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| 2. |
- Aksimsek, Sinan, et al.
(author)
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TEM wave scattering by a step discontinuity on the outer wall of a coaxial waveguide
- 2013
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In: IEEE transactions on microwave theory and techniques. - : IEEE Press. - 0018-9480 .- 1557-9670. ; 61:8, s. 2783-2791
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Journal article (peer-reviewed)abstract
- In this paper, the propagation of TEM waves along a coaxial waveguide with a step discontinuity on its outer wall is investigated rigorously by applying the direct Fourier transform and reducing the problem into the solution of a modified Wiener–Hopf equation. The solution for the field terms are determined in terms of an infinite number of unknown coefficients, which satisfy an infinite set of linear algebraic equations. These equations are solved numerically and the effect of area ratio is presented graphically at the end of the analysis. The same problem is also analyzed by applying the mode-matching technique and the results of the two approaches are compared. It is observed numerically that the Wiener–Hopf technique provides a better convergence than the mode-matching technique.
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| 4. |
- Fhager, Andreas, 1976, et al.
(author)
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Image Reconstruction in Microwave Tomography Using a Dielectric Debye Model
- 2012
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In: IEEE Transactions on Biomedical Engineering. - 0018-9294 .- 1558-2531. ; 59:1, s. 156 - 166
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Journal article (peer-reviewed)abstract
- In this paper, quantitative dielectric image reconstruction based on broadband microwave measurements is investigated. A time-domain-based algorithm is derived where Debye model parameters are reconstructed in order to take into account the strong dispersive behavior found in biological tissue. The algorithm is tested with experimental and numerical data in order to verify the algorithm and to investigate improvements in the reconstructed image resulting from the improved description of the dielectric properties of the tissue when using broadband data. The comparison is made in relation to the more commonly used conductivity model. For the evaluation, two examples were considered, the first was a lossy saline solution and the second was less lossy tap water. Both liquids are strongly dispersive and used as a background medium in the imaging examples. The results show that the Debye model algorithm is of most importance in the tap water for a bandwidth of more than 1.5 GHz. Also the saline solution exhibits a dispersive behavior but since the losses restrict the useful bandwidth, the Debye model is of less significance even if somewhat larger and stronger artifacts can be seen in the conductivity model reconstructions.
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| 5. |
- Gustafsson, Mats, et al.
(author)
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Absorption Efficiency and Physical Bounds on Antennas
- 2010
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In: International Journal of Antennas and Propagation. - : Hindawi Limited. - 1687-5869 .- 1687-5877. ; 2010
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Journal article (peer-reviewed)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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| 6. |
- Gustafsson, Mats, et al.
(author)
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Optimal Antenna Currents for Q, Superdirectivity, and Radiation Patterns Using Convex Optimization
- 2013
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In: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 61:3, s. 1109-1118
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Journal article (peer-reviewed)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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| 8. |
- Gustafsson, Stefan
(author)
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Electromagnetic dispersion modeling and analysis for HVDC power cables
- 2012
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Licentiate thesis (other academic/artistic)abstract
- Derivation of an electromagnetic model, regarding the wave propagation in a very long (10 km or more) High Voltage Direct Current (HVDC) power cable, is the central part of this thesis. With an existing “perfect” electromagnetic model there are potentially a wide range of applications.The electromagnetic model is focused on frequencies between 0 and 100 kHz since higher frequencies essentially will be attenuated. An exact dispersion relation is formulated and the propagation constant is computed numerically. The dominating mode is the first Transversal Magnetic (TM) mode of order zero, denoted TM01, which is also referred to as the quasi-TEM mode. A comparison is made with the second propagating TM mode of order zero denoted TM02. The electromagnetic model is verified against real time data from Time Domain Reflection (TDR) measurements on a HVDC power cable. A mismatch calibration procedure is performed due to matching difficulties between the TDR measurement equipment and the power cable regarding the single-mode transmission line model.An example of power cable length measurements is addressed, which reveals that with a “perfect” model the length of an 80 km long power cable could be estimated to an accuracy of a few centimeters. With the present model the accuracy can be estimated to approximately 100 m.In order to understand the low-frequency wave propagation characteristics, an exact asymptotic analysis is performed. It is shown that the behavior of the propagation constant is governed by a square root of the complex frequency in the lowfrequency domain. This thesis also focuses on an analysis regarding the sensitivity of the propagation constant with respect to some of the electric parameters in the model. Variables of interest when performing the parameter sensitivity study are the real relative permittivityand the conductivity.
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| 9. |
- Gustafsson, Stefan, et al.
(author)
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Electromagnetic dispersion modeling and sensitivity analysis for HVDC power cables
- 2012
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Reports (other academic/artistic)abstract
- This paper addresses electromagnetic wave propagation in High Voltage Direct Current (HVDC) power cables. An electromagnetic model, based on long (10 km or more) cables with a frequency range of 0 to 100 kHz, is derived. Relating the frequency to the propagation constant a dispersion relation is formulated using a recursive approach. The propagation constant is found numerically with normalized residue calculation. The paper is concluded with a sensitivity analysis of the propagation constant with respect to the electrical parameters εr (the real relative permittivity) and σ (the conductivity)
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| 10. |
- Hashemzadeh, Parham, et al.
(author)
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A Cramer-Rao Bound Approach for Evaluating the Quality of Experimental Setups in Electrical Impedance Tomography
- 2011
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In: Proceeding (723) Biomedical Engineering.
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Conference paper (peer-reviewed)abstract
- In this report, we propose the application of the Cramer Rao Lower Bound (CRLB) as a performance measure for optimal design of experimental setups in electrical impedance tomography. In particular, we focus on the optimum positioning of electrodes. Cramer Rao Bound is bounded from below by the inverse of the Fisher information matrix (FIM). FIM incorporates all aspects of the forward problem, statistical properties of the measurement noise, and multi-frequency data. We consider the application of CRB in both the deterministic as well as the Bayesian setting. We first present the CRB for the case of the unbiased estimator and then the Bayesian Cramer Rao Bound (BCRB) for the case of the biased estimator. All CRB computations are performed using a measured noise model from a clinical experiment.
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