Gian Luigi Gragnani

Two-dimensional microwave imaging by a numerical inverse scattering solution

A numerical approach that aims to detect, by means of interrogating microwaves, the locations and the dielectric permittivities of unknown inhomogeneous dielectric cylindrical objects of arbitrary cross sections that might be present inside a fixed area of interest is proposed. An illumination is assumed with the electric field vector polarized along the cylindrical axis. The two-dimensional Lippman-Schwinger integral equation of electromagnetic scattering is transformed into matrix form by the moment method. The system obtained is solved by using a pseudoinversion algorithm to overcome ill-conditioning problems. The first-order Born approximation is also applied when the dielectric inhomogeneities are weakly scattering. Computer simulations have been performed by means of a numerical program. Results show the capabilities and limitations of the proposed approach. >

A multiview microwave imaging system for two-dimensional penetrable objects

A microwave imaging system is based on a multiview numerical solution to the integral equation of 2D transverse magnetic (TM) scattering is proposed. This solution is achieved by the moment method, and a pseudoinversion transformation is used to face ill-conditioning problems. All experimental setup is described that uses a scanning subsystem for measuring the values of the scattered electric field inside an observation domain located outside the investigation one (i.e., the area containing the cross sections of cylindrical dielectric scatters). Rotations of the investigation domain with respect to the scanning subsystem and the transmitting antenna allow a multiview imaging process. The imaging system does not require plane-wave illumination and does not use any first-order approximations; hence, it may be used even in the case of strong scatterers. The offline and once-and-for-all computation of the pseudoinverse matrix allows an inexpensive reconstruction in terms of computer resources. Some tests of the system were carried out, and the results are reported. >

Vivaldi Antennas for Microwave Imaging: Theoretical Analysis and Design Considerations

The geometrical and electrical features of the Vivaldi antenna are studied in the light of the frequency-independent antenna theory. A scaling principle is derived for the exponential tapering of the antenna, and a closed-form model for the current distribution is provided. Such theoretical results are in good agreement with several numerical simulations performed by using the NEC2 code. Furthermore, a practical feeding system, based on a double-Y balun, is developed and tested to obtain a more systematic approach to the design of the aforesaid antennas

Microwave imaging based on a Markov random field model

A new approach to microwave imaging of 2D inhomogeneous dielectric scatterers is presented. The method is developed in the space domain, and Markov random fields are used to obtain a model of the distributions of dielectric features in the scattering region. In this way, a-priori knowledge can be easily inserted in the imaging scheme. This stochastic approach gives rise to a functional equation that can be minimized by using a simulated annealing algorithm. An iterative scheme is derived that allows one to bypass the need for storing large matrices in the computer. Numerical simulation results, confirming the capabilities and effectiveness of the proposed method, are reported. Solutions have generally been obtained in few steps, and seem better than those obtained by other imaging techniques in the space domain. The capability of the algorithm to operate in a strongly noisy environment is also proved. >

Reconstruction of dielectric permittivity distributions in arbitrary 2-D inhomogeneous biological bodies by a multiview microwave numerical method

A numerical method for microwave imaging of two-dimensional inhomogeneous biological bodies illuminated by TM waves is presented. It is a spatial-domain multiview approach which makes use of the moment method to discretize the integral-equation formulation of inverse scattering. A pseudoinversion technique is applied to obtain a minimum-norm solution for the equivalent current density inside the cross-section of a scatterer. A multi-illumination-angle multiview process is used. The invariance of the Green matrix makes it possible to perform only one pseudoinverse (off line and once for all), independently of the number of views, thus reducing the need for computer resources. A pixel representation is adopted, and a look-up table is utilized to fast synthesize images. No plane-wave illumination is required and no first-order approximations are applied. Distortions in the dielectric reconstruction and noise effects are evaluated via some numerical simulations.

Inverse‐scattering method for dielectric objects based on the reconstruction of the nonmeasurable equivalent current density

Usual inverse-scattering methods for the reconstruction of dielectric objects are limited by the information that can be collected at the measurement locations. In this paper, an inverse-scattering method is proposed that allows one to reconstruct the unmeasured components of the equivalent current density and then to improve the dielectric reconstruction. The mathematical formulation is provided, and the theory is supported by the results of numerical simulations.

Iterative multi-resolution retrieval of non-measurable equivalent currents for the imaging of dielectric objects

In this paper, an iterative multi-resolution method for the reconstruction of the unmeasured components of the equivalent current density is proposed in order to improve the retrieval of the dielectric properties of an unknown scenario probed by interrogating electromagnetic waves. The mathematical formulation, concerned with lossless as well as lossy dielectric scatterers, is provided and the theory is supported by a set of representative examples dealing with synthetic as well as experimental scattering data.

Numerical electromagnetic inverse-scattering solutions for two-dimensional infinite dielectric cylinders buried in a lossy half-space

An approach to microwave imaging in a half-space geometry and to treating infinite dielectric cylinders buried in a lossy medium is proposed. The two-dimensional integral equation for the inverse-scattering problem is discretized by the moment method. The resulting ill-conditioned system is solved by pseudoinversion. A multi-incidence process based on the invariance of the Green matrix to the incident field is described. Results of some numerical simulations, assuming a noisy environment, are reported and discussed. >

An approach to microwave imaging using a multiview moment method solution for a two-dimensional infinite cylinder

An approach based on a multiview solution to the inverse-scattering problem of a two-dimensional infinite cylinder is developed in a space-frequency domain. Microwave imaging is simulated by a computer algorithm using the moment method. To overcome ill-conditioning and solve nonsquare systems, a pseudoinverse transformation is employed. The equivalent current density and the complex conductivity are considered as object functions for image formation. The results of some numerical simulations in a noisy environment are reported. and a discussion of monoview and multiview imaging techniques for a space-frequency domain is presented. >

A circuital approach to evaluating the electromagnetic field on rectangular apertures backed by rectangular cavities

In this paper, the problem of evaluating the electromagnetic field on rectangular apertures backed by rectangular cavities is investigated. The electromagnetic-field distribution is derived by using a circuital model of an aperture and suitable forcing terms introduced into the equations related to the aperture model. The effects of a rectangular cavity on the aperture-field distribution are assessed by considering the rectangular cavity as a load impedance. The impedance value is obtained by modeling the rectangular cavity as a length of rectangular waveguide back-ended by a short. The distribution of the electromagnetic field on the aperture is used as an exciting source to evaluate, through a modal expansion, the electromagnetic field inside the cavity. Numerical simulations are in a good agreement with both other theoretical models and experimental data.