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Sökning: LAR1:lu > Sjöberg Daniel > Lunds universitet > Kungliga Tekniska Högskolan

  • Resultat 1-6 av 6
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  • Gustafsson, Mats, et al. (författare)
  • Physical bounds on the partial realized gain
  • 2010
  • Konferensbidrag (refereegranskat)abstract
    • An antenna identity, derived from the forward scattering sum rule, shows that the partial realized gain of an antenna is related to the polarizability of the antenna structure. The partial realized gain contains the mismatch, directivity, efficiency, and polarization properties of the antenna. The antenna identity expresses how the performance depends on the electrical size and shape of the antenna structure. It is also the starting point for several antenna bounds. In this paper, the identity, its associated physical bounds, and computational aspects of the polarizability dyadics are discussed.
  • Ramprecht, Jörgen, et al. (författare)
  • Biased magnetic materials in RAM applications
  • 2007
  • Ingår i: Progress in Electromagnetics Research. - 1559-8985. ; 75, s. 85-117
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetization of a ferro- or ferri-magnetic material has been modeled with the Landau-Lifshitz-Gilbert (LLG) equation. In this model demagnetization effects are included. By applying a linearized small signal model of the LLG equation, it was found that the material can be described by an effective permeability and with the aid of a static external biasing field, the material can be switched between a Lorentz-like material and a material that exhibits a magnetic conductivity. Furthermore, the reflection coefficient for normally impinging waves on a PEC covered with a ferro/ferri-magnetic material, biased in the normal direction, is calculated. When the material is switched into the resonance mode, two distinct resonance frequencies in the reflection coefficient were found, one associated with the precession frequency of the magnetization and the other associated with the thickness of the layer. The former of these resonance frequencies can be controlled by the bias field and for a bias field strength close to the saturation magnetization, where the material starts to exhibit a magnetic conductivity, low reflection ( around - 20 dB) for a quite large bandwidth ( more than two decades) can be achieved.
  • Ramprecht, Jörgen, et al. (författare)
  • Magnetic Losses in Composite Materials
  • 2008
  • Ingår i: Journal of Physics D : Applied Physics. - 0022-3727. ; 41:13, s. 135005
  • Tidskriftsartikel (refereegranskat)abstract
    • We discuss some of the problems involved in homogenization of a composite material built from ferromagnetic inclusions in a nonmagnetic background material. The small signal permeability for a ferromagnetic spherical particle is combined with a homogenization formula to give an effective permeability for the composite material. The composite material inherits the gyrotropic structure and resonant behaviour of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. The magnetic losses are described by a magnetic conductivity which can be made independent of frequency and proportional to the volume fraction by choosing a certain bias. Finally, some concerns regarding particles of small size, i.e. nanoparticles, are treated and the possibility of exciting exchange modes are discussed. These exchange modes may be an interesting way to increase losses in composite materials.
  • Ramprecht, Jörgen, et al. (författare)
  • On the amount of magnetic material necessary in broadband magnetic absorbers
  • 2008
  • Ingår i: 2008 IEEE International Symposium on Antennas and Propagation (AP-S 2008). - 978-1-4244-2041-4 ; s. 1412-1415
  • Konferensbidrag (refereegranskat)abstract
    • In this paper we have examined the use of ferromagnetic particles for thin broadband RAMs. The permeability of the composite material was obtained from a well tested model for ferromagnetic materials and a result from homogenization theory. It was found that the resonance frequency of the composite is independent of the volume fraction of the particles and that a magnetic conductivity proportional to the volume fraction exists. Furthermore, we showed that a certain amount of magnetic material is required in order to maintain a low reflection and that there exists a lower bound on how thin we could expect the absorber to be. Numerical calculations show that, though thin, the RAM tends to be quite heavy, which might prevent the use of ferromagnetic material in RAM applications. These results may serve as guidelines when designing thin broadband RAMs.
  • Ramprecht, Jörgen, et al. (författare)
  • Scattering from a thin magnetic layer with a periodic lateral magnetization application to electromagnetic absorbers
  • 2008
  • Ingår i: Progress in Electromagnetics Research. - 1559-8985. ; 83, s. 199-224
  • Tidskriftsartikel (refereegranskat)abstract
    • A magnetized thin layer mounted on a PEC surface is considered as an alternative for an absorbing layer. The magnetic material is modeled with the Landau-Lifshitz-Gilbert equation, with a lateral static magnetization having a periodic variation along one lateral direction. The scattering problem is solved by means of an expansion into Floquet-modes, a propagator formalism and wave-splitting. Numerical results are presented, and for parameter values close to the typical values for ferro- or ferrimagnetic media, reflection coefficients below -20 dB can be achieved for the fundamental mode over the frequency range 1-4 GHz, for both polarizations. It is found that the periodicity of the medium makes the reflection properties for the fundamental mode almost independent of the azimuthal direction of incidence, for both normally and obliquely incident waves.
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