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[1] We present a 3D spherical bistatic experimental setup developed in the anechoic chamber of Institut Fresnel in Marseille, France, for scattering measurements of targets in the microwave domain, both in copolarization and cross-polarization. We first describe the geometry of the setup, the measurement protocol, and the calibration process. The results obtained for different metallic and dielectric targets are then compared to numerical simulations, computed either using the coupled dipole method or Mie theory. These comparisons are used to validate the algorithms and the setup, as well as to provide information on the accuracy of the measurements.Citation: Eyraud, C., J.-M. Geffrin, P. Sabouroux, P. C. Chaumet, H. Tortel, H. Giovannini, and A. Litman (2008), Validation of a 3D bistatic microwave scattering measurement setup, Radio Sci., 43, RS4018,

To overcome the ill-posedness of the inverse scattering problem, we introduce an appropriate way of representing the unknown permittivity profile which benefits from two a priori pieces of information which are commonly encountered. First, the unknown permittivity map has a limited spatial support which can be contained within a circular investigation area. Second, from physical reasoning based on the scattering operator, there is only a limited number of independent parameters that can be retrieved from the measured fields. Both pieces of a priori information can be adequately introduced by representing the unknown permittivity profile in terms of a Zernike polynomial expansion, correctly truncated according to the number of independent parameters. To investigate the effectiveness of such a Zernike representation, the reconstructions obtained from experimentally acquired data in the circular microwave scanner developed at the Institut Fresnel are analyzed.

Abstract-The calibration of the multistatic scattering matrix plays an important part in the construction of a quantitative microwave imaging system.For scattering measurement applications, the calibration must be performed on the amplitude and on the phase of the fields of interest. When the antennas are not completely identical, as for example with a multiplexed antennas array, a specific calibration procedure must be constructed. In the present work, we explain how a complex calibration matrix can be defined which takes advantage of the geometrical organization of the antennas. Indeed, for arrays of antennas positioned on a circle, the inherent symmetries of the configuration can be fully exploited by means of an adequate reorganization of the multistatic scattering matrix. In addition, the reorganization permits to detect antenna pairs which are not properly functioning and to estimate the signal-to-noise ratio. Experimental results obtained within a cylindrical cavity enclosed by a metallic casing are provided to assess the performance of the proposed calibration procedure.This calibration protocol, which is described here in detail, has already been applied to provide quantitative images of dielectric targets [1,2].

International audienceThis article deals with the inverse scattering problem from scattered field data measured inside a closed microwave scanner. This system is presently being developed to demonstrate the potentiality of a non-invasive microwave imaging system for volumetric water content monitoring. The final goal is to retrieve soil moisture information as it is an important parameter for understanding fluid flow modelling, as well as water uptake by plants roots. Based on the actual state of the setup, we are proposing appropriate numerical tools, in particular a finite element formalism combined with a Lagrangian minimization scheme to provide a fast and accurate imaging tool. We will also show how we can improve the reconstruction algorithms by changing in a very simpler manner the measurement configuration, using either off-centred information or impedance boundary matching environment

International audienceThis letter deals with the monitoring of the volumetric water content of a soil column in a fully controlled environment by means of a noninvasive microwave imaging system. Indeed, soil moisture is an important piece of information to improve fluid flow modeling or to better understand the water uptake by plant roots. In this letter, we address the problem of recovering the footprint of soil moisture evolution with respect to time using a built-in laboratory microwave setup coupled to a robust qualitative microwave imaging method: the linear sampling method (LSM). The evolution of the water content in the soil is ruled by the Richards equations and stored at different time steps. The associated maps of soil water content are converted into permittivity maps using the Dobson model. Electromagnetic scattered fields are then computed with finite-element software. We have tested the LSM in a situation that can be encountered in agricultural soils where the water content is not homogeneous. We show that the evolution of the soil water content can be qualitatively monitored with the LSM. We also point out that the source is more precisely located by considering the evolution in time of the singular system of the multistatic matrix (multiple signal classification method)

International audienceAn accurate knowledge of the complex permittivity value of materials is compulsory when performing experimental electromagnetic applications. Unfortunately, these values are not so obvious to determine in practice. In this letter, we propose a novel approach for determining the complex dielectric constant of materials. This method combines free-space far-field scattering pattern measurements with a Bayesian procedure, which fully exploits the measurement uncertainties. Therefore, the measured values weighted according to their experimental accuracy are incorporated in the permittivity determination algorithm. In this letter, the samples are all shaped as spheres in order to benefit from efficient Mie scattered field computations. The dielectric properties of typical plastic samples are first determined and compared to values found in the literature in order to assess the validity and the accuracy of the proposed methodology. A more “exotic” sample extracted from a microwave absorber, which is a polyurethane foam charged with carbon particles, is also analyzed

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