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Sökning: LAR1:lu > Gustafsson Mats > Linnéuniversitetet

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1.
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2.
  • Gustafsson, Mats, et al. (författare)
  • Absorption Efficiency and Physical Bounds on Antennas
  • 2010
  • Ingår i: International Journal of Antennas and Propagation. - : Hindawi Limited. - 1687-5869 .- 1687-5877. ; 2010
  • Tidskriftsartikel (refereegranskat)abstract
    • Abstract—The all spectrum absorption efficiency appears in the physical bounds on antennas expressed in the polarizability dyadics. Here, it is shown that this generalized absorption efficiency is close to 1/2 for small idealized dipole antennas and for antennas with a dominant resonance in their absorption. Also, the usefulness of this parameter is analyzed for estimation of antenna performance. The results are illustrated with numerical data for several antennas.
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3.
  • Gustafsson, Mats, et al. (författare)
  • Bandwidth, Q factor, and resonance models of antennas
  • 2006
  • Ingår i: Progress in Electromagnetics Research PIER. - 1070-4698. ; 62, s. 1-20
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, we introduce a first order accurate resonance model based on a second order Pade approximation of the reflection coefficient of a narrowband antenna. The resonance model is characterized by its Q factor, given by the frequency derivative of the reflection coefficient. The Bode-Fano matching theory is used to determine the bandwidth of the resonance model and it is shown that it also determines the bandwidth of the antenna for sufficiently narrow bandwidths. The bandwidth is expressed in the Q factor of the resonance model and the threshold limit on the reflection coefficient. Spherical vector modes are used to illustrate the results. Finally, we demonstrate the fundamental difficulty of finding a simple relation between the Q of the resonance model, and the classical Q defined as the quotient between the stored and radiated energies, even though there is usually a close resemblance between these entities for many real antennas.
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4.
  • Gustafsson, Mats, et al. (författare)
  • Cramer-Rao Lower Bounds for Inverse Scattering Problems of Multilayer Structures
  • 2006
  • Ingår i: Inverse Problems. - : IOP Publishing. ; 22:4, s. 1359-1380, s. 1-1
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, the inverse scattering problem of amultilayer structure is analysed with the Fisher information matrix and the Cramer–Rao lower bound (CRLB). The CRLB quantifies the ill-posedness of the inverse scattering problem in terms of resolution versus estimation accuracy based on the observation of noisy data. The limit for feasible inversion is identified by an asymptotic eigenvalue analysis of the Toeplitz Fisher information matrix and an application of the sampling theorem. It is shown that the resolution is inversely proportional to the bandwidth of the reflection data and that the CRLB increases linearly with the number of slabs. The transmission data give a rank-1 Fisher information matrix which can approximately reduce the CRLB by a factor of 4. Moreover, the effect of dispersive material parameters and simultaneous estimation of two material parameters are analysed. The results are illustrated with numerical examples.
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5.
  • Gustafsson, Mats, et al. (författare)
  • Optimal Antenna Currents for Q, Superdirectivity, and Radiation Patterns Using Convex Optimization
  • 2013
  • Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 61:3, s. 1109-1118
  • Tidskriftsartikel (refereegranskat)abstract
    • The high Q-factor (low bandwidth) and low efficiency make the design of small antennas challenging. Here, convex optimization is used to determine current distributions that provide upper bounds on the antenna performance. Optimization formulations for maximal gain Q-factor quotient, minimal Q-factor for superdirectivity, and minimum Q for given far-fields are presented. The effects of antennas embedded in structures are also discussed. The results are illustrated for planar geometries.
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6.
  • Gustafsson, Mats, et al. (författare)
  • Physical bounds and sum rules in scattering and antenna theory
  • 2009
  • Ingår i: Electromagnetics in Advanced Applications, 2009. ICEAA '09. International Conference on. ; , s. 600-603, s. 600-603
  • Konferensbidrag (refereegranskat)abstract
    • The objective of this paper is to review some recently developed sum rules and physical bounds in scattering and antenna theory. The sum rules are based on identities for Herglotz functions that relate the quantity of interest integrated over all wavelengths with its static polarizability dyadics. They are transformed to physical bounds by applying variational principles for the polarizability dyadics together with various estimates of the integrals. The theoretical findings are exemplified by numerical results for several configurations.
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7.
  • Ivanenko, Yevhen, et al. (författare)
  • Optical theorems and physical bounds on absorption in lossy media
  • 2019
  • Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 27:23, s. 34323-34342
  • Tidskriftsartikel (refereegranskat)abstract
    • Two different versions of an optical theorem for a scattering body embedded inside a lossy background medium are derived in this paper. The corresponding fundamental upper bounds on absorption are then obtained in closed form by elementary optimization techniques. The first version is formulated in terms of polarization currents (or equivalent currents) inside the scatterer and generalizes previous results given for a lossless medium. The corresponding bound is referred to here as a variational bound and is valid for an arbitrary geometry with a given material property. The second version is formulated in terms of the T-matrix parameters of an arbitrary linear scatterer circumscribed by a spherical volume and gives a new fundamental upper bound on the total absorption of an inclusion with an arbitrary material property (including general bianisotropic materials). The two bounds are fundamentally different as they are based on different assumptions regarding the structure and the material property. Numerical examples including homogeneous and layered (core-shell) spheres are given to demonstrate that the two bounds provide complimentary information in a given scattering problem.
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8.
  • Nordebo, Sven, 1963-, et al. (författare)
  • A priori modeling for gradient based inverse scattering algorithms
  • 2009
  • Ingår i: Progress in Electromagnetics Research B. - 1937-6472. ; 16, s. 407-432
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a Fisher information based Bayesian approach to analysis and design of the regularization and preconditioning parameters used with gradient based inverse scattering algorithms. In particular, a one-dimensional inverse problem is considered where the permittivity and conductivity profiles are unknown and the input data consist of the scattered field over a certain bandwidth. A priori parameter modeling is considered with linear, exponential and arctangential parameter scalings and robust preconditioners are obtained by choosing the related scaling parameters based on a Fisher information analysis of the known background. The Bayesian approach and a principal parameter (singular value) analysis of the stochastic Cramer-Rao bound provide a natural interpretation of the regularization that is necessary to achieve stable inversion, as well as an indicator to predict the feasibility of achieving successful reconstruction in a given problem set-up. In particular, the Tikhonov regularization scheme is put into a Bayesian estimation framework. A time-domain least-squares inversion algorithm is employed which is based on a quasi-Newton algorithm together with an FDTD-electromagnetic solver. Numerical examples are included to illustrate and verify the analysis.
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10.
  • Nordebo, Sven, et al. (författare)
  • Cylindrical multipole expansion for periodic sources with applications for three-phase power cables
  • 2018
  • Ingår i: Mathematical methods in the applied sciences. - : Wiley-Blackwell. - 0170-4214 .- 1099-1476. ; 41:3, s. 959-965
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a c ylindrical multipole expansion for periodic sources with applications for three-phase power cables.It is the aim of the contribution to provide some analytical solutions and techniques that can be useful in the calculation ofcable losses. Explicit analytical results are given for the fields generated by a three-phase helical current distribution andwhich can be computed efficiently as an input to other numerical methods such as, for example , the Method of Moments.It is shown that the field computations are numerically stable at low frequencies (such as 50 Hz) as well as in the quasi-magnetostatic limit provided that sources are divergence-free. The cylindrical multipole expansion is fur thermore usedto derive an efficient analytical model of a measurement coil to measure and estimate the complex valued permeability ofmagnetic steel armour in the presence of a strong skin-effect.
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