Lidé

prof. Ing. Miloslav Čapek, Ph.D.

Všechny publikace

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.

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 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.

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.

Characteristic Modes - Progress, Overview, and Emerging Topics

  • Autoři: Lau, B.K., prof. Ing. Miloslav Čapek, Ph.D., Hassan, A.M.
  • Publikace: IEEE Antennas and Propagation Magazine. 2022, 64(2), 14-22. ISSN 1045-9243.
  • Rok: 2022
  • DOI: 10.1109/MAP.2022.3145719
  • Odkaz: https://doi.org/10.1109/MAP.2022.3145719
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Over the past decade, characteristic mode analysis (CMA) research has grown from a niche topic to a mainstream topic, warranting a tutorial-style special issue to survey the significant progress that has been made in this field. In this introductory article (Paper 1), the focus is on providing the big picture. We start with a simple description of characteristic modes. Next, we examine the trends in this field, followed by providing further insights into CMA’s historical development. We will also address common myths surrounding the subject. Then, leaving the detailed coverage of major topics to the following papers, we summarize recent applications of CMA in scattering and other emerging topics. Finally, we conclude with some future perspectives on this field.

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.

Computational Aspects of Characteristic Mode Decomposition - An Overview

  • DOI: 10.1109/MAP.2021.3127527
  • Odkaz: https://doi.org/10.1109/MAP.2021.3127527
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Nearly all practical applications of the theory of characteristic modes (CMs) involve the use of computational tools. Here in Paper 2 of this Series on CMs, we review the general transformations that move CMs from a continuous theoretical framework to a discrete representation compatible with numerical methods. We also review key concepts encountered across a variety of numerical CM implementations. These include modal tracking, dynamic range, code validation, and techniques related to electrically large problems.

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

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.

Capacity Bounds and Degrees of Freedom for MIMO Antennas Constrained by Q-Factor

  • Autoři: Ehrenborg, C., Gustafsson, M., prof. Ing. Miloslav Čapek, Ph.D.,
  • Publikace: IEEE Transactions on Antennas and Propagation. 2021, 69(9), 5388-5400. ISSN 0018-926X.
  • Rok: 2021
  • DOI: 10.1109/TAP.2021.3069432
  • Odkaz: https://doi.org/10.1109/TAP.2021.3069432
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The optimal spectral efficiency and number of independent channels for MIMO antennas in isotropic multipath channels are investigated when bandwidth requirements are placed on the antenna. By posing the problem as a convex optimization problem restricted by the port Q-factor a semi-analytical expression is formed for its solution. The antennas are simulated by method of moments and the solution is formulated both for structures fed by discrete ports, as well as for design regions characterized by an equivalent current. It is shown that the solution is solely dependent on the eigenvalues of the so-called energy modes of the antenna. The magnitude of these eigenvalues is analyzed for a linear dipole array as well as a plate with embedded antenna regions. The energy modes are also compared to the characteristic modes to validate characteristic modes as a design strategy for MIMO antennas. The antenna performance is illustrated through spectral efficiency over the Q-factor, a quantity that is connected to the capacity. It is proposed that the number of energy modes below a given Q-factor can be used to estimate the degrees of freedom for that Q-factor.

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.

FOPS: A new framework for the optimization with variable number of dimensions

  • Autoři: Marek, M., Kadlec, P., prof. Ing. Miloslav Čapek, Ph.D.,
  • Publikace: International Journal of RF and Microwave Computer-Aided Engineering. 2020, 30(9), ISSN 1096-4290.
  • Rok: 2020
  • DOI: 10.1002/mmce.22335
  • Odkaz: https://doi.org/10.1002/mmce.22335
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A single- and multi-objective optimization package is presented and described in detail. It contains an ensemble of local and global optimization routines. Procedures controlling variable number of dimensions are implemented as well, which is a rare feature among optimization oriented packages. The package is provided as a MATLAB toolbox. It excels in versatility and extensibility, which is demonstrated on a series of examples covering classical electromagnetism and antenna design. It is taken for granted that defining parameters of the optimization method can be set prior to the simulation run. However, its effective performance can be changed during the optimization run thanks to the full control feature. Moreover, it opens new possibilities in merging various algorithms into hybrids, performing complex dynamic programming tasks, or exploiting third party software. These advantages render the package as a perfect tool to deal with nowadays challenging engineering tasks.

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.

Augmenting Characteristic Mode Orthogonality via Iteratively Applied Constraints

  • Autoři: Schab, K., prof. Ing. Miloslav Čapek, Ph.D., Gustafsson, M.
  • Publikace: 2019 13TH EUROPEAN CONFERENCE ON ANTENNAS AND PROPAGATION (EUCAP). IEEE (Institute of Electrical and Electronics Engineers), 2019. p. 1-3. ISSN 2164-3342. ISBN 9788890701887.
  • Rok: 2019
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Affine constraints are iteratively applied to enforce multiple orthogonality conditions on currents satisfying a general eigenvalue problem. As an example, the characteristic mode eigenvalue problem is iteratively solved with constraints simultaneously enforcing orthogonality in the loss, radiation, and reactance operators. The resulting modes share many properties with standard characteristic modes.

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.

Maximum Gain, Effective Area, and Directivity

  • Autoři: Gustafsson, M., prof. Ing. Miloslav Čapek, Ph.D.,
  • Publikace: IEEE Transactions on Antennas and Propagation. 2019, 67(9), 5282-5293. ISSN 0018-926X.
  • Rok: 2019
  • DOI: 10.1109/TAP.2019.2916760
  • Odkaz: https://doi.org/10.1109/TAP.2019.2916760
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Fundamental bounds on antenna gain are found via convex optimization of the current density in a prescribed region. Various constraints are considered, including self-resonance and only partial control of the current distribution. Derived formulas are valid for arbitrarily shaped radiators of a given conductivity. All the optimization tasks are reduced to eigenvalue problems, which are solved efficiently. The second part of the paper deals with superdirectivity and its associated minimal costs in efficiency and Q-factor. The paper is accompanied with a series of examples practically demonstrating the relevance of the theoretical framework and entirely spanning a wide range of material parameters and electrical sizes used in antenna technology. Presented results are analyzed from a perspective of effectively radiating modes. In contrast to a common approach utilizing spherical modes, the radiating modes of a given body are directly evaluated and analyzed here. All crucial mathematical steps are reviewed in the appendices, including a series of important subroutines to be considered making it possible to reduce the computational burden associated with the evaluation of electrically large structures and structures of high conductivity.

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.

Tradeoff Between Antenna Efficiency and Q-Factor

  • Autoři: Gustafsson, M., prof. Ing. Miloslav Čapek, Ph.D., Schab, K.
  • Publikace: IEEE Transactions on Antennas and Propagation. 2019, 67(4), 2482-2493. ISSN 0018-926X.
  • Rok: 2019
  • DOI: 10.1109/TAP.2019.2891448
  • Odkaz: https://doi.org/10.1109/TAP.2019.2891448
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The trade-off between radiation efficiency and antenna bandwidth, expressed in terms of Q-factor, for small antennas is formulated as a multi-objective optimization problem in current distributions of predefined support. Variants on the problem are constructed to demonstrate the consequences of requiring a self-resonant current as opposed to one tuned by an external reactance. The trade-offs are evaluated for sample problems and the resulting Pareto-optimal sets reveal the relative cost of valuing low radiation Q-factor over high efficiency, the cost in efficiency to require a self-resonant current, the effects of lossy parasitic loading, and other insights. Observations are drawn from the sample problems selected, though the tradeoff evaluation method is valid for studying arbitrary antenna geometries. In the examples considered here, we observe that small increases in Q-factor away from its lower bound allow for dramatic increases in efficiency toward its upper bound.

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.

AToM: A Versatile MATLAB Tool for Antenna Synthesis

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The Antenna Toolbox For MATLAB (AToM), originally an in-house academic tool, has been transformed into a complete MATLAB toolbox, capable of modeling, discretizing and calculating arbitrarily shaped planar radiators while analysing the results. All tasks can be performed directly in MATLAB. The majority of the code allows its direct modification. AToM supports the latest features, used predominantly in the realm of electrically small antennas, namely modal decompositions, evaluation of fundamental bounds, and other techniques based on the source concept.

Benchmark Problem Definition and Cross-Validation for Characteristic Mode Solvers

  • Autoři: Chen, Y., Schab, K., prof. Ing. Miloslav Čapek, Ph.D., Mašek, M., Lau, B.K., Aliakbari, H., Haykir, Y., Wu, Q., Strydom, W.J., Peitzmeier, N., Jovovic, M., Genovesi, S., Dicandia, F.A.
  • Publikace: 2018 12th European Conference on Antennas and Propagation. Bruxelles: The European Association on Antennas and Propagation, 2018. ISSN 0537-9989. ISBN 978-1-78561-815-4.
  • Rok: 2018
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In October 2016, the Special Interest Group on Theory of Characteristic Modes (TCM) initiated a coordinated effort to perform benchmarking work for characteristic mode (CM) analysis. The primary purpose is to help improve the reliability and capability of existing CM solvers and to provide the means for validating future tools. Significant progress has already been made in this joint activity. In particular, this paper describes several benchmark problems that were defined and analyzes some results from the cross-validations of different CM solvers using these problems. The results show that despite differences in the implementation details, good agreement is observed in the calculated eigenvalues and eigencurrents across the solvers. Finally, it is concluded that future work should focus on understanding the impact of common parameters and output settings to further reduce variability in the results.

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.

Implementation of the Theory of Characteristic Modes into Antenna Modeling Tools AToM and Visual Antenna

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Two software packages suitable for the modeling of antennas and scatterers are presented. The approach of modal decomposition utilized leads to a better understanding of the radiation properties of the antennas studied and promises to reduce the length of the design process. Two examples are shown and the results are compared to an analytical solution, as well as to results from other simulators.

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.

Analytical Representation of Characteristic Modes Decomposition

  • DOI: 10.1109/TAP.2016.2632725
  • Odkaz: https://doi.org/10.1109/TAP.2016.2632725
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Aspects of the theory of characteristic modes, based on their variational formulation, are presented and an explicit form of a related functional, involving only currents in a spatial domain, is derived. The new formulation leads to deeper insight into the modal behavior of radiating structures as demonstrated by a detailed analysis of three canonical structures: a dipole, an array of two dipoles and a loop, cylinder and a sphere. It is demonstrated that knowledge of the analytical functional can be utilized to solve important problems related to the theory of characteristic modes decomposition such as the resonance of inductive modes or the benchmarking of method of moments code.

Characteristic Modes of Electrically Small Antennas in the Presence of Electrically Large Platforms

  • DOI: 10.1109/PIERS.2017.8262407
  • Odkaz: https://doi.org/10.1109/PIERS.2017.8262407
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The theory of characteristic modes can be applied to both electrically small and large objects. Its utilization, based on the electric-field integral equation using the Method of Moments and the Fast Multipole Method numerically implemented in Matlab, is described in this paper. Preliminary results and a comparison of both methods are presented on an example of a perfectly electrical patch.

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.

Minimization of Antenna Quality Factor

  • Autoři: prof. Ing. Miloslav Čapek, Ph.D., Gustafsson, M., Schab, K.
  • Publikace: IEEE Transactions on Antennas and Propagation. 2017, 65(8), 4115-4123. ISSN 0018-926X.
  • Rok: 2017
  • DOI: 10.1109/TAP.2017.2717478
  • Odkaz: https://doi.org/10.1109/TAP.2017.2717478
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Optimal currents on arbitrarily shaped radiators with respect to the minimum quality factor are found using a simple and efficient procedure. The solution starts with a reformulation of the problem of minimizing quality factor Q as an alternative, so-called dual, problem. Taking advantage of modal decomposition and group theory, it is shown that the dual problem can easily be solved and always results in minimal quality factor Q. Moreover, the optimization procedure is generalized to minimize quality factor Q for embedded antennas, with respect to the arbitrarily weighted radiation patterns, or with prescribed magnitude of the electric and magnetic near-fields. The obtained numerical results are compatible with previous results based on composition of modal currents, convex optimization, and quasi-static approximations; however, using the methodology in this paper, the class of solvable problems is significantly extended.

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.

An Introduction to the Source Concept for Antennas

  • DOI: 10.13164/re.2016.0012
  • Odkaz: https://doi.org/10.13164/re.2016.0012
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Antenna parameters particularly relevant to electrically small antenna design are reviewed in this paper. Source current definitions are accentuated leading to the introduction of the source concept which advantageously utilize only spatially bounded quantities. The framework of the source concept incorporates powerful techniques such as structural and modal decomposition, operator’s inversion and current optimization, thus opening new, challenging possibilities for antenna design, analysis and synthesis.

Bandwidth optimization of linear arrays above ground

  • DOI: 10.1109/EuCAP.2016.7481267
  • Odkaz: https://doi.org/10.1109/EuCAP.2016.7481267
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Two different array of dipoles above infinite electric ground plane are optimized for bandwidth using particle swarm optimizer, concept of self and mutual impedances and impedance quality factor. It is found that it is the interaction of mutual radiated power that limits the bandwidth, rather than stored energies.

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.

Some numerical aspects of characteristic mode decomposition

  • DOI: 10.1109/EuCAP.2016.7481187
  • Odkaz: https://doi.org/10.1109/EuCAP.2016.7481187
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Characteristic mode decomposition represents popular and powerful numerical method which offers deep physical insight into radiation mechanisms and an effective framework for small antenna design. The quality of resultant characteristic basis is strongly dependent on the properties of the impedance matrix but also on utilized factorization technique, or the exact shape of the radiator. In this contribution, selected numerical aspects like incompleteness of the characteristic basis will be discussed. These observations will be proved to be important for practical usage of the characteristic modes.

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.

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.

Evaluating radiation efficiency from characteristic currents

  • DOI: 10.1049/iet-map.2013.0473
  • Odkaz: https://doi.org/10.1049/iet-map.2013.0473
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This study describes an effective technique for calculating modal radiation efficiency calculation based on decomposition into characteristic modes. The key assumption is that the current distribution on the perfect electric conductor is almost the same as in the case of a very good conductor, for example, metals such as copper, aluminium and silver. This assumption is verified against the conventional technique, the impedance boundary condition (IBC). The proposed approach does not require any modification of the formulation of method of moments for perfectly conducting surfaces, which is assumed for the modal decomposition. Modal efficiencies provide an additional insight that is useful especially for the design of small antennas. Taking the feeding into account, the modal losses can be summed up to obtain the total efficiency. The technique works perfectly for common metals, is fully comparable with the IBC, and can easily be incorporated into any present-day in-house solver. A numerical analysis of three antennas is presented to demonstrate the merits of the approach. Radiation efficiency of coupled dipoles, an electrically small meandered dipole, and PIFA were investigated by the presented method. The results are in perfect agreement with the reference commercial package.

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 Antenna Toolbox for Matlab (AToM)

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This contribution presents recent development of the Antenna Toolbox for Matlab (AToM). The toolbox utilize novel theoretical findings related to characteristic mode decomposition and the so-called source concept, by which all antenna characteristics are represented solely by means of currents.

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 Method for Evaluating Radiation Efficiency Based on a Modal Approach

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Radiation efficiency is an important parameter, especially for electrically small antennas. It can be computed directly from the method-of-moments impedance matrix, and also more generally by employing modal decomposition through characteristic modes. This advantageously enables us to study separately the effect of the radiating shape and the feeding. The proposed method does not require any modification of the Electric Field Integral Equation implementation. A perfectly conducting antenna is considered as the only input to the modal decomposition. The conductive losses are taken into account once the surface currents are calculated. Good agreement is observed for the proposed treatment of the skin effect. Verification of the method is numerically performed on an fractal antenna, a thinstrip Sierpinski curve of various iterations. A full-wave software FEKO with implemented IBC is used for comparison purposes.

Aspects of mesh generation for characteristic-mode analysis

  • DOI: 10.1109/MAP.2014.6867702
  • Odkaz: https://doi.org/10.1109/MAP.2014.6867702
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper deals with practical aspects of mesh generation for the theory of characteristic modes. First, we describe a tool for surface-mesh generation in MATLAB. The tool is afterwards used for an analysis of relative convergence of modal results computed by an in-house modal analyzer in MATLAB. Different meshing scenarios are selected for a dipole, a rectangular patch, and a rectangular patch with a slot, and a recommendation for a mesh-refinement strategy is given. The study is supported by a simple error analysis, considering the approximation error in the evaluation of moment-matrix elements. It is shown that the results are also applicable for the commercial implementation in FEKO software.

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 Radiation Q-Factor of a Horizontal lambda/2 Dipole Above Ground Plane

  • DOI: 10.1109/LAWP.2014.2329421
  • Odkaz: https://doi.org/10.1109/LAWP.2014.2329421
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Closed-form equations are given for radiation quality factor of a thin-wire =2 dipole above an infinite electric ground plane. Particular results of interest include a simple formula for a small separation between the dipole and the ground and for one dipole in free space.

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.

Acceleration Techniques in Matlab for EM Community

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper describes some acceleration techniques in Matlab. Here, we strongly focus on acceleration of computational routines via High Performance Computing (HPC), which - in terms of Matlab software - includes parallel and distributive computing on CPU(s) and GPU computing. We also introduce an in-house antenna tool which is based on the Method of Moments and Theory of Characteristic Modes. Two numerically challenging examples are presented and then solved in Matlab

Comments to Reactive Energies, Impedance, and Q Factor of Radiating Structures by G. Vandenbosch

Implementation of the Theory of Characteristic Modes in MATLAB

  • DOI: 10.1109/MAP.2013.6529342
  • Odkaz: https://doi.org/10.1109/MAP.2013.6529342
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper describes the implementation of a complex MATLAB tool to calculate the characteristic modes and associated antenna parameters. The first code, written in FORTRAN, was presented in the early seventieths by Harrington and Mautz. Here, we utilize MATLAB, which is widely known and used in the antenna community these days. Because eigen-decomposition is time consuming, parallel and distributed computing is used. Thanks to the hundreds of built-in functions in MATLAB, computation of the surface currents from the eigenvectors obtained, as well as other important characteristics, are very easy and effective. The practical features are discussed with two examples.

Simulation of Electromagnetic Field of a Fast Moving Target Close to Antennas

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Experiments have shown that Doppler signatures of a missile moving in a close vicinity of the antenna is quite complex. Experimentally obtained spectrograms exhibit clusters of Doppler frequencies instead of a single one. This is due to the fact that radial velocities differ significantly for different parts of the missile. Partial numerical simulations of this complex problem have been carried out for thin long fast-moving targets (wires). Simulation results compare well with measured Doppler signatures and enable better understanding of the problem and therefore improve the ability to distinguish dangerous targets from ordinary ones.

A Method for the Evaluation of Radiation Q Based on Modal Approach

  • DOI: 10.1109/TAP.2012.2207329
  • Odkaz: https://doi.org/10.1109/TAP.2012.2207329
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A new formula for the evaluation of the modal radiation Q factor is derived. The total Q of selected structures is to be calculated from the set of eigenmodes with associated eigen-energies and eigen-powers. Thanks to the analytical expression of these quantities, the procedure is highly accurate, respecting arbitrary current densities flowing along the radiating device. The electric field integral equation, Delaunay triangulation, method of moments, Rao-Wilton-Glisson basis function and the theory of characteristic modes constitute the underlying theoretical background. In terms of the modal radiation Q, all necessary relations are presented and the essential points of implementation are discussed. Calculation of the modal energies and Q factors enable us to study the effect of the radiating shape separately to the feeding. This approach can be very helpful in antenna design. A few examples are given, including a thin-strip dipole, two coupled dipoles a bowtie antenna and an electrically small meander folded dipole. Results are compared with prior estimates and some observations are discussed. Good agreement is observed for different methods.

Active Low Noise Differentially Fed Dipole Antenna

  • Autoři: Eichler, J., Segovia-Vargas, D., doc. Ing. Pavel Hazdra, Ph.D., prof. Ing. Miloslav Čapek, Ph.D., González-Posadas, V.
  • Publikace: Proceedings of the 10th International Symposium on Antennas, Propagation, and EM Theory. Piscataway: IEEE, 2012. pp. 1-4. ISSN 1045-9243. ISBN 978-1-4673-1799-3.
  • Rok: 2012
  • DOI: 10.1109/ISAPE.2012.6408766
  • Odkaz: https://doi.org/10.1109/ISAPE.2012.6408766
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An active antenna based on a printed dipole directly connected to a differential amplifier is presented. The antenna covers the frequency band from 1.7-2.6GHz meaning the relative bandwidth of 41.86%. As the input impedance is mainly defined by the amplifier output, S 11 < -10dB was obtained in the whole frequency range. By designing both the active and the passive parts together a low equivalent noise temperature of 169K was obtained. The gain of the active antenna is higher than 15dBi. Antenna prototype was manufactured and measured. A good agreement between simulation and measurement was observed.

Modal Resonant Frequencies and Radiation Quality Factors of Microstrip Antennas

  • DOI: 10.1155/IJAP
  • Odkaz: https://doi.org/10.1155/IJAP
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The chosen rectangular and fractal microstrip patch antennas above an infinite ground plane are analyzed by the Theory of characteristic modes. The resonant frequencies and radiation Q are evaluated. An novel method by G. Vandenbosch for rigorous evaluation of the radiation Q is employed for modal currents on a Rao-Wilton-Glisson (RWG) mesh. It is found that the resonant frequency of a rectangular patch antenna with a dominant mode presents quite complicated behaviour including having a minimum at a specific height. Similarly, as predicted from the simple wire model, the radiation Q exhibits a minimum too. It is observed that the presence of out-of-phase currents flowing along the patch antenna leads to a significant increase of the Q-factor.

On the Modal Resonant Properties of Microstrip Antennas

  • DOI: 10.1109/EuCAP.2012.6206511
  • Odkaz: https://doi.org/10.1109/EuCAP.2012.6206511
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The chosen rectangular and fractal microstrip patch antennas above an infinite ground plane are analyzed by the Theory of characteristic modes. The resonant frequencies and radiation Q are evaluated. An method by G. Vandenbosch for rigorous evaluation of the radiation Q is employed for modal currents on a Rao-Wilton-Glisson (RWG) mesh. It is found that the resonant frequency of a rectangular patch antenna with a dominant mode presents quite complicated behaviour including having a minimum at a specific height. Similarly, as predicted from the simple wire model, the radiation Q exhibits a minimum too. It is observed that the presence of out-of-phase currents flowing along the patch antenna leads to a significant increase of the Q-factor.

The Evaluation of Total Radiation Q Based on Modal Aproach

  • DOI: 10.1109/EuCAP.2012.6205873
  • Odkaz: https://doi.org/10.1109/EuCAP.2012.6205873
  • Pracoviště: Katedra matematiky, Katedra elektromagnetického pole
  • Anotace:
    The total Q of selected structures is to be calculated from the set of eigenmodes with associated eigen-energies and eigen-powers. Thanks to the analytical expression of these quantities, the procedure is highly accurate, respecting arbitrary current densities flowing along the radiating device. The electric field integral equation, Delaunay triangulation, method of moments, Rao-Wilton-Glisson basis function and the theory of characteristic modes constitute the underlying theoretical background. Calculation of the modal energies and Q factors enable us to study the effect of the radiating shape separately to the feeding. To outline some benefits of proposed method, the total radiation Q of a Huyghens source is calculated for several distances between a loop and an dipole.

A Method for Tracking Characteristic Numbers and Vectors

  • DOI: 10.2528/PIERB11060209
  • Odkaz: https://doi.org/10.2528/PIERB11060209
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A new method for tracking characteristic numbers and vectors appearing in the Characteristic Mode Theory is presented in this paper. The challenge here is that the spectral decomposition of the moment impedance-matrix doesn't always produce well ordered eigenmodes. This issue is addressed particularly to nite numerical accuracy and slight nonsymmetry of the frequency- dependent matrix. At specic frequencies, the decomposition problem might be ill-posed and non-uniquely dened as well. Hence an advanced tracking procedure has been developed to deal with noisy modes, non-continuous behavior of eigenvalues, mode swapping etc. Proposed method has been successfully implemented into our in-house Characteristic Mode software tool for the design of microstrip patch antennas and tested for some interesting examples.

Design of a Dual-Band Orthogonally Polarized L-Probe-Fed Fractal Patch Antenna Using Modal Methods

  • DOI: 10.1109/LAWP.2011.2178811
  • Odkaz: https://doi.org/10.1109/LAWP.2011.2178811
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Modal methods are used to effectively design a dual-band orthogonally polarized fractal patch antenna. This letter summarizes the workflow from generating a fractal motif through modal analysis to feeding design and full-wave analysis. As the antenna's feeding and matching structure, a dual L-probe was proposed to widen its bandwidth. The full-wave simulation is in very good agreement with the measurement. The motif size is 50 50 mm, and the antenna operates at 1.25 and 2.1 GHz. The relative bandwidths are 4.18% and 11.4%, respectively.

Multiobjective optimization of Fractal Radiating Structures Using Modal Decomposition

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The paper describes in-house developed software tools for design of microstrip patch fractal antennas. As the fractal geometry is quite complex, IFSMaker application was developed purposedly for an easy maintaining of planar fractal structures. In the next step, we are using cavity model technique for modal analysis of internal fields of the patch. This simplified model is reasonably fast, so that optimization loop may be employed in order to reduce the fundamental mode resonant frequency. Particle swarm optimization is used and the PSOoptimizer tool was created to handle it. Theory of characteristic modes (TCM) is then used to refine the results. The crucial problem of use of TCM in the optimization loop is the proper sorting of characteristic modes. The sorting problem is discussed and new method is presented.

Radiation Efficiency and Q Factor Study of Franklin Antenna Using the Theory of Characteristic Modes

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents the analysis of Franklin antenna by the Theory of Characteristic Modes. Modal radiation efficiencies are calculated and used to obtain the overall radiation efficiency. For each mode also the Q factor is calculated. On these results the influence of distance between arms of the folded part of antenna is demonstrated.

Radiation Q-Factors of Thin-Wire Dipole Arrangements

  • DOI: 10.1109/LAWP.2011.2158050
  • Odkaz: https://doi.org/10.1109/LAWP.2011.2158050
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this letter, we present an investigation of the radiation -factors of two coupled thin dipole antennas with sinusoidal current distribution. The approach is based on novel rigorous equations for radiated power and stored energies recently derived by Vandenbosch. First, we study the validity of the used thin-wire approximation with a reduced kernel. Good agreement between the assumed sinusoidal current distribution and the real cylindrical antenna modeled with the full-wave method of moments (MoM) is observed. Then, radiation -factors are evaluated for half-wave side-by-side coupled dipole antennas with different feeding configurations. It is found that every such combination of studied coupled dipoles presents minimum for specific feeding arrangement and separation distance.

Small dual-band fractal antenna with orthogonal polarizations

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Modal methods are effectively used to design dual band fractal patch antenna. This contribution summarizes the workflow from generating fractal motif through choosing proper feeding to final antenna completion. As an antenna's feeding and matching structure, dual L-probe was proposed to widen its bandwidth. Moreover, the polarizations on operating bands are mutually orthogonal. This behavior is easily explained by modal decomposition of the surface currents. Antenna was fabricated and measured. © 2011 EurAAP.

Software Tools for Efficient Generation, Modeling and Optimisation of Fractal Radiating Structures

  • DOI: 10.1049/iet-map.2010.0269
  • Odkaz: https://doi.org/10.1049/iet-map.2010.0269
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The paper describes in-house developed software tools for design of microstrip patch fractal antennas. As the fractal geometry is quite complex, IFSMaker application was developed purposedly for an easy maintaining of planar fractal structures. In the next step, we are using cavity model technique for modal analysis of internal fields of the patch. This simplified model is reasonably fast, so that optimisation loop may be employed in order to reduce the fundamental mode resonant frequency. This is accomplished with the valInFEM, PSOoptimizer and IFSLimiter applications. Theory of characteristic modes is then used to refine the results and finally antenna is simulated with full-wave CST simulator, built and measured.

Advanced Modal Techniques for Microstrip Patch Antenna Analysis

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper shows the advantage of modal methods utilization in the area of microstrip patch antennas design. Namely, the Theory of Characteristic Modes (TCM) is developed starting from the Electric Field Integral Equation and formulating proper functional equation for stored and radiated power. Developed in-house TCM analyzer is presented and employed to investigate properties of the linearly and circularly polarized rectangular patch antenna.

Design of IFS Patch Antenna Using Particle Swarm Optimization

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Paper describes usage of the IFS (Iterated Function System) fractals employed as a planar microstrip patch antennas. To generate the IFS fractals, versatile tool named IFSMaker has been evolved. Efficient design of patches will be shown. After fractal pre-design, one can start to optimize the IFS collage by using the PSO algorithm. This procedure is managed by the PSOptimizer tool. Optimalization loop (which includes IFS generator, cavity model (CM) solver and PSO evaluation) works generally with any number of optimalization conditions. This feature is provided by IFSLimiter tool. Resonant frequencies (eigenvalue problem) are calculated with cavity model in Comsol Multiphysics environment. Finally we discuss the radiation pattern calculation and usage of theory of characteristic modes as well.

Optimization Tool for Fractal Patches Based on the IFS Algorithm

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents set of software tool for the design and optimization of fractal patches describes by the IFS.

PSO Optimalization of IFS Fractal Patch Antennas

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Paper describes the use of IFS (Iterated Function System) fractals employed as a planar microstrip patch antennas. The fractal shapes have some interesting features when used as a radiator - like reduction of resonant frequencies of multiband behaviour. To generate the IFS fractals, versatile tool named IFSMaker (based on Matlab Object-Oriented Programming), has been evolved. After fractal predesign, one can start to optimize the IFS collage by using the PSO algorithm. This procedure is managed by the PSOptimizer tool. Optimalization loop (which includes IFS generator, cavity model solver and PSO evaluation) may works generally with any number of optimalization conditions. This feature is provided by IFSLimiter tool. The resonant frequencies are calculated with cavity model in comsol Multiphysics environment.

PSO optimalizace v MATLABU

  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Příspěvek se věnuje implementaci PSO algoritmu v Matlabu. PSO je stále relativně novou, velice rychlou a robustní technikou, která podává ve většině případů výborné výsledky. Algoritmus je rozšířen o tzv. zdi, které celou optimalizaci zrychlují. Probrány jsou i jednotlivé parametry, které průběh optimalizace ovlivňují.

Za stránku zodpovídá: Ing. Mgr. Radovan Suk