| 21. |
- Sjöberg, Daniel, et al.
(författare)
-
Cramér-Rao bounds for determination of permittivity and permeability in slabs
- 2010
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We compute the Cramér-Rao lower bounds for determination of isotropic permittivity and permeability in slabs using scattering data such as reflection and transmission coefficients. Assuming only the fundamental mode is propagating in the slab, the results are formally the same regardless if the slab is in free space, or inside a rectangular or coaxial waveguide. The bounds depend only on signal quality and not the actual inversion method used, making them suitable to evaluate a particular measurement setup. The results are illustrated with several measurements in a rectangular waveguide setting.
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| 22. |
- Sjöberg, Daniel
(författare)
-
Dispersive effective material parameters for Maxwell's equations
- 2006
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We study how effective material parameters can be defined for Maxwell's equations when taking dispersion into account. The reasoning is based on the concept of dispersion relations, and is consequently primarily concerned with lossless media. We essentially require that the effective material parameters should produce the same dispersion relations as the heterogeneous problem, which implies the effective material is primarily connected to the phase velocity of the waves. Material parameters which exhibit temporal dispersion only, can be defined if the propagation direction is fixed.
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| 23. |
- Sjöberg, Daniel
(författare)
-
Exact and asymptotic dispersion relations for homogenization of stratified media with two phases
- 2005
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Using exact dispersion relations for electromagnetic wave propagation in layered, periodic media, consisting of two phases, we derive explicit asymptotic solutions for small wavenumbers. These solutions are compared to the numerical solutions of the exact dispersion relations, and applications to homogenization problems are discussed. The results can be used as test cases for homogenization techniques intended for finite scale homogenization, that is, where the wavelength is not assumed infinitely large compared to the microscale.
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| 24. |
- Sjöberg, Daniel
(författare)
-
Homogenization of dispersive material parameters for Maxwell's equations using a singular value decomposition
- 2004
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We find effective, or homogenized, material parameters for Maxwell’s equations when the microscopic scale becomes small compared to the scale induced by the frequencies of the imposed currents. After defining a singular value decomposition of the non-selfadjoint partial differential operator, we expand the electromagnetic field in the modes corresponding to the singular values, and show that only the smallest singular values make a significant contribution to the total field when the scale is small. The homogenized material parameters can be represented with the mean values of the singular vectors through a simple formula, which is valid for wavelengths not necessarily infinitely large compared to the unit cell.
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| 25. |
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| 26. |
- Sjöberg, Daniel
(författare)
-
Nonlinear waveguides
- 2000
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the propagation of electromagnetic waves in a waveguide filled with a nonlinear material. The electromagnetic field is expanded in the usual eigenmodes of the waveguide, and the coupling between the modes is quanti fied. We derive the wave equations governing each mode, with special emphasis on the situation with a dominant TE-mode. The result is a strictly hyperbolic system of nonlinear partial differential equations for the dominating mode, whereas the minor modes satisfy hyperbolic systems of linear, nonstationary, partial differential equations. A growth estimate is given for the minor modes.
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| 27. |
- Sjöberg, Daniel
(författare)
-
On uniqueness and continuity for the quasi-linear, bianisotropic Maxwell equations, using an entropy condition
- 2001
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The quasi-linear Maxwell equations describing electromagnetic wave propagation in nonlinear media permit several weak solutions, which may be discontinuous (shock waves). It is often conjectured that the solutions are unique if they satisfy an additional entropy condition. The entropy condition states that the energy contained in the electromagnetic fields is irreversibly dissipated to other energy forms, which are not described by the Maxwell equations. We use the method employed by Kruˇzkov to scalar conservation laws to analyze the implications of this additional condition in the electromagnetic case, i.e., systems of equations in three dimensions. It is shown that if a certain term can be ignored, the solutions are unique.
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| 28. |
- Sjöberg, Daniel, et al.
(författare)
-
Physical bounds on the all-spectrum transmission through periodic arrays: oblique incidence
- 2010
-
Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075. ; 92, s. 1-34009
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The performance of a low-pass screen designed to block electromagnetic waves in a stop-band is shown to have an upper bound defined by the static electric and magnetic polarizability per unit area of the screen. The bound is easy to calculate for all angles of incidence and polarizations, and applies regardless of how complicated the screen's microstructure is. For a homogeneous dielectric sheet the bound for TM polarization is more restrictive than the bound for TE, but this is not generally true for a screen with microstructure. The results are verified by measurements and simulations of oblique transmission through an array of split ring resonators, printed on a dielectric substrate.
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| 29. |
- Sjöberg, Daniel, et al.
(författare)
-
Realization of a matching region between a radome and a ground plane
- 2010
-
Ingår i: Progress In Electromagnetics Research Letters. - 1937-6480. ; 17, s. 1-10
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In order to reduce the monostatic signature of the junction between a radome and the metallic structure to which it is attached, a tapered resistive sheet can be used. In this paper, we describe an easy method to realize this tapering using geometric variations on a subwavelength scale, with a significant reduction of the number of processing steps as a result.
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| 30. |
- Sjöberg, Daniel
(författare)
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Reconstruction of nonlinear material properties for homogeneous, isotropic slabs using electromagnetic waves
- 1998
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This paper addresses the inverse problem of reconstructing a medium’s instantaneous, nonlinear response to electromagnetic excitation. Using reflection and transmission data for an almost arbitrary incident field on a homogeneous slab, we are able to obtain the nonlinear constitutive relations for both electric and magnetic fields, with virtually no assumptions made on the specific form of the relations. It is shown that for a nonmagnetic material, reflection data suffices to obtain the electrical nonlinear response. We also show that the algorithms are well posed. Numerical examples illustrate the analysis presented in this paper.
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