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Sökning: LAR1:lu > Gustafsson Mats > Linnéuniversitetet > (2008) > Lunds universitet

  • Resultat 1-4 av 4
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  • Gustafsson, Mats, et al. (författare)
  • Physical bounds on the antenna scattering matrix
  • 2008
  • Konferensbidrag (refereegranskat)abstract
    • The antenna scattering matrix is based on a spherical vector wave expansion and contains a complete description of the matching, transmission, receiving, and scattering properties of an antenna. It is commonly utilized in near-field measurements and it can also be used to model MIMO antennas. Here, an approach based on the holomorphic properties of the antenna scattering matrix is used to derive physical bounds on the bandwidth of lossless antennas. The resulting bounds are expressed in the radius of the smallest circumscribing sphere and the polarizability dyadics of the antenna. The derivation and final results resemble both the classical work by Chu (1948) and a recently developed theory based on the forward scattering. However, instead of estimating the Q-factor through the stored energy, the low-frequency expansion of the scattering matrix is used to obtain a set of summation rules from which bounds on the bandwidth are derived. The use of Cauchy integrals and the low-frequency expansion in terms of the polarizability dyadics are similar with the approach in (Chu, 1948 and Gustafsson et al., 2007).
  • Nordebo, Sven, et al. (författare)
  • A Systematic Approach to Robust Preconditioning for Gradient Based Inverse Scattering Algorithms
  • 2008
  • Ingår i: Inverse Problems. - 0266-5611. ; 24:2
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a systematic approach to robust preconditioning for gradient-based nonlinear inverse scattering algorithms. In particular, one- and two-dimensional inverse problems are considered where the permittivity and conductivity profiles are unknown and the input data consist of the scattered field over a certain bandwidth. A time-domain least-squares formulation is employed and the inversion algorithm is based on a conjugate gradient or quasi-Newton algorithm together with an FDTD-electromagnetic solver. A Fisher information analysis is used to estimate the Hessian of the error functional. A robust preconditioner is then obtained by incorporating a parameter scaling such that the scaled Fisher information has a unit diagonal. By improving the conditioning of the Hessian, the convergence rate of the conjugate gradient or quasi-Newton methods are improved. The preconditioner is robust in the sense that the scaling, i.e. the diagonal Fisher information, is virtually invariant to the numerical resolution and the discretization model that is employed. Numerical examples of image reconstruction are included to illustrate the efficiency of the proposed technique.
  • Sohl, Christian, et al. (författare)
  • A general approach for deriving bounds in electromagnetic theory
  • 2008
  • Konferensbidrag (refereegranskat)abstract
    • This paper reports on a systematic procedure for deriving physical bounds in electromagnetic theory. The approach is based on the holomorphic properties of certain Herglotz functions and their asymptotic expansions in the low- and high-frequency regimes. A family of integral identities or summation rules is obtained with values governed by the coefficients in the low- and high-frequency expansions. In particular, summation rules for plane-wave scattering by a homogenous and isotropic sphere is derived and verified numerically by computing the extinction cross section and the bistatic radar cross section in the forward direction. It is concluded that summation rules based on Herglotz functions show great potential for deriving new physical bounds in, e.g., scattering and antenna problems.
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