properties, and offers its basic numerical validation.

Unified Theory of Characteristic Modes - Part II: Tracking, Losses, and FEM Evaluation

  • DOI: 10.1109/TAP.2022.3209264
  • Odkaz: https://doi.org/10.1109/TAP.2022.3209264
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This is the second component of a two-part article dealing with a unification of characteristic mode decomposition. This second part addresses modal tracking, interpolation, and the role of ohmic losses, and presents several numerical examples for surface-based method-of-moment (MoM) formulations. A new tracking algorithm based on the algebraic properties of the transition matrix is developed, achieving excellent precision and requiring a very low number of frequency samples when compared with procedures previously reported in the literature. The transition matrix is further used to show that characteristic mode decomposition of lossy objects fails to deliver orthogonal far-fields and to demonstrate how characteristic modes can be evaluated using the finite element method (FEM).

Upper bounds on focusing efficiency

  • DOI: 10.1364/OE.472558
  • Odkaz: https://doi.org/10.1364/OE.472558
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Upper bounds on the focusing efficiency of aperture fields and lens systems are formulated using integral equation representations of Maxwell’s equations and Lagrangian duality. Two forms of focusing efficiency are considered based on lens exit plane fields and optimal polarization currents within lens design regions of prescribed shape and available materials. Bounds are compared against the performance of classical prescriptions of ideal lens aperture fields, hyperbolic lens designs, and lenses produced by inverse design. Results demonstrate that, without regularization, focusing efficiency based solely on lens exit plane fields is unbounded, similar to the problem of unbounded antenna directivity. Additionally, results considering extruded two-dimensional dielectric geometries driven by out-of-plane electric fields for the calculation of bounds and inverse design demonstrate that aperture fields based on time-reversal do not necessarily yield optimal lens focusing efficiency, particularly in the case of near-field (high numerical aperture) focusing.

A Role of Symmetries in Evaluation of Fundamental Bounds

  • DOI: 10.1109/TAP.2021.3070103
  • Odkaz: https://doi.org/10.1109/TAP.2021.3070103
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A problem of the erroneous duality gap caused by the presence of symmetries is solved in this paper utilizing point group theory. The optimization problems are first divided into two classes based on their predisposition to suffer from this deficiency. Then, the classical problem of Q-factor minimization is shown in an example where the erroneous duality gap is eliminated by combining solutions from orthogonal sub-spaces. Validity of this treatment is demonstrated in a series of subsequent examples of increasing complexity spanning the wide variety of optimization problems, namely minimum Q-factor, maximum antenna gain, minimum total active reflection coefficient, or maximum radiation efficiency with self-resonant constraint. They involve problems with algebraic and geometric multiplicities of the eigenmodes, and are completed by an example introducing the selective modification of modal currents falling into one of the symmetry-conformal sub-spaces. The entire treatment is accompanied with a discussion of finite numerical precision, and mesh grid imperfections and their influence on the results. Finally, the robust and unified algorithm is proposed and discussed, including advanced topics such as the uniqueness of the optimal solutions, dependence on the number of constraints, or an interpretation of the qualitative difference between the two classes of the optimization problems.

Excitation of Orthogonal Radiation States

  • DOI: 10.1109/TAP.2021.3061161
  • Odkaz: https://doi.org/10.1109/TAP.2021.3061161
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A technique of designing antenna excitation realizing orthogonal states is presented. It is shown that a symmetric antenna geometry is required in order to achieve orthogonality with respect to all physical quantities. A maximal number of achievable orthogonal states and a minimal number of ports required to excite them are rigorously determined from the knowledge of an antenna’s symmetries. The number of states and number of ports are summarized for commonly used point groups (a rectangle, a square, etc.). The theory is applied to an example of a rectangular rim where the positions of ports providing the best total active reflection coefficient, an important metric in multi-port systems, are determined. The described technique can easily be implemented in existing solvers based on integral equations.

Finding Optimal Total Active Reflection Coefficient and Realized Gain for Multi-port Lossy Antennas

  • DOI: 10.1109/TAP.2020.3030941
  • Odkaz: https://doi.org/10.1109/TAP.2020.3030941
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A numerically effective description of the total active reflection coefficient and realized gain are studied for multi-port antennas. Material losses are fully considered. The description is based on operators represented in an entire-domain port-mode basis, i.e., on matrices with favorably small dimensions. Optimal performance is investigated and conditions on optimal excitation and matching are derived. The solution to the combinatorial problem of optimal ports’ placement and optimal feeding synthesis is also accomplished. Four examples of various complexity are numerically studied, demonstrating the advantages of the proposed method. The final formulas can easily be implemented in existing electromagnetic simulators using integral equation solver.

Fundamental Bounds for Multi-Port Antennas

  • DOI: 10.23919/EuCAP51087.2021.9411454
  • Odkaz: https://doi.org/10.23919/EuCAP51087.2021.9411454
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Current-density fundamental bounds are formulated for N-port radiators utilizing a port mode basis. A prescription is given for the transformation of full-wave method-of-moments matrix operators into port matrices. Fundamental bound on the total active reflection coefficient is presented as an example. Significant reduction of size of underlying matrices allows for simultaneous optimization of ports' placement and excitation amplitudes.

Fundamental Bounds on Cloaking Based on Convex Optimization

  • DOI: 10.1109/Metamaterials52332.2021.9577149
  • Odkaz: https://doi.org/10.1109/Metamaterials52332.2021.9577149
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A convex optimization framework over contrast current density is developed to calculate fundamental bounds on the performance of linear passive cloaks. The formulation uses the method of moments applied to the electric field integral equation while using extincted power as the optimized metric. The presented results show that high cloaking efficiency requires cloaks made of low-loss and high-contrast materials.

Fundamental bounds on the performance of monochromatic passive cloaks

  • DOI: 10.1364/OE.428536
  • Odkaz: https://doi.org/10.1364/OE.428536
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Fundamental bounds on the performance of monochromatic scattering-cancellation and field-zeroing cloaks made of prescribed linear passive materials occupying a predefined design region are formulated by projecting field quantities onto a sub-sectional basis and applying quadratically constrained quadratic programming. Formulations are numerically tested revealing key physical trends as well as advantages and disadvantages between the two classes of cloaks. Results show that the use of low-loss materials with high dielectric contrast affords the highest potential for effective cloaking.

Maximum Radiation Efficiency of Implanted Antennas Employing a Novel Hybrid Method

  • DOI: 10.1109/APS/URSI47566.2021.9704153
  • Odkaz: https://doi.org/10.1109/APS/URSI47566.2021.9704153
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A hybrid combining T-matrix method and electric field integral equation is used to formulate fundamental bounds on radiation efficiency of an implanted antenna. Resulting quadratic optimization problem is solved using a dual formulation. The results present the versatility of the described computational scheme and show the optimal current densities that are the least impaired by dissipation in the tissue.

Topology Optimization of Electrically Small Antennas With Shape Regularity Constraints

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The existent framework for shape optimization of electrically small antennas is extended by a new set of geometrical operators. They are capable of operating over shapes directly, controlling their regularity, amount of used material, etc. The formulation is compatible with existent physical fitness functions and known fundamental bounds. A simple example of Q-factor minimization is presented.

Unified Approach to Characteristic Modes

  • Autoři: prof. Ing. Miloslav Čapek, Ph.D., doc. Ing. Lukáš Jelínek, Ph.D., Losenický, V., Gustafsson, M., Schab, K.
  • Publikace: Proceedings of the 2021 IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting. Piscataway (New Jersey): IEEE, 2021. p. 1-2. ISBN 978-1-7281-4670-6.
  • Rok: 2021
  • DOI: 10.1109/APS/URSI47566.2021.9703910
  • Odkaz: https://doi.org/10.1109/APS/URSI47566.2021.9703910
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An algebraic link between transition matrix and impedance matrix is utilized in this work to unify characteristic mode decomposition originally proposed by Garbacz with a framework proposed later on by Harrington and Mautz. The resulting formulation is unique to all method of moments formulations, valid for all linear and lossless materials, and has many favorable properties. The unified prescription is applicable to surface equivalence, body-of-revolution, or volumetric method of moments formulations. The unification paves a way to establish characteristic modes as a standalone technique in time-harmonic domain being independent of method of moment paradigm, i.e., to utilize - for example - finite element method instead.

Design of a Linear Antenna Array: Variable Number of Dimensions Approach

  • DOI: 10.1109/RADIOELEKTRONIKA49387.2020.9092422
  • Odkaz: https://doi.org/10.1109/RADIOELEKTRONIKA49387.2020.9092422
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A linear antenna array design is usually formulated as a binary optimization task. A global optimization algorithm is then used to turn on/off a fixed number of elements positioned on a uniform grid. This paper introduces a new formulation of the problem: the optimizer searches for distances between individual elements and their total amount at the same time. Such a formulation has to be solved using the optimization with a variable number of dimensions that enables to work with decision space vectors having different lengths. Here, the Particle Swarm Optimization algorithm is used to solve different formulations of the linear antenna array design problem. The “uniform grid” formulation is compared with the “variable number of dimensions” formulation on several optimization tasks encompassing the minimization of selected antenna parameters: the Side-Lobe Level, the First Null Beam Width and the number of used (active) elements.

Feeding Positions Providing the Lowest TARC of Uncorrelated Channels

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper, point group theory is utilized for the simultaneous block-diagonalization of all linear operators representing the underlying symmetrical structure. This procedure is utilized for designing orthogonal channels suitable, for example, for MIMO systems. Within these uncorrelated channels, the total active reflection coefficient is further formulated within the method of moments framework and is used to find position of feeders that provide orthogonal channels with maximum radiation.

Fundamental Bounds For Volumetric Structures and Their Feasibility

  • DOI: 10.23919/EuCAP48036.2020.9135082
  • Odkaz: https://doi.org/10.23919/EuCAP48036.2020.9135082
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Fundamental bounds on antenna and scattering metrics are presented in this paper utilizing volumetric method of moments. This makes it possible to investigate scenarios not solvable with classical surface method of moments which assumes that good conductors as used. One practical example is the study of plasmonic devices whose operation relies on the interaction between material properties and radiation mechanisms. The implementation of the code is briefly summarized, including some implementation hints which allow for fast evaluation of necessary matrix operators. Two optimization problems are introduced and solved for scattering and antenna problems. Feasibility of the bounds will be investigated with topology optimization and the results will be presented during the conference.

Hybrid MoM/T-Matrix Method for Analysis of Interaction Between Objects

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A hybrid method for analysis of an interaction between electromagnetic scatterers is introduced. The method connects the method of moments and T-matrix method and represents a promising candidate capable of solving problems associated with 5G or antennas close to the human body. Two specific cases of the mutual position of the objects are shown. Preliminary results are demonstrated on two examples. The advantages and limitations of the method are discussed.

Modal Tracking Based on Group Theory

  • DOI: 10.1109/TAP.2019.2943354
  • Odkaz: https://doi.org/10.1109/TAP.2019.2943354
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Issues in modal tracking in the presence of crossings and crossing avoidances between eigenvalue traces are solved via the theory of point groups. The von~Neumann-Wigner theorem is used as a key factor in predictively determining mode behavior over arbitrary frequency ranges. The implementation and capabilities of the proposed procedure are demonstrated using characteristic mode decomposition as a motivating example. The procedure is, nevertheless, general and can be applied to an arbitrarily parametrized eigenvalue problems. A treatment of modal degeneracies is included and several examples are presented to illustrate modal tracking improvements and the immediate consequences of improper modal tracking. An approach leveraging a symmetry-adapted basis to accelerate computation is also discussed. A relationship between geometrical and physical symmetries is demonstrated on a practical example.

Sub-Structure Limits to Optical Phenomena

  • DOI: 10.1109/Metamaterials49557.2020.9285045
  • Odkaz: https://doi.org/10.1109/Metamaterials49557.2020.9285045
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Fundamental limits on scattering metrics such as absorption, extinction, and Purcell’s factor are formulated by separating a scattering structure into controllable and uncontrollable regions. We present each bound as a special case of a general methodology based on the method of moments and convex optimization.

Trade-offs in absorption and scattering by nanophotonic structures

  • DOI: 10.1364/OE.410520
  • Odkaz: https://doi.org/10.1364/OE.410520
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Trade-offs between absorption and scattering cross sections of lossy obstacles confined to an arbitrarily shaped volume are formulated as a multi-objective optimization problem solvable by Lagrangian-dual methods. Solutions to this optimization problem yield a Pareto-optimal set, the shape of which reveals the feasibility of achieving simultaneously extremal absorption and scattering. Two forms of the trade-off problems are considered involving both pre-assigned loss and reactive material parameters. Numerical comparisons between the derived multi-objective bounds and several classes of realized structures are made. Additionally, low-frequency (electrically small, long wavelength) limits are examined for certain special cases.

Upper bounds on absorption and scattering

  • DOI: 10.1088/1367-2630/ab83d3
  • Odkaz: https://doi.org/10.1088/1367-2630/ab83d3
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A general framework for determining fundamental bounds in nanophotonics is introduced in this paper. The theory is based on convex optimization of dual problems constructed from operators generated by electromagnetic integral equations. The optimized variable is a contrast current defined within a prescribed region of a given material constitutive relations. Two power conservation constraints analogous to the optical theorem are utilized to tighten the bounds and to prescribe either losses or material properties. Thanks to the utilization of matrix rank-1 updates, modal decompositions, and model order reduction techniques, the optimization procedure is computationally efficient even for complicated scenarios. No dual gaps are observed. The method is well-suited to accommodate material anisotropy and inhomogeneity. To demonstrate the validity of the method, bounds on scattering, absorption, and extinction cross sections are derived first and evaluated for several canonical regions. The tightness of the bounds is verified by comparison to optimized spherical nanoparticles and shells. The next metric investigated is bi-directional scattering studied closely on a particular example of an electrically thin slab. Finally, the bounds are established for Purcell's factor and local field enhancement where a dimer is used as a practical example.

Approaching Q-Factor Bounds by Combining TM and TE Modes on a Cylindrical Shell

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Surface current densities on cylindrical shells are studied in terms of fundamental bounds on Q-factor. It is shown that the optimal current solution is not feasible by means any of practical realization. A solution with the second lowest Q-factor is found using the resonant composition of modes. This current solution is proved to be attainable as a cylindrical helix. Optimal modal composition is studied with respect to cylinder eccentricity and diameter to length ratio. It is shown that a circular cylinder yields a Q-factor close to fundamental bounds, however, the limiting case of a rectangular plate yields only a Q-factor associated with the TM bound. The physical reasons for this behaviour are explained.

Inversion-Free Evaluation of Nearest Neighbors in Method of Moments

  • DOI: 10.1109/LAWP.2019.2912459
  • Odkaz: https://doi.org/10.1109/LAWP.2019.2912459
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A recently introduced technique of topology sensitivity in method of moments is extended by the possibility of adding degrees-of-freedom (reconstruct) into underlying structure. The algebraic formulation is inversion-free, suitable for parallelization and scales favorably with the number of unknowns. The reconstruction completes the nearest neighbors procedure for an evaluation of the smallest shape perturbation. The performance of the method is studied with a greedy search over a Hamming graph representing the structure in which initial positions are chosen from a random set. The method is shown to be an effective data mining tool for machine learning-related applications.

Inversion-Free Evaluation of Small Geometry Perturbation in Method of Moments

  • DOI: 10.1109/APUSNCURSINRSM.2019.8888974
  • Odkaz: https://doi.org/10.1109/APUSNCURSINRSM.2019.8888974
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Topology sensitivity is introduced and defined to be utilized within method of moments. It is based on a series of the smallest perturbations of the investigated structure and, thanks to the application of the Woodbury matrix identity, its evaluation is inversion-free, suitable for vectorization and parallelization, and results in a fast and versatile tool for analyzing antenna optimality.

Modal Crossing Treatment Using Group Theory

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Detecting modal crossings and crossing avoidances between eigenvalue traces in modal tracking is solved based on group theory. Rules based on the presence of symmetry are applied to predictively determine mode behavior over a frequency range. The procedure can be used for a large class of generalized eigenvalue problems and is demonstrated by an example of characteristic modes. An approach leveraging this procedure to accelerate computation is also discussed.

Optimal Planar Electric Dipole Antennas: Searching for antennas reaching the fundamental bounds on selected metrics

  • DOI: 10.1109/MAP.2019.2920088
  • Odkaz: https://doi.org/10.1109/MAP.2019.2920088
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Considerable time is often spent optimizing antennas to meet specific design metrics. Rarely, however, are the resulting antenna designs compared to rigorous physical bounds on those metrics. Here we study the performance of optimized planar meander line antennas with respect to such bounds. Results show that these simple structures meet the lower bound on radiation Q-factor (maximizing single resonance fractional bandwidth), but are far from reaching the associated physical bounds on efficiency. The relative performance of other canonical antenna designs is compared in similar ways, and the quantitative results are connected to intuitions from small antenna design, physical bounds, and matching network design.

Resonance Tuning Cost in Radiation Efficiency of Electrically Small Antennas

  • DOI: 10.1109/APUSNCURSINRSM.2019.8888674
  • Odkaz: https://doi.org/10.1109/APUSNCURSINRSM.2019.8888674
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Numerical tools are used to predict the upper-bounds on radiation efficiency with and without a constraint on self-resonance. It is demonstrated that the assumption of lossless external tuning produces unrealistic results. The main result of this contribution is that, when realistic (e.g., finite conductivity) materials are used, the cost of resonance tuning in the radiation efficiency of small antenna systems is high and is manifested by a dissipation factor which scales as a fourth power of electrical size.

Shape Synthesis Based on Topology Sensitivity

  • DOI: 10.1109/TAP.2019.2902749
  • Odkaz: https://doi.org/10.1109/TAP.2019.2902749
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A method evaluating the sensitivity of a given parameter to topological changes is proposed within the method of moments paradigm. The basis functions are used as degrees of freedom which, when compared to the classical pixeling technique, provide important advantages, one of them being impedance matrix inversion free evaluation of the sensitivity. The devised procedure utilizes port modes and their superposition which, together with only a single evaluation of all matrix operators, leads to a computationally effective procedure. The proposed method is approximately one hundred times faster than contemporary approaches, which allows the investigation of the sensitivity and the modification of shapes in real-time. The method is compared with known approaches and its validity and effectiveness is verified using a series of examples. The procedure can be implemented in up-to-date EM simulators in a straightforward manner. It is shown that the iterative repetition of the topology sensitivity evaluation can be used for gradient-based topology synthesis. This technique can also be employed as a local step in global optimizers.

Topology Sensitivity in Method of Moments

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Topology sensitivity is derived for the method of moments. It is a powerful technique for studying possible shape modifications of an antenna, showing how sensitive a structure is with respect to a given parameter when the smallest possible shape perturbation is performed. The method utilizes port modes and Woodbury identity. This makes it possible to evaluate sensitivity without the necessity of inverting the impedance matrix. No modification of the existent method of moments kernel is needed. Several examples are presented with discussion of different shapes and antenna parameters. The method can be extended so it consecutively removes degrees of freedom while performing shape optimization.

Utilization of Symmetries in Method of Moments

  • DOI: 10.1109/APUSNCURSINRSM.2019.8888654
  • Odkaz: https://doi.org/10.1109/APUSNCURSINRSM.2019.8888654
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The utilization of symmetries on operators within method of moments is presented. Three examples are presented and commented upon -- (i) the separation of the problem into a set of smaller sub-problems solved independently making significant reduction in computational time, (ii) effective modal tracking where a decision on modal crossing is made in a deterministic manner, and (iii) a design of feeding schemes to obtain orthogonal channels to be used in MIMO systems.

Accurate and Efficient Evaluation of Characteristic Modes

  • DOI: 10.1109/TAP.2018.2869642
  • Odkaz: https://doi.org/10.1109/TAP.2018.2869642
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A new method to improve the accuracy and efficiency of characteristic mode (CM) decomposition for perfectly conducting bodies is presented. The method uses the expansion of the Green dyadic in spherical vector waves. This expansion is utilized in the method of moments (MoM) solution of the electric field integral equation (EFIE) to factorize the real part of the impedance matrix. The factorization is then employed in the computation of CMs, which improves the accuracy as well as the computational speed. An additional benefit is a rapid computation of far fields. The method can easily be integrated into existing MoM solvers. Several structures are investigated illustrating the improved accuracy and performance of the new method.

Accurate Evaluation of Characteristic Modes

  • Autoři: prof. Ing. Miloslav Čapek, Ph.D., Tayli, D., Akrou, L., Losenický, V., doc. Ing. Lukáš Jelínek, Ph.D., Gustafsson, M.
  • Publikace: 2018 12th European Conference on Antennas and Propagation. Bruxelles: The European Association on Antennas and Propagation, 2018. p. 1-3. ISSN 0537-9989. ISBN 978-1-78561-815-4.
  • Rok: 2018
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The well-known issue of ill-conditioned characteristic modes decomposition is challenged in this contribution. Spherical waves are utilized to decompose the resistance operator as a product of two identical matrices. The modal decomposition can be reformulated using this projection matrix to double the numerical precision and, consequently, the number of properly found modes. It has notable properties related to the numerical convergence or a priori estimation of the number of attainable modes.

Dissipation Factors of Spherical Current Modes on Multiple Spherical Layers

  • DOI: 10.1109/TAP.2018.2841408
  • Odkaz: https://doi.org/10.1109/TAP.2018.2841408
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Radiation efficiencies of modal current densities distributed on a spherical shell are evaluated in terms of dissipation factor. The presented approach is rigorous, yet simple and straightforward, leading to closed-form expressions. The same approach is utilized for a two-layered shell and the results are compared with other models existing in the literature. Discrepancies in this comparison are reported and reasons are analyzed. Finally, it is demonstrated that radiation efficiency potentially benefits from the use of internal volume which contrasts with the case of the radiation Q-factor.

Energy Stored by Radiating Systems

  • DOI: 10.1109/ACCESS.2018.2807922
  • Odkaz: https://doi.org/10.1109/ACCESS.2018.2807922
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Though commonly used to calculate Q-factor and fractional bandwidth, the energy stored by radiating systems (antennas) is a subtle and challenging concept that has perplexed researchers for over half a century. Here, the obstacles in defining and calculating stored energy in general electromagnetic systems are presented from first principles as well as using demonstrative examples from electrostatics, circuits, and radiating systems. Along the way, the concept of unobservable energy is introduced to formalize such challenges. Existing methods of defining stored energy in radiating systems are then reviewed in a framework based on technical commonalities rather than chronological order. Equivalences between some methods under common assumptions are highlighted, along with the strengths, weaknesses, and unique applications of certain techniques. Numerical examples are provided to compare the relative margin between methods on several radiating structures.

Modal Tracking Based on Group Theory

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The tracking of characteristic modes is solved based on group theory. Recent findings on symmetry aspects and the occurrence of crossing avoidance mechanism are applied and related to particular boundary conditions. A novel approach leads to the acceleration of the calculation and precise tracking of eigenmodes. The current understanding of tracking issues is broadened and the necessity of tracking is reviewed based on physical insight into the problem. The procedure can be used for a large class of generalized eigenvalue problems and is validated by an example of an antenna array made of identical radiating elements.

Optimal Currents and Optimal Antennas

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This contribution studies the realizability of already known fundamental bounds on small radiating structures using a single discrete feeder and the geometry optimization of a radiator’s shape. Shape optimization is performed by using pixelization technique powered by heuristic optimization. The feeding optimization is carried out directly. The model of an antenna is parametrized by a map between pixels of varying shape and Rao-Wilton-Glisson (RWG) basis functions. Robust post-processing suppresses the occurrence of single pixels and unwanted artifacts. Some known or recently theorized statements are verified and the complexity of the optimization problem is investigated.

Radiation Efficiency Cost of Resonance Tuning

  • DOI: 10.1109/TAP.2018.2870492
  • Odkaz: https://doi.org/10.1109/TAP.2018.2870492
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Existing optimization methods are used to calculate the upper-bounds on radiation efficiency with and without the constraint on self-resonance. These bounds are used for the design and assessment of small electric-dipole-type antennas. We demonstrate that the assumption of lossless, lumped, external tuning skews the true nature of radiation efficiency bounds when practical material characteristics are used in the tuning network. A major result is that, when realistic (e.g., finite conductivity) materials are used, small antenna systems exhibit dissipation factors which scale as (ka)–4, rather than (ka)–2 as previously predicted under the assumption of lossless external tuning.

Evaluation of polarisability tensors of arbitrarily shaped highly conducting bodies

  • DOI: 10.1049/iet-map.2016.0793
  • Odkaz: https://doi.org/10.1049/iet-map.2016.0793
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A full-wave numerical scheme of polarizability (polarisability) tensors evaluation is presented. The method accepts highly conducting bodies of arbitrary shape and explicitly accounts for the radiation as well as ohmic losses. The method is verified on canonical bodies with known polarizability tensors, such as a sphere and a cube, as well as on realistic scatterers. The theoretical developments are followed by a freely available code whose sole user input is the triangular mesh covering the surface of the body under consideration.

Minimum energy storage in dissipative electromagnetic systems

  • DOI: 10.1109/APUSNCURSINRSM.2017.8072063
  • Odkaz: https://doi.org/10.1109/APUSNCURSINRSM.2017.8072063
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This contribution discusses the concept of recoverable energy of a general electromagnetic system and practical way of its evaluation. The paper also shows how the concept is linked to fractional bandwidth, and, more significantly, how it is related to minimum energy storage and a minimum-phase-shift of Darlington's synthesis.

Numerical benchmark based on characteristic modes of a spherical shell

  • Autoři: prof. Ing. Miloslav Čapek, Ph.D., Losenický, V., doc. Ing. Lukáš Jelínek, Ph.D., Gustafsson, M., Tayli, D.
  • Publikace: Proceedings of International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting. Piscataway, NJ: IEEE, 2017. p. 965-966. ISSN 1947-1491. ISBN 978-1-5386-3284-0.
  • Rok: 2017
  • DOI: 10.1109/APUSNCURSINRSM.2017.8072525
  • Odkaz: https://doi.org/10.1109/APUSNCURSINRSM.2017.8072525
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Characteristic modes of a spherical shell are found analytically and compared with numerical solutions acquired from both in-house and commercial packages. These studies led to a proposal of several independent benchmarks, all with analytically known results. Dependence on mesh size, electrical size and other parameters can easily be incorporated. It is observed that all contemporary implementations have limitations.

Optimal Currents on Arbitrarily Shaped Surfaces

  • DOI: 10.1109/TAP.2016.2624735
  • Odkaz: https://doi.org/10.1109/TAP.2016.2624735
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An optimization problem has been formulated to find a resonant current extremizing various antenna parameters. The method is presented on, but not limited to, particular cases of gain G, quality factor Q, gain to quality factor ratio G/Q, and radiation efficiency $\eta$ of canonical shapes with conduction losses explicitly included. The Rao-Wilton-Glisson basis representation is used to simplify the underlying algebra while still allowing surface current regions of arbitrary shape to be treated. By switching to another basis generated by a specific eigenvalue problem, it is finally shown that the optimal current can, in principle, be found as a combination of a few eigenmodes. The presented method constitutes a general framework in which the antenna parameters, expressed as bilinear forms, can automatically be extremized.

Polarizability tensors of highly conductive bodies

  • DOI: 10.1109/APUSNCURSINRSM.2017.8073025
  • Odkaz: https://doi.org/10.1109/APUSNCURSINRSM.2017.8073025
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A freely available code, whose sole user input is the triangular mesh covering the surface of the body under consideration and which provides full-wave extraction of polarizability tensors, is presented. The code accepts highly conductive bodies of arbitrary shape and is verified on canonical bodies with known polarizability tensors.

Validating the Characteristic Modes Solvers

  • DOI: 10.1109/TAP.2017.2708094
  • Odkaz: https://doi.org/10.1109/TAP.2017.2708094
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Characteristic modes of a spherical shell are found analytically as spherical harmonics normalized to radiate unitary power and to fulfill specific boundary conditions. The presented closed-form formulas lead to a proposal of precise synthetic benchmarks which can be utilized to validate the method of moments matrix or performance of characteristic mode decomposition. Dependence on the mesh size, electrical size and other parameters can systematically be studied, including the performance of various mode tracking algorithms. A noticeable advantage is the independence on feeding models. Both theoretical and numerical aspects of characteristic mode decomposition are discussed and illustrated by examples. The performance of state-of-the-art commercial simulators and academic packages having been investigated, we can conclude that all contemporary implementations are capable of identifying the first dominant modes while having severe difficulties with higher-order modes. Surprisingly poor performance of the tracking routines is observed notwithstanding the recent ambitious development.

Comments on "On Stored Energies and Radiation Q"

  • DOI: 10.1109/TAP.2015.2473678
  • Odkaz: https://doi.org/10.1109/TAP.2015.2473678
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The commented paper [1] claims to provide a new expression for an energy stored around a general radiator. The major purpose of this comment is to show that the claim is unjustified. Alongside with this issue, it is pointed out that some of the core formulas of [1] are not completely correct, and that their correct form has in fact been derived elsewhere, though for the purpose of evaluating the quality factor QZ and not the stored energies.

Optimal composition of characteristic modes for minimal quality factor Q

  • DOI: 10.1109/APS.2016.7695709
  • Odkaz: https://doi.org/10.1109/APS.2016.7695709
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This work describes an efficient and powerful technique to obtain optimal currents with respect to the lower bounds of quality factor Q for an arbitrarily shaped electrically small antenna made of perfect conductor. The resulting optimal currents are expressed in terms of characteristic mode basis with the minimum always being obtained by either one mode tuned to the resonance, or, by properly combining two modes. The calculation of the stored electromagnetic energy is based on matrix representation of the integro-differential expressions derived by Vandenbosch, an approach which simplifies the entire development resulting in an uncomplicated numerical evaluation.

Optimal Composition of Modal Currents For Minimal Quality Factor Q

  • DOI: 10.1109/TAP.2016.2617779
  • Odkaz: https://doi.org/10.1109/TAP.2016.2617779
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This work describes a powerful, yet simple, procedure how to acquire a current approaching the lower bound of quality factor Q. This optimal current can be determined for an arbitrarily shaped electrically small radiator made of a perfect conductor. Quality factor Q is evaluated by Vandenbosch’s relations yielding stored electromagnetic energy as a function of the source current density. All calculations are based on a matrix representation of the integro-differential operators. This approach simplifies the entire development and results in a straightforward numerical evaluation. The optimal current is represented in a basis of modal currents suitable for solving the optimization problem so that the minimum is approached by either one mode tuned to the resonance, or, by two properly combined modes. An overview of which modes should be selected and how they should be combined is provided and results concerning rectangular plate, spherical shell, capped dipole antenna and fractal shapes of varying geometrical complexity are presented. The reduction of quality factor Q and the G/Q ratio are studied and, thanks to the modal decomposition, the physical interpretation of the results is discussed in conjunction with the limitations of the proposed procedure.

Optimal currents in the characteristic modes basis

  • DOI: 10.1109/APS.2016.7695919
  • Odkaz: https://doi.org/10.1109/APS.2016.7695919
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An optimization scheme yielding a current density in a given spatial region exhibiting the lowest tuned radiation quality factor is presented. Rather than assuming an external tuning by a lumped element, the current density is constrained to be self-resonant. The solution is presented as a weighted sum of characteristic modes and some important remarks are given with respect to their Q-optimality.

Stored Electromagnetic Energy and Quality Factor of Radiating Structures

  • DOI: 10.1098/rspa.2015.0870
  • Odkaz: https://doi.org/10.1098/rspa.2015.0870
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper deals with the old yet unsolved problem of defining and evaluating the stored electromagnetic energy - a quantity essential for calculating the quality factor, which reflects the intrinsic bandwidth of the considered electromagnetic system. A novel paradigm is proposed to determine the stored energy in the time domain leading to the method, which exhibits positive semi-definiteness and coordinate independence, i.e. two key properties actually not met by the contemporary approaches. The proposed technique is compared with an up-to-date frequency domain method that is extensively used in practice. Both concepts are discussed and compared on the basis of examples of varying complexity.

A Novel Scheme for Stored Energy Evaluation

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The old and still unsolved problem of defining and evaluating stored electromagnetic energy is treated in this contribution. A novel time-domain scheme is proposed for evaluating the stored electromagnetic energy for non-stationary fields. In order to obtain the stored energy of the total electromagnetic energy, the well-known definition of radiated energy is utilized, and the subtraction is performed in the time domain during a transient state of the system under study.

A System of Split Ring Resonators with Randomly Dispersed Resonance Frequencies

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper reports on a 3D system of broad side coupled split ring resonators with randomly dispersed resonance frequencies. This dispersion is caused by the varied geometric and material parameters of the resonators. The resonance frequency band of the effective permeability of the metamaterial is significantly widened, the value of the real part of the permeability is reduced, or the band of negative permeability even disappears. The paper comments on the design of a lens aimed at improving the imaging properties of the MRI system. The proposed homogenization method is verified by measuring the scattering parameters of the metamaterial in a rectangular waveguide.

An Analytical Evaluation of The Quality Factor Qz for Dominant Spherical Modes

  • DOI: 10.1049/iet-map.2014.0302
  • Odkaz: https://doi.org/10.1049/iet-map.2014.0302
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper describes an analytical evaluation of the quality factor Qz in a separable system in which the vector potential is known. The proposed method uses a potential definition of active and reactive power, implicitly avoiding infinite entire space integration and extraction of radiation energy. As a result, all the used quantities are finite, and the calculated Qz is always non-negative function of frequency. The theory is presented on the canonical example of the currents flowing on a spherical shell. The Qz for the dominant spherical TM and TE mode and their linear combination are found in closed forms, including both internal and external energies. The proposed analytical method and its results are compared to previously published limits of the quality factor Q.

An electron-light wave analogy for Cartesian and spherical systems

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The analogy between ballistic electrons and electromagnetic waves for heterostructures exhibiting planar and spherical symmetry is presented. Theory exposing similar behavior of the electromagnetic field and quantum mechanical waves is presented, together with subsequent possible applications.

Bound States Within the Continuum - A Metamaterial-Semiconductor Analogy

  • Autoři: Hrebíková, I., doc. Ing. Lukáš Jelínek, Ph.D., Silveirinha, M. G.
  • Publikace: 9th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics Proceedings. Louvain-la-Neuve: UCL, Laboratoire d´Hyperfréquences, 2015. pp. 112-114. ISBN 978-88-941141-0-2.
  • Rok: 2015
  • DOI: 10.1109/MetaMaterials.2015.7342540
  • Odkaz: https://doi.org/10.1109/MetaMaterials.2015.7342540
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The existence of bound states within the continuum has been known since the early years of quantum mechanics. However a realistic quantum system supporting such bound states has not yet been designed. Here we present a core–shell semiconductor heterostructure composed of Hg(1-x)Cd(x)Te compounds, which is able to support the bound states within the continuum using zero-valued effective mass within the outer shell. The working principle is analogous to that of the recently proposed open core-shell plasmonic resonators for light waves.

Embedded Energy State in an Open Semiconductor Heterostructure

  • Autoři: Hrebíková, I., doc. Ing. Lukáš Jelínek, Ph.D., Silveirinha, M. G.
  • Publikace: Physical Review B. 2015, 92(15), 155303-155311. ISSN 1098-0121.
  • Rok: 2015
  • DOI: 10.1103/PhysRevB.92.155303
  • Odkaz: https://doi.org/10.1103/PhysRevB.92.155303
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper, we show that HgCdTe heterostructures may supporttextcolor{red}{, within the envelope function approximation,} bound electronic states embedded in the continuum, such that the discrete energy spectrum overlaps the continuous spectrum. Although the proposed heterostructures are generally penetrable by an incoming electron wave, it is shown that they may support spatially localized trapped stationary states with an infinite lifetime. We discuss the possibility of a free electron being captured by the proposed open resonator, and present a detailed study of the trapping lifetime in the case of a detuned resonator.

Metamaterial Made of BC-SRRs with Randomly Dispersed Resonance Frequencies

  • DOI: 10.1109/MWSYM.2015.7166793
  • Odkaz: https://doi.org/10.1109/MWSYM.2015.7166793
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper reports on an investigation of a metamaterial designed as a 3D system of broad side coupled split ring resonators. Imperfections in the fabrication process cause dispersion of the geometrical and material parameters of the resonators, and thus dispersion of their resonance frequencies. Homogenization of a 3D cubic lattice of these imperfect resonators reveals that the resulting resonance frequency band of the effective permeability is significantly widened, but at the cost of reducing its magnitude or even completely disappearing the negative values of its real part. An experiment performed in the rectangular waveguide verifies the theoretical predictions.

New aspects of artificial diamagnetics

  • Autoři: Lapine, M., doc. Ing. Lukáš Jelínek, Ph.D.,
  • Publikace: Advanced Electromagnetic Materials in Microwaves and Optics (METAMATERIALS), 2015 9th International Congress on. Rome: METAMORPHOSE VI AISBL, 2015. pp. 172-174. ISSN 1045-9243. ISBN 978-1-4799-7836-6.
  • Rok: 2015
  • DOI: 10.1109/MetaMaterials.2015.7342562
  • Odkaz: https://doi.org/10.1109/MetaMaterials.2015.7342562
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Artificial diamagnetic metamaterials are known to produce impressively low permeability values in a broad frequency range. As an observable feature however, the same is true for good conductors. We analyse performance of artificial diamagnetics with regards to applications such as levitation, in comparison to bulk metals, and show that as long as the fields in the exterior of the body are concerned, non-structured metals outperform metamaterial structure. Furthermore, we calculate the response of practical samples of artificial diamagnetics, taking their discrete structure and finite size into account, and find that predictions of effective medium theory are much better applicable to non-resonant structures than to resonant ones.

On the Functional Relation Between Quality Factor and Fractional Bandwidth

  • DOI: 10.1109/TAP.2015.2414472
  • Odkaz: https://doi.org/10.1109/TAP.2015.2414472
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The functional relation between the fractional bandwidth and the quality factor of a radiating system is investigated in this note. Several widely used definitions of the quality factor are compared on two examples of RLC circuits that serve as a simplified model of a single resonant antenna tuned to its resonance. It is demonstrated that for a first-order system, only the quality factor based on differentiation of the input impedance has unique proportionality to the fractional bandwidth, whereas e.g. the classical definition of the quality factor, i. e. the ratio of the stored energy to the lost energy per one cycle, is not uniquely proportional to the fractional bandwidth. In addition, it is shown that for higher-order systems the quality factor based on differentiation of the input impedance ceases to be uniquely related to the fractional bandwidth.

On the Properties of Stored Electromagnetic Energy

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In our talk, we will review all the available concepts that have attempted to determine stored energy. It will be pointed out that all these concepts fail in at least one of the "must have" properties mentioned above. Finally, it will be concluded that no fully consistent de nition of stored electromagnetic energy is yet known. This of course raises the question whether the very idea of stored (and radiated) energy is well-posed.

On the Stored and Radiated Energy Density

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This contribution reviews and discusses several concepts of stored and radiated energy density of an electromagnetic field. The contribution exposes the most important differences between the up to date definitions and discusses consequences in terms of the energy density of the dominant spherical TE mode.

The Quality Factor QZ of the Combined TE10 / TM10 Spherical Mode

  • DOI: 10.1109/APS.2015.7304920
  • Odkaz: https://doi.org/10.1109/APS.2015.7304920
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This contribution describes an evaluation of the quality factor QZ of the combined TE10 / TM10 mode, including the internal and the external region of the bounding sphere. The proposed method uses a potential definition of complex power, implicitly avoiding infinite entire space integration. It is shown that QZ of the combined mode is not (unlike classical Q) limited to one half of the value for the pure TM10 mode. It is shown that the QZ factor does not have a strict lower bound other than zero.

The Relation Between Fractional Bandwidth and Q Factor

  • DOI: 10.1109/APS.2015.7304816
  • Odkaz: https://doi.org/10.1109/APS.2015.7304816
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The functional relation between the fractional bandwidth and the quality factor of a radiating system is studied from simple first-order-like RLC-like systems to higher-order radiating systems. It is demonstrated that, considering a firstorder system, only the quality factor based on differentiation of the input impedance has unique proportionality to the fractional bandwidth. In the general case of higher-order systems, it is however shown that this property is lost.

Time Domain Scheme for Stored Energy Evaluation

  • DOI: 10.1109/APS.2015.7305242
  • Odkaz: https://doi.org/10.1109/APS.2015.7305242
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A novel method for evaluation of the stored energy contained in a non-stationary electromagnetic field is presented in this paper. The method utilizes the classical definition of the radiated energy, which subtraction from the total electromagnetic energy is carried out in time domain during a transient state of the system under study.

A Perfect Lens for Ballistic Eectrons: An Electron-Light Wave Analogy

  • DOI: 10.1016/j.photonics.2013.08.005
  • Odkaz: https://doi.org/10.1016/j.photonics.2013.08.005
  • Pracoviště: Katedra elektromagnetického pole, Katedra mikroelektroniky
  • Anotace:
    The analogy between electromagnetic waves and ballistic electrons within the Kane's model is developed and subsequently applied to a theoretical description of a quantum version of a metamaterial planar lens. Restrictions imposed on the perfect lens and the poor man's lens by available semiconductor band structures are discussed. A realistic implementation is proposed for the quantum poor man's lens, which uses specific properties of the HgTe compound. The properties of the lens are presented on the basis of a calculated transmission of oblique electrons through the lens structure.

A Polarizability Measurement Method for Electrically Small Particles

  • DOI: 10.1109/LAWP.2014.2327152
  • Odkaz: https://doi.org/10.1109/LAWP.2014.2327152
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A simple and precise free space polarizability measurement method for electrically small particles is presented. The method is based on measuring the scattering parameters of a waveguide loaded by a particle, and on knowledge of the dipolar polarizabilities of two calibration standards. The method is verified on a metallic disk and a resonant conducting ring terminated by a capacitor.

Applicability of nonresonant artificial diamagnetics

  • DOI: 10.1103/PhysRevB.90.104413
  • Odkaz: https://doi.org/10.1103/PhysRevB.90.104413
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Artificial diamagnetics are prominent for achieving extraordinarily strong diamagnetism in a wide frequency range. However, as far as the magnetic fields outside the artificial medium are concerned, bulk conductors show a very similar pattern. The question arises whether the complicated internal structure of artificial diamagnetics can, to this end, be replaced by a simpler object.We show that for an electrically small body, internal structuring is likely to make the effective diamagnetic response weaker than that of a simple conducting object.

Lower bounds of the quality factor QZ

  • DOI: 10.1109/APS.2014.6904358
  • Odkaz: https://doi.org/10.1109/APS.2014.6904358
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper describes an analytical evaluation of the measurable quality factor QZ of a system in which the vector potential is known. The method is presented on a canonical example of the currents flowing on a spherical shell, which leads to the analytical prescription for the lower bound of QZ.

The Measurable Q Factor and Observable Energies of Radiating Structures

  • DOI: 10.1109/TAP.2013.2287519
  • Odkaz: https://doi.org/10.1109/TAP.2013.2287519
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    New expressions are derived to calculate the Q factor of a radiating device. The resulting relations link Q based on the frequency change of the input impedance at the input port (QX, QZ) with expressions based solely on the current distribution on an radiating device. The question of which energies of a radiating system are observable is reviewed, and then the proposed Q factor as defined in this paper is physical. The derivation is based on potential theory rather than fields. This approach hence automatically eliminates all divergent integrals associated with electromagnetic energies in infinite space. The new formulas allow us to study the radiation Q factor for antennas without feeding (through e.g. Characteristic Modes) as well as fed by an arbitrary number of ports. The new technique can easily be implemented in any numerical software dealing with current densities. To present the merits of proposed technique, three canonical antennas are studied. Numerical examples show excellent agreement between the measurable QZ derived from input impedance and the new expressions.

The source definition of the quality factor QZ, 2014 IEEE International Symposium on Antennas and Propagation

  • DOI: 10.1109/APS.2014.6904357
  • Odkaz: https://doi.org/10.1109/APS.2014.6904357
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    New expressions for calculating of the quality factor QZ are presented. The resulting relations link QZ, based on the frequency change of the input impedance at the input port, with expressions based solely on the current distribution on a radiator. The derivation is based on electromagnetic potentials, automatically eliminating all divergent integrals associated with electromagnetic energies in infinite space.

A Building Element of an Active Negative Permeability Metamaterial

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents an active particle designed as a building block of an active volumetric metamaterial showing negative magnetic polarizability. The particle is composed of a conducting ring loaded by an impedance with a negative real part. This impedance is produced by a one-port transistor circuit. Measurements of the fabricated element verified its working principle in the RF range.

A Perfect Lens for Ballistic Electrons

  • Autoři: Hrebíková, I., doc. Ing. Lukáš Jelínek, Ph.D., doc. RNDr. Jan Voves, CSc., Baena, J. D.
  • Publikace: Proceedings of Metamaterials´ 2013: The 7th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics. Louvain-la-Neuve: UCL, Laboratoire d´Hyperfréquences, 2013. ISBN 978-952-67611-5-2.
  • Rok: 2013
  • Pracoviště: Katedra elektromagnetického pole, Katedra mikroelektroniky
  • Anotace:
    The analogy between plane electromagnetic waves and electron waves is presented and subsequently applied to the description of a quantum perfect lens. The natural constraints resulting from available semiconductor compounds are discussed. The final lens design relies on the inverted bandstructure of the HgTe compound, and uses a voltage gate to created the lens structure.

Coupled-Wave Surface-Impedance Analysis of Extraordinary Transmission Through Single and Stacked Metallic Screens

  • Autoři: Delgado, V., Marques, R., doc. Ing. Lukáš Jelínek, Ph.D.,
  • Publikace: IEEE Transactions on Antennas and Propagation. 2013, 61(3), 1342-1351. ISSN 0018-926X.
  • Rok: 2013
  • DOI: 10.1109/TAP.2012.2227657
  • Odkaz: https://doi.org/10.1109/TAP.2012.2227657
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper, we present an efficient coupled-wave surface-impedance method for the analysis of extraordinary optical transmission (EOT) through single and stacked realistic metallic screens under normal and oblique incidence, including possible dielectric interlayers. The proposed theory is valid for the complete frequency range where EOT has been reported, including microwaves and optics. Electromagnetic simulations validate the results of the model, which enable fast and accurate characterization of the analyzed structures.

Loss compensation in RF metamaterials by single transistor circuits

  • DOI: 10.1109/MetaMaterials.2013.6809005
  • Odkaz: https://doi.org/10.1109/MetaMaterials.2013.6809005
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper shows that reduction of losses, or even addition of gain, in RF metamaterials can be achieved by a single transistor circuit that exploits its conditional stability. The presented solution offers not only extreme simplicity, but also absolute control over stability and achieved gain values.

New results on extraordinary optical transmission and Fishnet Metamaterials

  • Autoři: Delgado, V., Marques, R., doc. Ing. Lukáš Jelínek, Ph.D.,
  • Publikace: Proceedings of the 7th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (METAMATERIALS). Piscataway: IEEE, 2013. p. 55-57. ISBN 978-1-4799-1232-2.
  • Rok: 2013
  • DOI: 10.1109/MetaMaterials.2013.6808951
  • Odkaz: https://doi.org/10.1109/MetaMaterials.2013.6808951
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A generalized transverse waveguide analysis for Extraordinary Transmission (ET) and Fishnet Metamaterials (FMs) in realistic metallic screens has recently been published by the authors [1]. The model is valid for lossy metallic screens thanks to the Surface Impedance (SI) approximation (e.g. [2]). A standard scattering matrixes formalism makes the model applicable to stacked screens with possible dielectric boards. A number of reported phenomena [3, 4] related to screens with a periodic array of slits or holes are reproduced efficiently with the presented analysis.

Precise Measurement Using Coaxial-to-Microstrip Transition Through Radiation Suppression

  • DOI: 10.1109/TMTT.2013.2272380
  • Odkaz: https://doi.org/10.1109/TMTT.2013.2272380
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents a study about the radiation problem of coaxial-to-microstrip launchers and suggests their improvement with a novel design. The proposed solution is based on a coaxial-to-microstrip transition enclosed in a parallel-plate transmission line that has its cutoff frequency above the working frequency band of interest. Any radiated field is quickly attenuated because it is propagating inside a subcritical parallel-plate transmission line. The proposed method is extensively analyzed in the CST Microwave Studio and simulation results are verified on fabricated test-fixture by multiple measurements. The proposed solution improves the accuracy and reduces the uncertainty during measurements on microstrip.

Surface mesoscopic effects in finite metamaterials

  • DOI: 10.1364/OE.20.018297
  • Odkaz: https://doi.org/10.1364/OE.20.018297
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    While the effective medium treatment of unbounded metamaterials appears to be well established and firmly proven, related phenomena in finite structures have not received sufficient attention. We report on mesoscopic effects associated with the boundaries of finite discrete metamaterial samples, which can invalidate an effective medium description. We show how to avoid such effects by proper choice of boundary configuration. As all metamaterial implementations are naturally finite, we are confident that our findings are crucial for future metamaterial research.

An FET-Based Unit Cell for an Active Magnetic Metamaterial

  • DOI: 10.1109/LAWP.2011.2167311
  • Odkaz: https://doi.org/10.1109/LAWP.2011.2167311
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A particle that can be used to create an active magnetic metamaterial has been designed using an FET transistor loaded in its gate by a conducting ring and in its source by a parallel resonance circuit. The design procedure is discussed, and the working principle is experimentally demonstrated in the RF range.

Bulk Metamaterials Made of Resonant Rings

  • Autoři: Marques, R., doc. Ing. Lukáš Jelínek, Ph.D., Freire, J.R., Baena, J.D., Lapine, M.
  • Publikace: Proceedings of the IEEE. 2011, 99(10), 1660-1668. ISSN 0018-9219.
  • Rok: 2011
  • DOI: 10.1109/JPROC.2011.2141970
  • Odkaz: https://doi.org/10.1109/JPROC.2011.2141970
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this brief review, we present the fundamentals of bulk resonant ring metamaterial (RRM) theory. Metamaterials made of resonant rings are discussed, and some basic design rules are provided. Homogenization (including spatial dispersion) of 3-D resonant ring latices is reviewed, with emphasis in isotropic designs. Edge effects in finite size metamaterial samples are discussed. Finally, possible applications and future trends are briefly reviewed.

Extraordinary transmission through dielectric screens with 1D sub-wavelength metallic inclusions

  • DOI: 10.1364/OE.19.013612
  • Odkaz: https://doi.org/10.1364/OE.19.013612
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Extraordinary optical transmission (EOT) through dielectric screens periodically loaded with sub-wavelength 1D discontinuities, such as apertures or metallic insets is analyzed. The results of the analysis and computational electromagnetic simulations show that the transmission is higher for for metallic inclusions than for empty slits. This effect confirms that EOT is a quite general property of weakly transparent periodic diffraction screens and opens the door to optically induced EOT in photo-conductive semiconductor screens.

Free Space Polarizability Measurement Method

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., prof. Ing. Jan Macháč, DrSc.,
  • Publikace: Proceedings of Metamaterials´ 2011: The Fifth International Congress on Advanced Electromagnetic Materials in Microwaves and Optics. Louvain-la-Neuve: UCL, Laboratoire d´Hyperfréquences, 2011. pp. 967-969. ISBN 978-952-67611-0-7.
  • Rok: 2011
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A simple and precise method for free space polarizability measurement of electrically small particles is presented. The method is based on a measurement of the scattering parameters of a waveguide loaded by a particle, and on knowledge of the dipolar polarizabilities of two calibration standards.

Further support for the alignment of cattle along magnetic field lines: reply to Hert et al. response

  • Autoři: Heřt, J., doc. Ing. Lukáš Jelínek, Ph.D., Pekárek, L., Pavlíček, A.
  • Publikace: Journal of Comparative Physiology A. 2011, 197(12), 1135-1136. ISSN 0340-7594.
  • Rok: 2011
  • DOI: 10.1007/s00359-011-0688-8
  • Odkaz: https://doi.org/10.1007/s00359-011-0688-8
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    We have carefully read the text of the comments of Begall et al. (2011) on our publication (Hert et al. 2011), and meticulously considered the objections concerning our paper as well as the results of the re-evaluation, re-assessment and re-interpretation of our data. In the following, we present our response.

Metamaterial-inspired Perfect Tunnelling in Semiconductor Heterostructures

  • DOI: 10.1088/1367-2630/13/8/083011
  • Odkaz: https://doi.org/10.1088/1367-2630/13/8/083011
  • Pracoviště: Katedra elektromagnetického pole, Katedra mikroelektroniky
  • Anotace:
    In this paper, we use a formal analogy of the electromagnetic wave equation and the Schrödinger equation in order to study the phenomenon of perfect tunnelling (tunnelling with unitary transmittance) in a one-dimensional semiconductor heterostructure. Using the Kane model of a semiconductor, we show that this phenomenon can indeed exist, resembling all the interesting features of the corresponding phenomenon in classical electromagnetism in which metamaterials (substances with negative material parameters) are involved. We believe that these results can pave the way toward interesting applications in which metamaterial ideas are transferred into the semiconductor domain.

No Alignment of Cattle Along Geomagnetic Field Lines Found

  • Autoři: Hert, J., doc. Ing. Lukáš Jelínek, Ph.D., Pekárek, L., Pavlíček, A.
  • Publikace: Journal of Comparative Physiology A. 2011, 197(6), 677-682. ISSN 0340-7594.
  • Rok: 2011
  • DOI: 10.1007/s00359-011-0628-7
  • Odkaz: https://doi.org/10.1007/s00359-011-0628-7
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents a study of the body orientation of domestic cattle on free pastures in everal European states, based on the satellite photographs. In sum, 232 herds with 3,412 individuals were evaluated. Two independent groups participated in our study and came to the same conclusion that in contradiction to the recent findings of other researchers, no alignment of the animals and of their herds along geomagnetic field lines could be found. Several possible reasons for this discrepancy should be taken into consideration: poor quality of satellite photographs, difficulties in determining body axis, selection of herds or animals within herds, lack of blinding in the evaluation, possible subconscious bias, and, most importantly, high sensitivity of the calculated main directions of the Rayleigh vectors to some kind of bias or to some overlooked or ignored confounder. This factor could easily have led to an unsubstantiated positive conclusion about the existence of magnetoreception.

Perfect Tunneling in Semiconductor Heterostructures

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Baena, J.D., doc. RNDr. Jan Voves, CSc., Marques, R.
  • Publikace: Proceedings of Metamaterials´ 2011: The Fifth International Congress on Advanced Electromagnetic Materials in Microwaves and Optics. Louvain-la-Neuve: UCL, Laboratoire d´Hyperfréquences, 2011. pp. 1021-1023. ISBN 978-952-67611-0-7.
  • Rok: 2011
  • Pracoviště: Katedra elektromagnetického pole, Katedra mikroelektroniky
  • Anotace:
    We study the phenomenon of perfect tunneling (tunneling with unitary transmittance) in a 1D semiconductor heterostructure using a formal analogy of the electromagnetic wave equation and the Schrodinger equation. The Kane model of a semiconductor is used and it is shown that this phenomenon can indeed exist, resembling all the interesting features of the corresponding phenomenon in classical electromagnetism in which metamaterials are involved.

A Resonant Particle for a Volumetric DNG Metamaterial

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper studies a combination of electric and magnetic particles in order to obtain simultaneously negative electric and magnetic polarisability under irradiation by an electromagnetic wave. The magnetic particle is represented by a split ring resonator, and electric particle is represented by an electric dipole terminated by an inductor. They are combined together in a way that obtains a resonant element with inversion symmetry. The internal coupling between these resonators located in a waveguide is subjected to a special study. The response of this particle is strongly anisotropic, but an isotropic response can be achieved by a proper spatial arrangement.

Analytical Theory of Extraordinary Optical Transmission Through Realistic Metallic Screens

  • DOI: 10.1364/OE.18.006506
  • Odkaz: https://doi.org/10.1364/OE.18.006506
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An analytical theory of extraordinary optical transmission (EOT) through realistic metallic screens perforated by a periodic array of subwavelength holes is presented. The theory is based on our previous work on EOT through perfect conducting screens and on the surface impedance concept. The proposed theory is valid for the complete frequency range where EOT has been reported, including microwaves and optics. A reasonably good agreement with electromagnetic simulations is shown in all this frequency range. We feel that the proposed theory may help to clarify the physics underlying EOT and serve as a first step to more accurate analysis.

Artificial Magnetism and Left-handed Media from Dielectric Rings and Rods

  • DOI: 10.1088/0953-8984/22/2/025902
  • Odkaz: https://doi.org/10.1088/0953-8984/22/2/025902
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    It is shown that artificial magnetism with relatively large frequency bandwidth can be obtained from periodic arrangements of dielectric rings. Combined with dielectric rods, dielectric rings can provide 3D isotropic left-handed metamaterials which are an advantageous alternative to metallic split ring resonators (SRRs) and/or metallic wires when undetectability by low frequency external magnetic fields is desired. Furthermore it is shown that, unlike conventional SRRs, dielectric rings can also be combined with natural plasma-like media to obtain a left-handed metamaterial.

Edge Effects in Finite Size Resonant Ring Metamaterials

  • Autoři: Marqués, R., Lapine, M., doc. Ing. Lukáš Jelínek, Ph.D.,
  • Publikace: Metamaterials 2010. Karlsruhe: Universität Karlsruhe, 2010. pp. 330-332. ISBN 978-952-92-7734-6.
  • Rok: 2010
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    It is reported how finite size resonant ring metamaterials may present strong deviations from the expected behavior in the negative magnetic permeability region, even for moderate losses. This effect seems to be related with the excitation of magnetoinductive quasi-static resonances which are closely related to the "unphysical" resonances that may appear near sharp wedges in negative magnetic permeability continuous media.

Exact Modelling Method for Discrete Finite Metamaterial Lens

  • Autoři: Lapine, M., doc. Ing. Lukáš Jelínek, Ph.D., Marques, M., Freire, M.J.
  • Publikace: IET Microwaves, Antennas & Propagation. 2010, 4(8), 1132-1139. ISSN 1751-8725.
  • Rok: 2010
  • DOI: 10.1049/iet-map.2009.0598
  • Odkaz: https://doi.org/10.1049/iet-map.2009.0598
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The authors analyse the properties of metamaterial lens composed of capacitively loaded ring resonators, with the help of an efficient rigorous model suitable for calculating the properties of finite metamaterial samples. This approach takes into account the discrete structure and finite extent of realistic metamaterials. The authors show that the discrete model reveals the effects, which can be missed by a continuous model based on effective parameters and that the results are in close agreement with the experimental data.

Fishnet Metamaterials - Rules for Refraction and Limits of Homogenization

  • DOI: 10.1364/OE.18.017940
  • Odkaz: https://doi.org/10.1364/OE.18.017940
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The perfectly conducting stacked fishnet metamaterial is studied in this paper. The analysis is based on a combination of the mode matching method together with the generalized eigenvalue problem, and takes into account wave propagation along all three Cartesian axes. The analysis has been developed for a fishnet of square lateral periodicity and for two particular polarizations, namely TE and TM, corresponding to the two most common excitations. The 1D and 2D dispersion characteristics are calculated for both polarizations, showing that the TM waves undergo negative refraction in a narrow frequency band just below Wood's anomaly, whereas TE polarized waves exhibit ordinary positive refraction. Finally, possible homogenization of the fishnet metamaterial is considered, showing that only for small angles of incidence and in the immediate vicinity of Wood's anomaly can the fishnet be seen as homogenizable uniaxial medium.

Homogenization of Fishnet Metamaterials: Possibilities and Restrictions

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The possibilities and restrictions of homogenization in fishnet metamaterials are studied. By means of 1D and 2D dispersion diagrams obtained by combination of mode-matching with generalized eigenvalue problem it is shown that in narrow frequency region just below Wood's anomaly the approximation of fishnet by uniaxial medium is allowed. It is also shown that this is the frequency band of possible negative refraction.

Realistic Metamaterial Lenses: Limitations Imposed by Discrete Structure

  • DOI: 10.1103/PhysRevB.82.165124
  • Odkaz: https://doi.org/10.1103/PhysRevB.82.165124
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    We study the peculiarities of a metamaterial "superlens," caused by its discrete structure and finite size. We show that precise modeling of the lens provides remarkable distinctions from continuous medium approximation. In particular, we address the problem of highest resolution that can be achieved with a realistic electrically thin metamaterial lens. We conclude that discrete structure imposes essential limitations on the resolution and that the resolution cannot be improved by decreasing dissipation in the system. Further implications related to effective medium description of discrete structures are discussed.

Accurate modeling of split ring metamaterial lenses for magnetic resonance imaging applications

  • DOI: 10.1063/1.3067788
  • Odkaz: https://doi.org/10.1063/1.3067788
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The usefulness of thin split ring metamaterial slabs for imaging applications, including magnetic resonance imaging applications, has attracted some attention in the past years. However, the small number of unit cells across these thin slabs prevents the direct application of continuous medium models for its characterization. The main aim of this contribution is to provide a rigorous model for these structures, also clarifying the usefulness of continuous medium approach for their characterization. The proposed model is a generalization of the classical Lorentz procedure to two dimensions and is able to deal with electrically thin slabs made of small resonant closed current loops. The obtained results are validated by full-wave electromagnetic simulations and compared with the continuous model approximation of the slab.

Analytical theory of extraordinary transmission through metallic diffraction screens perforated by small holes

  • DOI: 10.1364/OE.17.005571
  • Odkaz: https://doi.org/10.1364/OE.17.005571
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this letter, the problem of extraordinary (ET) transmission of electromagnetic waves through opaque screens perforated with subwavelength holes is addressed from an analytical point of view. Our purpose was to find a closed-form expression for the transmission coefficient in a simple case in order to explore and clarify, as much as possible, the physical background of the phenomenon. The solution of this canonical example, apart from matching quite well with numerical simulations given by commercial solvers, has provided new insight in extraordinary transmission as well as Wood's anomaly. Thus, our analysis has revealed that one of the key factors behind ET is the continuous increase of excess electric energy around the holes as the frequency approaches the onset of some of the higher-order modes associated with the periodicity of the screen. The same analysis also helps to clarify the role of surface modes -or spoof plasmons- in the onset of ET.

Analytical Theory of Wave Propagation through Stacked Fishnet Metamaterials

  • DOI: 10.1364/OE.17.011582
  • Odkaz: https://doi.org/10.1364/OE.17.011582
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This work analyzes the electromagnetic wave propagation through periodically stacked fishnets from zero frequency to the first Wood's anomaly. It is shown that, apart from Fabry-Perot resonances, these structures support two transmission bands that can be backward under the appropriate conditions. The first band starts at Wood's anomaly and is closely related to the well-known phenomena of extraordinary transmission through a single fishnet. The second band is related to the resonances of the fishnet holes. In both cases, the in-plane periodicity of the fishnet cannot be made electrically small, which prevents any attempt of homogenization of the structure along the fishnet planes. However, along the normal direction, even with very small periodicity transmission is still possible. An homogenization procedure can then be applied along this direction, thus making that the structure can behave as a backward-wave transmission line for such transmission bands.

Characterization of miniaturized metamaterial resonators coupled to planar transmission lines through parameter extraction

  • Autoři: Aznar, F., Gil, M., Bonache, J., doc. Ing. Lukáš Jelínek, Ph.D., Baena, J., Marques, R., Martin, F.
  • Publikace: Journal of Applied Physics. 2008, 104(11), ISSN 0021-8979.
  • Rok: 2008
  • DOI: 10.1063/1.3021109
  • Odkaz: https://doi.org/10.1063/1.3021109
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper, a method for obtaining the electrical characteristics of metamaterial resonators coupled to planar transmission lines is proposed. This parameter extraction technique is based on the comparison between the measured (or full wave electromagnetic simulated) transmission and reflection characteristics of a host line loaded with such resonators and those obtained from its lumped element equivalent circuit model (previously reported by some of the authors). The resonant particles considered in this study are split ring resonators, spiral resonators, and other electrically small resonant particles based on two metal levels. The interest in this technique lies in the lack of analytical models providing the electrical parameters of several of the considered ultrasmall resonator topologies (due to their complexity). From the extracted parameters, it is concluded that the circuit models predict very accurately the frequency responses of the considered structures for the different resonators under study. There is an increasing interest in the synthesis of metamaterial transmission lines with extremely small unit cell size. In order to achieve dimensions close to (or even below) lambda(g)/100, it is imperative to use complex resonator topologies that combine broadside coupling and extremely elongated metallic strips (etched in different metal levels connected through vias). This justifies the proposed technique. Thus, this work is useful in aiding the synthesis of microwave components based on resonant type metamaterial transmission lines.

Experimental demonstration of a mu=-1 metamaterial lens for magnetic resonance imaging

  • DOI: 10.1063/1.3043725
  • Odkaz: https://doi.org/10.1063/1.3043725
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this work a mu=-1 metamaterial (MM) lens for magnetic resonance imaging (MRI) is demonstrated. MRI uses surface coils to detect the radio frequency (rf) energy absorbed and emitted by the nuclear spins in the imaged object. The proposed MM lens manipulates the rf field detected by these surface coils so that the coil sensitivity and spatial localization are substantially improved. Beyond this specific application, we feel that the reported results are the experimental confirmation of a new concept for the manipulation of rf field in MRI, which paves the way to many other interesting applications.

Periodic arrangements of chiral scatterers providing negative refractive index bi-isotropic media

  • DOI: 10.1103/PhysRevB.77.205110
  • Odkaz: https://doi.org/10.1103/PhysRevB.77.205110
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The main goal of this contribution is to show that periodic arrangements of chiral scatterers can be the basis for the development of three dimensional and isotropic negative refractive index artificial media. Three dimensionality and isotropy are key issues in this context since only three dimensional structures can be properly viewed as a "medium," and only in isotropic media the refractive index is unambiguously defined. The proposed arrangements are cubic lattices of chiral split ring resonators conveniently designed to yield an isotropic behavior. The reported structures are shown to provide a significant frequency band of negative refraction with good matching to free space.

Unified homogenization theory for magnetoinductive and electromagnetic waves in split-ring metamaterials

  • DOI: 10.1103/PhysRevA.78.013842
  • Odkaz: https://doi.org/10.1103/PhysRevA.78.013842
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A unified homogenization procedure for split-ring metamaterials taking into account time and spatial dispersion is introduced. It is shown that electromagnetic and magnetoinductive waves propagating in the metamaterial are obtained from this analysis. Therefore, the proposed time and spatially dispersive permeability accounts for the characterization of the complete spectrum of waves of the metamaterial. Finally, it is shown that the proposed theory is in good quantitative and qualitative agreement with full wave simulations.

A Magnetic Metamaterial Composed of Randomly Oriented SRRs

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The concept, manufacture and testing of a volumetric isotropic magnetic metamaterial composed of randomly distributed broadside coupled split ring resonators is presented in this paper. An estimate of the isotropy based on the measured transmissions is made in terms of the arithmetic mean value and the standard deviation of the sample. We offer conclusions and recommendations for the development of a genuine bulk isotropic medium of various densities and distributions of the particles, based on the 64 measured frequency responses of each sample.

Characterization of Resonant Modes in a Cube of SRR´s

  • Autoři: Baena, J.D., doc. Ing. Lukáš Jelínek, Ph.D., Marqués, R., Zehentner, J.
  • Publikace: PIERS 2007 Progress In Electromagnetics Research Symposium - Proceedings - CN. Cambridge, MA: The Electromagnetics Academy, 2007. pp. 469. ISSN 1559-9450.
  • Rok: 2007
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A general quasistatic circuit model of the cubical unit cell consisting of six particles deposited on the walls of a cube is presented in this paper. The matrix description of the equivalent circuit of the cube, incorporating the self and mutual impedances, is applied to determining of that unit cell response. It is shown that the resonance splitting due to the EM coupling between faces of the cube does not take place and all unit cells of this kind possess only one magnetic resonance as does the original SRR. The theoretical prediction of that unit cell behavior has been experimentally demonstrated.

Isotropic frequency selective surfaces made of cubic resonators

  • Autoři: Baena, J., doc. Ing. Lukáš Jelínek, Ph.D., Marques, R., Mock, J., Gollub, J., Smith, D.
  • Publikace: Applied Physics Letters. 2007, 91(19), ISSN 0003-6951.
  • Rok: 2007
  • DOI: 10.1063/1.2806915
  • Odkaz: https://doi.org/10.1063/1.2806915
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Isotropic frequency selective surface (FSS) made of cubic arrangements of split ring resonators (SRRs) is proposed and analyzed. For this purpose, a suitable isotropic modification of the SRR was used in the design of a cubic unit element invariant under the tetrahedral point symmetry group. It was experimentally demonstrated that the transmission through such a FSS is angle and polarization independent. For comparison, another FSS, whose unit elements do not satisfy necessary symmetries, was measured, showing clearly anisotropic behavior. We feel then that symmetries play an important role. Potential device applications are envisioned for antenna technology at microwave and terahertz frequencies.

Metamaterials - A Challenge for Contemporary Advanced Technology

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Metamaterials are artificial composite media exhibiting electromagnetic responses that natural materials do not provide. They enable their permittivity and permeability to be cast down to negative values. Metamaterials are known in two forms: volumetric medium and planar version. The first form is characterized by field theory, while the second form utilizes transmission line theory. There is a brief report on the authors' innovative findings. It concerns the utilization of split ring resonators in isotropic negative permeability metamaterials, an isotropic negative permittivity medium, CPW supporting a left-handed wave, and wire media exhibiting negative permittivity. Finally, prospects for further developments in this interesting field are presented.

Negative refraction from balanced quasi-planar chiral inclusions

  • Autoři: Marques, R., doc. Ing. Lukáš Jelínek, Ph.D., Mesa, F.
  • Publikace: Microwave and Optical Technology Letters. 2007, 49(10), 2606-2609. ISSN 0895-2477.
  • Rok: 2007
  • DOI: 10.1002/mop.22736
  • Odkaz: https://doi.org/10.1002/mop.22736
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This work proposes a quasi-planar chiral resonator suitable for the design of negative refraction index metamaterials. It is presented as an analytical model to determine the metamaterial polarizabilities, which is also the basis of a further study of the viability of negative refraction in chiral and racemic arrangements of inclusions made up with the proposed quasi-planar chiral resonator. The present analysis is expected to pave the way for the design and building feasible negative refractive index metamaterials whose inclusions can be manufactured by means of standard photo-etching techniques.

Quasi-isotropic Frequency Selective Surface

  • Autoři: Baena, J.D., Mock, J., doc. Ing. Lukáš Jelínek, Ph.D., Smith, D.R., Marqués, R., Gollub, J.
  • Publikace: PIERS 2007 Progress In Electromagnetics Research Symposium - Proceedings - CN. Cambridge, MA: The Electromagnetics Academy, 2007. pp. 478. ISSN 1559-9450.
  • Rok: 2007
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The cubic cell, satisfying the inversion symmetry removing the magnetoelectric coupling, was proposed and is presented in this paper. The quasi-isotropic frequency selective surface was designed, manufactured and measured under different angle and polarization of the incident wave between horn antennas. A weak dependence of the transmission on the incident angle and polarization of the illumination has been observed. A simple model based on surface impedance deduced from the polarizability of a single unit cell is given in the presentation.

Towards a systematic design of isotropic bulk magnetic metamaterials using the cubic point groups of symmetry

  • DOI: 10.1103/PhysRevB.76.245115
  • Odkaz: https://doi.org/10.1103/PhysRevB.76.245115
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper, a systematic approach to the design of bulk isotropic magnetic metamaterials is presented. The roles of the symmetries of both the constitutive element and the lattice are analyzed. For this purpose, it is assumed that the metamaterial is composed of cubic split ring resonators (SRRs) arranged in a cubic lattice. The minimum symmetries needed to ensure an isotropic behavior are analyzed, and some particular configurations are proposed. Besides, an equivalent circuit model is proposed for the considered cubic SRRs. Experiments are carried out in order to validate the proposed theory. We hope that this analysis will pave the way to the design of bulk metamaterials with strong isotropic magnetic response, including negative permeability and left-handed metamaterials.

3D Magnetic Resonators for Isotropic Metamaterial Design

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Baena, J.D., Marques, R., Zehentner, J.
  • Publikace: Radioelektronika 2006 - Conference Proceedings. Bratislava: Slovak University of Technology, 2006. pp. 264-266. ISBN 80-227-2388-6.
  • Rok: 2006
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The paper presents 3D-isotropic magnetic resonators, made of 6 planar resonator placed on the faces of a cube so that the whole particle satisfies necessary spatial symmetries to reduce all second order (pseudo-)tensors to (pseudo-)scalars. The isotropy is experimentally confirmed by measuring the transmission coefficent through a waveguide loaded with such 3D particle in several orientations.

Computer Controlled Measurement of Electromagnetic Fields

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents a system for measuring high frequency electromagnetic fields fully controlled by a computer. A measuring probe is driven in one direction by a step motor, and in two other directions it is set manually. The paper collects some experimental data on field distributions taken along a transmission line and a leaky wave antenna.

Direct Polarisability Extraction Method

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Baena, J.D., Marques, R., Zehentner, J.
  • Publikace: Proceedings of the 36th European Microwave Conference. Piscataway: IEEE, 2006. ISBN 2-9600551-6-0.
  • Rok: 2006
  • DOI: 10.1109/EUMC.2006.281087
  • Odkaz: https://doi.org/10.1109/EUMC.2006.281087
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The method of finding the polarisability of a particle under test is described in the paper. To this point the particle is placed inside the waveguide and its polarisability is calculated either from known field at the input and the output port or from scattering parameters. The method is verified by electromagnetic simulation in the case of particle with known polarisability.

Electrically Small Isotropic Three-Dimensional Magnetic Resonators for Metamaterial Design

  • Autoři: Baena, J.D., doc. Ing. Lukáš Jelínek, Ph.D., Marqués, R., Zehentner, J.
  • Publikace: Applied Physics Letters. 2006, 88(134108), 134108-1-134108-3. ISSN 0003-6951.
  • Rok: 2006
  • DOI: 10.1063/1.2190442
  • Odkaz: https://doi.org/10.1063/1.2190442
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The problem of the design of artificial magnetic resonators for isotropic metamaterials is addressed. The internal symmetries that ensure an isotropic behavior of such resonators are analyzed and some specific designs based on the proper arrangement of modified split ring resonators are proposed. These proposals are validated by electromagnetic simulations and experiments. The reported results are likely to have applications in the design of devices such as negative refractive index materials, super-lenses, and meta-surfaces with isotropic response

On the Design of Isotropic Magnetic Metamaterials

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Zehentner, J., Baena, J.D., Marques, R.
  • Publikace: The 13th IEEE Mediterranean Electrotechnical Conference - MELECON 2006 - Electronic Proceedings. Piscataway: IEEE, 2006. pp. 250-253. ISBN 1-4244-0088-0.
  • Rok: 2006
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper new 3D-isotropic magnetic resonators are proposed. These new particles possess spatial symmetries under whose all second order (pseudo-)tensors are reduced to (pseudo-)scalars. If then the contribution of higher order tensors is neglected, the particles are isotropic. To confirm that proposed particles still generate magnetic response, a quasistatic model, showing the qualitative behavior of the reported resonators, is proposed. Finally, the isotropy is experimentally checked by measuring the transmission coefficient through a waveguide loaded with such 3D particle in several orientations and results are compared with some previous proposals lacking these symmetries.

Analysis of 2D- and 3D-Isotropic Split Ring Resonators

  • Autoři: Baena, J.D., doc. Ing. Lukáš Jelínek, Ph.D., Marqués, R., Zehentner, J.
  • Publikace: ICECom 2005 - Conference Proceedings 18th International Conference on Applied Electromagneics and Communications. Dubrovnik: IEEE Croatia Section, 2005. pp. 325-328. ISBN 953-6037-44-0.
  • Rok: 2005
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Previously proposed 2D isotropic split ring resonators are analyzed. Furthermore, a new isotropic 3D particle is proposed and analyzed. To this end a quasi-static LC circuit model, that provides the frequency of resonance, is developed. Theoretical computations of resonant frequency are in good agreement with the values obtained with a commercial electromagnetic solver. Finally the complete isotropy of the proposed 3D particle is demonstrated by its symmetries and is confirmed by numerical simulations.

Near-perfect Tunneling and Amplification of Evanescent Electromagnetic Waves in a Waveguide

  • Autoři: Baena, J.D., doc. Ing. Lukáš Jelínek, Ph.D., Marqués, R., Medina, F.
  • Publikace: Physical Review B. 2005, 72(7), 075116-1-075116-8. ISSN 1098-0121.
  • Rok: 2005
  • DOI: 10.1103/PhysRevB.72.075116
  • Odkaz: https://doi.org/10.1103/PhysRevB.72.075116
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Amplification of evanescent electromagnetic waves by a metamaterial slab is still a controversial issue. Metallic waveguides operating in cutoff provide a unique physical environment to produce and study these waves because isolated evanescent modes can be excited in such environment. In this paper, the tunneling of electromagnetic waves through a rectangular waveguide partially filled by a metamaterial is analyzed. Enhanced tunneling due to the amplification of evanescent waves in the metamaterial filled waveguide section is demonstrated. Since evanescent modes in rectangular waveguides are the superposition of two plane waves, the aforementioned results also show the amplification and tunneling of evanescent plane waves by a metamaterial slab in free space.

On the Resonances and Polarizabilities of Split Ring Resonators

  • Autoři: Baena, J.D., Marqués, R., doc. Ing. Lukáš Jelínek, Ph.D., García-García, J., Martín, F.
  • Publikace: Journal of Applied Physics. 2005, 98(033103), 1-9. ISSN 0021-8979.
  • Rok: 2005
  • DOI: 10.1063/1.2006224
  • Odkaz: https://doi.org/10.1063/1.2006224
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this paper, the behavior at resonance of split ring resonators SRRs and other related topologies, such as the nonbianisotropic SRR and the broadside-coupled SRR, are studied. It is shown that these structures exhibit a fundamental resonant mode the quasistatic resonance and other higher-order modes which are related to dynamic processes. The excitation of these modes by means of a properly polarized time varying magnetic and/or electric fields is discussed on the basis of resonator symmetries. To verify the electromagnetic properties of these resonators, simulations based on resonance excitation by nonuniform and uniform external fields have been performed. Inspection of the currents at resonances, inferred from particle symmetries and full-wave electromagnetic simulations, allows us to predict the first-order dipolar moments induced at the different resonators and to develop a classification of the resonances based on this concept.

Perfect Tunneling and Amplification of Evanescent EM Waves in a Waveguide

  • Autoři: doc. Ing. Lukáš Jelínek, Ph.D., Baena, J.D.
  • Publikace: 9th International Student Conference on Electrical Engineering. Praha: České vysoké učení technické v Praze, Fakulta elektrotechnická, 2005. pp. C1.
  • Rok: 2005
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Metallic waveguides operating in cutoff provide a unique physical environment for producing and studying evanescent waves: isolated evanescent modes can be excited in such environment. Tunneling of electromagnetic waves through the in-cutoff rectangular waveguide, filled by a metamaterial, shall be analyzed in this paper and at the same time, enhanced near-perfect tunneling caused by the amplification of evanescent waves in the metamaterial shall be demonstrated.

Perfect Tunneling and Amplification of Evanescent EM Waves in a Waveguide

  • Autoři: Baena, J.D., doc. Ing. Lukáš Jelínek, Ph.D., Marques, R., Medina, F.
  • Publikace: Odborné semináře - Sborník za rok 2004/2005. Praha: Československá sekce IEEE, 2005. pp. 22-26. ISBN 80-86582-17-5.
  • Rok: 2005
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Metallic waveguides operating in cutoff provide a unique physical environment for producing and studying evanescent waves: isolated evanescent modes can be excited in such environment. Tunneling of electromagneic waves through the in-cutoff rectangular waveguide, filled by a metamaterial, shall be analyzed in this paper and at the same time, enhanced near-perfect tunneling caused by the amplification of evanescent waves in the metamaterial shall be demonstrated.

Reducing Losses and Dispersion Effects in Multilayer Metamaterial Tunnelling Devices

  • DOI: 10.1088/1367-2630/7/1/166
  • Odkaz: https://doi.org/10.1088/1367-2630/7/1/166
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper focuses on reduction of losses and dispersion effects on tunnelling through waveguides filled with metamaterial. It will be shown that these unwanted effects could be reduced by dividing the metamaterial into several regions separated by air slabs. In the first part, these effects will be studied for isotropic left-handed media (LHM). Later this will be substituted by an anisotropic magnetic medium which will lead to a practical realization with broadside coupled split ring resonators (BC-SRRs). Finally, it is shown that quasi-perfect tunnelling is possible, even in the presence of unavoidable losses and dispersion in the metamaterial.

Za stránku zodpovídá: Ing. Mgr. Radovan Suk