Computational Electromagnetics (CEM)

CEM group concentrates on theoretical research in classical electromagnetic theory. Among the currently developed topics belong radiation properties of electrically small radiators and scatterers and their optimizations. Members of the group have for many years been also involved in classical antenna theory and field propagation in artificial composite materials.

Current research activities are strongly entangled with a “source concept” paradigm, in which all involved field quantities as well as engineering metrics are described solely in terms of electromagnetic sources. The idea of the source concept is sketched in the following figure.

CEM team collaborates with other department’s teams.

Past Members

Vit Losenicky
Michal Masek
Jakub Liska
Jonas Tucek
Vojtech Neuman

Martin Strambach
Lamyae Akrou

Projects

Source Concept of Electrically Small Antenna Synthesis  (2015-2017, GA15-10280Y)

Active External Collaborations

University of Lund, Sweden (Mats Gustafson, Doruk Tayli, Casimir Ehrenborg)
University of Santa Clara, USA (Kurt Schab)
KTH Royal Institute of Technology, Sweden (Lars Johnsson)
Catholic University of Leuven, Belgium (Guy Vandenbosch)
Université Nice Sophia Antipolis (Fabien Ferrero)

Journal Papers (Last Three Years)

Current and Antenna Optimization

 M. Gustafsson, M. Capek, and K. Schab, "Trade-off Between Antenna Efficiency and Q-Factor", 2017, eprint arXiv: 1802.01476. [Online]. Available: https://arxiv.org/abs/1802.01476 Abstract 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 resulting Pareto-optimal sets reveal the relative cost of valuing low Q-factor over high efficiency, the cost in efficiency to require a self-resonant current, and other insights. L Jelinek, K. Schab, and M. Capek, "The Radiation Efficiency Cost of Resonance Tuning", IEEE Trans. Antennas Propag, 2018, eprint arXiv: 1712.02613. [Online]. Available: https://arxiv.org/abs/1712.02613 Abstract Existing optimization methods are used to calculate the upper-bounds on radiation efficiency with and without the constraint of 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. M. Capek, M. Gustafsson, and K. Schab, "Minimization of antenna quality factor", IEEE Trans. Antennas Propag., vol. 65, pp. 4115 - 4123, 2017. Abstract The optimal currents on arbitrarily shaped radiators with respect to the minimum quality factor Q 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. L. Jelinek and M. Capek, "Optimal currents on arbitrarily shaped surfaces," IEEE Trans. Antennas Propag., vol. 65, pp. 329 - 341, 2017. Abstract 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. M. Capek and L. Jelinek, "Optimal composition of modal currents for minimal quality factor Q", IEEE Trans. Antennas Propag., vol. 64, pp. 5230 - 5242, 2016. Abstract 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.