In statistical theory, the Huber function yields robust estimations reducing the effect of outliers. In this paper, we employ the Huber function as regularization in a challenging inverse problem: quantitative microwave imaging. Quantitative microwave tomography aims at estimating the permittivity profile of a scattering object based on measured scattered fields, which is a nonlinear, ill-posed inverse problem. The results on 3D data sets are encouraging: the reconstruction error is reduced and the permittivity profile can be estimated from fewer measurements compared to state-of-the art inversion procedures.

A microwave nondestructive testing technique to measure the steel fiber content in hardened concrete slabs is presented. The technique is based on an open-ended coaxial probe reflectometry method for measuring the effective permittivity of the steel fiber reinforced concrete and on a classical homogenization approach for determining the fiber content. Two different full-wave models for the probe-aperture admittance as a function of the permittivity are applied: a spectral domain model for the forward problem and a rational function approximation for the inverse problem. A Maxwell-Garnett type of mixing rule is derived for randomly oriented conducting prolate spheroids in a dielectric host medium. A coaxial probe has been constructed, calibration aspects are discussed, and experimental results are presented for concrete slabs with various fiber contents. It can be concluded that the presented technique is a promising one.

A two-dimensional mixed-potential integral equation formulation is used to analyze the eigenmodes of microstrip lines. The new method involves a fast evaluation scheme for the space domain Green's functions of the substrate by using perfectly matched layers to obtain closed-form expressions as a series of leaky and Berenger modes. Efficient summation of these series is performed by means of the Shanks transform. The modal series can be applied to calculate all field interactions analytically, except for the selfpatch and the nearest neighbor contributions. Examples show an important reduction in CPU time for the new perfectly matched layers approach, as compared to the classical evaluation of the continuous Sommerfeld integrals.

A practical method is developed for selecting the minimal number of lumped elements needed to represent a lossy transmission line if a certain accuracy is desired in a well-defined frequency range. This method, which uses dimensionless transmission line parameters, can be used in a wide range of applications and is also extended to hybrid equivalent circuits, consisting of ideal single lossless lines and resistors. For completeness, a lumped element model for coupled lossy lines is presented which uses the same dimensionless parameters and the same criteria as proposed for single lines. An example of coupled transmission line structure including skin-effect losses illustrates the approach.>