1. 
 Fhager, Andreas, et al.
(författare)

Image Reconstruction in Microwave Tomography Using a Dielectric Debye Model
 2012

Ingår i: IEEE Transactions on Biomedical Engineering.  00189294. ; 59:1, s. 156166

Tidskriftsartikel (refereegranskat)abstract
 In this paper, quantitative dielectric image reconstruction based on broadband microwave measurements is investigated. A timedomainbased 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


2. 
 Gustafsson, Mats, et al.
(författare)

Absorption Efficiency and Physical Bounds on Antennas
 2010

Ingår i: International Journal of Antennas and Propagation.  Hindawi Publishing Corporation.  16875869. ; 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.


3. 
 Gustafsson, Mats, et al.
(författare)

An overview of some recent physical bounds in scattering and antenna theory
 2009

Konferensbidrag (refereegranskat)abstract
 The objective of this paper is to give an overview of some recently developed sum rules and physical bounds in scattering and antenna theory. The sum rules are based on integral identities for Herglotz functions that relate the quantity of interest with its low and highfrequency behavior. The sum rules are transformed to bounds by estimating the integrals and applying variational results to the parameters that appear in the asymptotic expansions. The theoretical findings are exemplified by numerical results for various scattering and antenna configurations.


4. 
 Gustafsson, Mats, et al.
(författare)

Bandwidth, Q factor, and resonance models of antennas
 2006

Ingår i: Progress in Electromagnetics Research PIER. ; 62, s. 120

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 BodeFano 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.


5. 
 Gustafsson, Mats, et al.
(författare)

CramerRao Lower Bounds for Inverse Scattering Problems of Multilayer Structures
 2006

Ingår i: Inverse Problems. ; 22, s. 13591380

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 illposedness 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 rank1 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.


6. 
 Gustafsson, Mats, et al.
(författare)

On the spectral efficiency of a sphere
 2007

Ingår i: Progress in Electromagnetics ResearchPier.  EMW Publishing.  10704698. ; 67, s. 275296

Tidskriftsartikel (refereegranskat)abstract
 In many cases it is desired to have both high capacity and small antennas in wireless communication systems. Unfortunately, the antenna performance deteriorate when the antennas get electrically small. In this paper fundamental limitations from antenna theory and broadband matching are used to analyze the spectral efficiency of an arbitrary antenna inserted inside a sphere.


7. 
 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 Propagation19630101+01:00.  IEEEInstitute of Electrical and Electronics Engineers Inc..  0018926X. ; 61:3, s. 11091118

Tidskriftsartikel (refereegranskat)abstract
 The high Qfactor (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 Qfactor quotient, minimal Qfactor for superdirectivity, and minimum Q for given farfields are presented. The effects of antennas embedded in structures are also discussed. The results are illustrated for planar geometries.


8. 
 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 nearfield 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 Qfactor through the stored energy, the lowfrequency 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 lowfrequency expansion in terms of the polarizability dyadics are similar with the approach in (Chu, 1948 and Gustafsson et al., 2007).


9. 
 Gustafsson, Stefan, et al.
(författare)

Electromagnetic Dispersion Modeling and Measurements for HVDC Power Cables
 2014

Ingår i: IEEE Transactions on Power Delivery.  IEEEInstitute of Electrical and Electronics Engineers Inc..  19374208. ; 29:6, s. 24392447

Tidskriftsartikel (refereegranskat)abstract
 This paper provides a general framework for electromagnetic (EM) modeling, sensitivity analysis, computation, and measurements regarding the wave propagation characteristics of highvoltage directcurrent (HVDC) power cables. The modeling is motivated by the potential use with transient analysis, partialdischarge measurements, fault localization and monitoring, and is focused on very long (10 km or more) HVDC power cables with transients propagating in the lowfrequency regime of about 0100 kHz. An exact dispersion relation is formulated together with a discussion on practical aspects regarding the computation of the propagation constant. Experimental timedomain measurement data from an 80kmlong HVDC power cable are used to validate the electromagnetic model, and a mismatch calibration procedure is devised to account for the connection between the measurement equipment and the cable. Quantitative sensitivity analysis is devised to study the impact of parameter uncertainty on wave propagation characteristics. The sensitivity analysis can be used to study how material choices affect the propagation characteristics, and to indicate which material parameters need to be identified accurately in order to achieve accurate fault localization. The analysis shows that the sensitivity of the propagation constant due to a change in the conductivity in the three metallic layers (the inner conductor, the intermediate lead shield, and the outer steel armor) is comparable to the sensitivity with respect to the permittivity of the insulating layer. Hence, proper modeling of the EM fields inside the metallic layers is crucial in the lowfrequency regime of 0100 kHz.


10. 
 Nordebo, Sven, et al.
(författare)

A Green's function approach to Fisher information analysis and preconditioning in microwave tomography
 2010

Ingår i: Inverse Problems in Science and Engineering.  Taylor & Francis.  17415985. ; 18:8, s. 10431063

Tidskriftsartikel (refereegranskat)abstract
 The Fisher Information Integral Operator (FIO) and related sensitivity analysis is formulated in a variational framework that is suitable for analytical Green's function and gradientbased approaches in microwave tomography. The main application considered here is for parameter sensitivity analysis and related preconditioning for gradientbased quasiNewton inverse scattering algorithms. In particular, the Fisher information analysis can be used as a basic principle yielding a systematic approach to robust preconditioning, where the diagonal elements of the FIO kernel are used as targets for sensitivity equalization. The infinitedimensional formulation has several practical advantages over the finitedimensional Fisher Information Matrix (FIM) analysis approach. In particular, the FIO approach avoids the need of making a priori assumptions about the underlying discretization of the material such as the shape, orientation and positions of the assumed image pixels. Furthermore, the integral operator and its spectrum can be efficiently approximated by using suitable quadrature methods for numerical integration. The eigenfunctions of the integral operator, corresponding to the identifiable parameters via the significant eigenvalues and the corresponding Cram<img src="http://www.informaworld.com/cache/entities/14/000000/ffffff/arial/md/00e9.png" />rRao bounds, constitute a suitable global basis for sensitivity and resolution analysis. As a generic numerical example, a twodimensional inverse electromagnetic scattering problem is analysed and illustrates the spectral decomposition and the related resolution analysis. As an application example in microwave tomography, a simulation study has been performed to illustrate the parameter sensitivity analysis and to demonstrate the effect of the related preconditioning for gradientbased quasiNewton inverse scattering algorithms.

