| 1. |
- Nel, Ben, et al.
(author)
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Stored Energies and Q-factor expressed in Material derivatives
- 2024
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In: IEEE Antennas and Wireless Propagation Letters. - 1548-5757. ; 23:1, s. 19-23
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Journal article (peer-reviewed)abstract
- Stored energies of radiating systems have generated research interest for several decades due to their relationship with Q-factor and fractional bandwidth. In this paper material derivatives are used to provide a new interpretation of widely used stored energy expressions. This provides a fundamental relationship between stored energies of radiating systems and their electromagnetic material properties. It is shown that electric stored energy is related to the electric material parameter (permittivity) derivative. Similarly, magnetic stored energy is linked to the derivative of the permeability. Further, as an extension Cauchy-Riemann equations are used to relate stored energy to dissipation.
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| 2. |
- Capek, Miloslav, et al.
(author)
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Characteristic Mode Decomposition Using the Scattering Dyadic in Arbitrary Full-Wave Solvers
- 2023
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In: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 71:1, s. 830-839
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Capek, Miloslav, et al.
(author)
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Optimal Inverse Design Based on Memetic Algorithms - Part 1 : Theory and Implementation
- 2023
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In: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 71:11, s. 8806-8816
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Journal article (peer-reviewed)abstract
- 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-moments 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 paper 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.
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| 4. |
- Capek, Miloslav, et al.
(author)
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Optimal Inverse Design Based on Memetic Algorithms - Part II : Examples and Properties
- 2023
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In: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 71:11, s. 8817-8829
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Journal article (peer-reviewed)abstract
- 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.
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| 5. |
- Gustafsson, Mats, et al.
(author)
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Degrees of Freedom and Characteristic Modes
- 2023
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Reports (other academic/artistic)abstract
- The number of degrees of freedom is a crucial parameter in many electromagnetic problems. In for example modern communication systems spatial diversity is often employed through multiple beams to enhance capacity and reliability. However, while the degrees of freedom can be computed, their connection to physical quantities is not as easily understood. To address this issue, this paper proposes a scattering-based formulation of characteristic mode analysis that can estimate the degrees of freedom of arbitrarily-shaped objects. The relation between the number of dominant characteristic modes and physical characteristics differs for electrically large and small objects. Specifically, for large objects, it is linked to the average shadow area, while for small objects, it is linked to their average polarizability through the forward scattering sum rule. Therefore, the average shadow area and polarizability are fundamental parameters that provide insight into the number of degrees of freedom for any object. These basic parameters also provide straightforward estimates of the minimum size of a device region required to support a desired number of electromagnetic degrees of freedom across a given spectral response.
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| 6. |
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| 7. |
- Lundgren, Johan, et al.
(author)
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Iterative Calculation of Characteristic Modes Using Arbitrary Full-Wave Solvers
- 2023
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In: IEEE Antennas and Wireless Propagation Letters. - 1536-1225. ; 22:4, s. 799-803
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Journal article (peer-reviewed)abstract
- 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.
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| 8. |
- Nel, Ben, et al.
(author)
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Q-factor Bounds for Microstrip Patch Antennas
- 2023
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In: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 71:4, s. 3430-3440
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Journal article (peer-reviewed)abstract
- Antenna bounds are a useful tool in assessing the feasibility or performance of an antenna design. Microstrip patch antennas are often limited by their relatively narrow bandwidth, and therefore Q-factor is an important design parameter, as it is related to the inverse of the fractional bandwidth. This article presents the first tight lower Q-factor bounds on microstrip patch antennas supported by an infinite dielectric substrate. The derived lower Q-factor bounds are orders of magnitude tighter than the Chu limit and introduce a new scaling rule. These bounds consider all possible geometries on the predefined design region. Moreover, it is shown that well-known patch antennas have Q-factors near the bounds and have thus a near-optimal bandwidth. The computation of the bounds is done using a method of moments (MoM) formulation. However, an approximation to these bounds using commonly available simulation tools is provided.
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| 9. |
- Nosratabadi, Ali Reza, 1964-, et al.
(author)
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Airway contraction and cytokine release in isolated rat lungs induced by wear particles from the road and tire interface and road vehicle brakes
- 2023
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In: Inhalation Toxicology. - : Taylor & Francis. - 0895-8378 .- 1091-7691. ; 35:13-14, s. 309-323
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Journal article (peer-reviewed)abstract
- The dominant road traffic particle sources are wear particles from the road and tire interface, and from vehicle brake pads. The aim of this work was to investigate the effect of road and brake wear particles on pulmonary function and biomarkers in isolated perfused rat lungs. Particles were sampled from the studded tire wear of three road pavements containing different rock materials in a road simulator; and from the wear of two brake pad materials using a pin-on-disk machine. Isolated rat lungs inhaled the coarse and fine fractions of the sampled particles resulting in an estimated total particle lung dose of 50 μg. The tidal volume (TV) was measured during the particle exposure and the following 50 min. Perfusate and BALF were analyzed for the cytokines TNF, CXCL1 and CCL3. The TV of lungs exposed to rock materials was significantly reduced after 25 min of exposure compared to the controls, for quartzite already after 4 min. The particles of the heavy-duty brake pads had no effect on the TV. Brake particles resulted in a significant elevation of CXCL1 in the perfusate. Brake particles showed significant elevations of all three measured cytokines, and quartzite showed a significant elevation of TNF in BALF. The study shows that the toxic effect on lungs exposed to airborne particles can be investigated using measurements of tidal volume. Furthermore, the study shows that the choice of rock material in road pavements has the potential to affect the toxicity of road wear PM10.
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| 10. |
- Schab, Kurt, et al.
(author)
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Characteristic Modes of Frequency-Selective Surfaces and Metasurfaces from S-Parameter Data
- 2023
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In: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 71:12, s. 9696-9706
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Journal article (peer-reviewed)abstract
- Characteristic modes of arbitrary 2-D 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 (MoM) 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.
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