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We introduce and discuss a new approach to the phase retrieval of fields radiated by continuous aperture sources having a circular support, which is of interest in many applications including the detection of shape deformations on reflector antennas. The approach is based on a decomposition of the actual 2-D problem into a number of 1-D phase retrieval problems along diameters and concentric rings of the visible part of the spectrum. In particular, the 1-D problems are effectively solved by using the spectral factorization method, while discrimination arguments at the crossing points allows to complete the retrieval of the 2-D complex field. The proposed procedure, which just requires a single set of far field amplitudes, takes advantage from up to now unexplored field properties and it is assessed in terms of reflector aperture fields.

Code related to the example presented in the paper entitled "4-Beams Reconfigurable Circular Ring Arrays Exploiting Orbital Angular Momentum for Monopulse Radar Applications"

Breast cancer represents one of the main reasons of death among women. As an alternative to the gold standard techniques for breast cancer diagnosis, microwave imaging has been proposed from research community and many microwave systems have been designed mainly to work at low microwave frequencies. Based both on the results of recent dielectric characterization campaigns on human breast ex-vivo tissues up to 50 GHz and on the promising feasibility studies of mm-wave imaging systems, in this article, we propose and test inverse scattering techniques as effective tool to process mm-wave data to image breast cancer. Differently from radar techniques so far adopted in conjunction with mm-wave imaging system, inverse scattering techniques turn out to be more versatile and robust with respect to the reduction of the amount of data and eventually also able to characterize the anomaly in terms of electromagnetic properties. In particular, in the above, two image reconstruction techniques, the Linear Sampling Method and the Born Approximation, are proposed and compared against both simulated and experimental data.

In this article, a new inversion model for 2-D microwave imaging is introduced by means of a convenient rewriting of the usual Lippmann–Schwinger integral scattering equation. Such a model is derived by decomposing Green’s function and the corresponding internal radiation operator in two different contributions, one of them easily computed from the collected scattered data. In the case of lossless backgrounds, the resulting model turns out to be more convenient than the traditional one, as it exhibits a lower degree of nonlinearity with respect to parameters embedding the unknown dielectric characteristics. This interesting property suggests its exploitation in the solution of the inverse scattering problem. The achievable performance is tested by comparing the proposed model with the one based on the usual Lippman–Schwinger equation in both cases of linearly approximated and full nonlinear frameworks. Both numerical and experimental data are considered.

We propose an innovative approach to the phase retrieval of 2-D discrete complex signals. The solution strategy profitably exploits some fundamental results available for the phase retrieval of one-dimensional discrete signals by following a simple yet effective philosophy resembling the solution of crossword puzzles. The resulting procedure is completely deterministic and allows identifying all the solutions of the problem by using just the spectral amplitude data, knowledge of the support of the source, and a few additional information. As a distinguishing characteristic, it does not exploit neither global-optimization algorithms nor iterative techniques. Numerical examples witnessing the interest of the proposed ideas are given for antenna applications.

Ground-penetrating radar (GPR) technology for underground exploration consists of the transmission of an electromagnetic signal in the ground for sensing the presence of buried objects. While monostatic or bistatic configurations are usually adopted, a limited number of multistatic GPR systems have been proposed in the scientific literature. In this article, we investigate the recovery performance of a specific and unconventional contactless multistatic GPR system, designed at the Georgia Institute of Technology for the subsurface imaging of antitank and antipersonnel plastic mines. In particular, for the first time, tomographic approaches are tested against this experimental multistatic GPR system, while most GPR processing in the scientific literature processes multimonostatic experimental data sets. First, by mimicking the system at hand, an accurate theoretical as well as numerical analysis is performed in order to estimate the data information content and the performance achievable. Two different tomographic linear approaches are adopted, i.e., the linear sampling method and the Born approximation (BA) method, this latter enhanced by means of the compressive sensing (CS) theoretical framework. Then, the experimental data provided by the Georgia Institute of Technology are processed by means of a multifrequency CS- and BA-based method, thus generating very accurate 3D maps of the investigated underground scenario.

A hybrid effective approach is proposed to focus the intensity of a vector field generated by an arbitrary fixed-geometry array antenna into a target point and keep it bounded elsewhere. To overcom...

We propose a new approach to solve the problem of optimal power synthesis of array antennas, so that maximum possible bandwidth can be granted to fixed sidelobe-level performances. The prop...

At the local scale, environmental parameters often require monitoring by means of affordable measuring techniques and technologies given they need to be frequently surveyed. Streamflow in riverbeds or in channels is a hydrological variable that needs to be monitored in order to keep the runoff regimes under control and somehow forecast floods, allowing prevention of damage for people and infrastructure. Moreover, measuring such a variable is always extremely important for the knowledge of the environmental status of connected aquatic ecosystems. This paper presents a new approach to assessing hydrodynamic features related to a given channel by means of a beamforming technique that was applied to video sensing. Different features have been estimated, namely the flow velocity, the temperature, and the riverbed movements. The applied beamforming technique works on a modified sum and delay method, also using the Multiple Signal Classification algorithm (MUSIC), by acting as Synthetic Aperture Radar (SAR) post-processing. The results are very interesting, especially compared to the on-site measured data and encourage the use of affordable video sensors located along the channel or river course for monitoring purposes. The paper also illustrates the use of beamforming measurements to be calibrated by means of conventional techniques with more accurate data. Certainly, the results can be improved; however, they indicate some margins of improvements and updates. As metrics of assessment, a histogram of greyscale/pixels was adopted, taking into account the example of layers and curve plots. They show changes according to the locations where the supporting videos were obtained.

The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) causes electromagnetic field variations which increase the specific absorption rate (SAR) local level and also imaging artifacts degrading the quality of MRI images. For this reason, different approaches have been proposed in literature, which aim at simultaneously improving the B 1 + homogeneity and reduce the SAR level in the body. However, most of them do not involve the determination of the complex excitation currents pertaining to the RF coil of MRI systems. In this contribution, starting from a recently introduced field intensity shaping technique, we propose a novel procedure for B 1 + shimming which is also able to control the purity of polarization of the total RF field as well as to limit the SAR level within the biological tissues. This procedure is based on convex optimization and is assessed in the following in case of brain imaging.

In this contribution, we focus on the possibility to reduce the degree of nonlinearity in inverse scattering problems by means of convenient rewritings of the Lippman Schwinger basic equation. In detail, two recently introduced rewritings are briefly reviewed and compared by emphasizing similarities and the differences. The first one, known as NIE model, is derived by extracting a local effect of the induced currents, while the second more recent one is based on the redefinition of the internal radiation operator and is known as Y0 model. Then, an effective joint exploitation of the ideas underlying the two models is discussed.

Hyperthermia treatment planning is a deeply patient-specific task which includes the optimal determination of the excitations of an array applicator. To enhance flexibility, various solutions exploiting different frequencies, antenna element, number, and applicator geometries have been proposed in the literature. Amongst them, increasing the frequency and the number of radiating elements have been effective for achieving more conformal heating. However, as each radiating element requires a power amplifier to control it, increasing the number of antennas considerably impacts the overall cost and complexity of the system. Accordingly, a procedure capable of selecting an optimal patient-specific subset of antennas from an oversized phased array applicator (with more antenna elements than available amplifiers) could help improve cost-effectiveness. In this study, we present an original approach, which allows improving performance by adaptively selecting the optimal subset of antennas to be activated for a given (redundant) applicator and a given patient. The proposed approach takes inspiration from the compressive sensing theory by embedding the sparsity promotion paradigm into a treatment planning procedure, which casts power deposition as a constrained convex optimization. Performances were demonstrated for the case of head and neck hyperthermia and benchmarked against the antenna selection procedure presently used in clinical practice.

We propose a new approach to the mask-constrained synthesis of shaped beams through generic array antennas having arbitrary layouts and element patterns. The introduced general methodology allows c...

The localization and tracking of human targets are cast as linear inverse obstacle problems and solved by means of the factorization method. The proposed approach is validated against indoor monitoring and multifrequency sensing, where Green’s function pertaining to the involved realistic scenarios has been determined through full-wave simulations. The results, which include the analysis of the impact on final performance of the number of employed transceivers as well as preliminary 2-D processing of realistic data simulated in 3-D geometry, show good robustness to noise and model errors.

A new compressive-sensing-inspired approach is proposed to the detection of faulty radiators in planar antenna arrays from a reduced set of phaseless far-field measurements. In order to deal with whatever kind of arrays, the problem is cast as the recovery of the aperture-field distribution, and two new convex-programming solution procedures are introduced. The first one relies on the improvement of the Total Variation norm technique, while the second one exploits a new aperture field expansion, which allows drastically reducing the number of unknowns of the problem. A wide set of numerical experiments involving benchmark scenarios, as well as full-wave data relative to a realistic antenna, is provided in support of the given theory.

A new approach to Phase Retrieval (PR) problem, which is of interest in antenna testing and synthesis, is introduced and discussed. The proposed strategy can be interpreted as a proper combination of solutions of simpler problems, which are herein 1-D PR problems. In particular, the 2-D PR problem is split into a number of 1-D PR problems (along diameters and circumferences) solved through the Spectral Factorization technique, and a subsequent crosswords-like selection scheme. It is worth to note that in the possible applications to antenna testing, opposite to the most widespread existing techniques, the approach just requires a single measurement surface plus the knowledge of the source’s support and of the field’s phase in a couple of sampling points.

We propose a new approach to the retrieval or synthesis of continuous sources having a circular support and generating a given shaped power pattern. The framework can be applied for whatever (asymmetrical) distribution exhibited by the measured or desired power distribution. The method relies on overlooked tools from the aperture antenna theory as well as from known tools for array antenna synthesis.

Breast cancer imaging represents a relevant and promising application area of microwave imaging technology. As breast tissues exhibit different electromagnetic properties at microwave frequencies depending on their typology and physio-pathological status, retrieving the complex-valued permittivity allows differentiating normal and diseased tissues. Recently, efforts have been made to obtain highquality prior information in order to improve both the accuracy and resolution of microwave imaging reconstructions. In this work, the use of prior information based on a combination of linear/orthogonal sampling method results is proposed in order to generate spatial priors to be used within the Contrast Source Inversion scheme. Results are compared to contrast source inversion images generated using prior information obtain from a non-iterative eigenfunction-based inversion method.

A new approach for determining the complex scalar field radiated by a finite-dimensional source starting from its modulus is proposed and assessed. First, by introducing a suitable relaxation, we show that the original problem can be conveniently tackled in terms of a finite number of different convex programing problems. Then, an effective procedure for dealing with a small number of hierarchically ordered optimizations is introduced and discussed. Numerical experiments, including consideration of an actual radiating source and of noisy data, are provided with reference to the canonical case of far-field data and array antennas.

In this contribution, the problem of designing artificial materials based devices is dealt with. In particular, a novel approach based on the Scattering Matrix Method is proposed as a potentially efficient and effective synthesis procedure.

A new approach to the problem of focusing the intensity of vector fields into a target point (subject to sidelobe constraints elsewhere) is proposed. While allowing the adoption of completely arbitrary array antennas as radiating systems, the method exploits a nested optimization procedure properly acting on both the field polarization on the target point and the array excitations. By so doing, it is able to improve the performances of existing procedures (that usually handle scalar fields) and to deal with both the near-field (NF) and the far-field (FF) focusing problems in complex inhomogeneous 3-D media.

In this paper, we report two different techniques that show how it is possible to manipulate the scattered energy from dielectric objects in order to reduce the outgoing electromagnetic fields as sensed by external observers. The first method is based on a multi-harmonic scattering cancellation, as generalized for dielectric objects, able to suppress the scattered waves for a fixed given direction of the incoming wave. While external fields are suppressed towards perfect zero cancellation, internal fields within the dielectric particle are demonstrated to increase due to energy conservation. The second technique exploits the relationship between the scattered field and the spectral content of the overall system (object plus cloak) under certain approximations to design coats able to shift the energy content outside the visible range. Both methodologies highlight the amount of scattering reduction and the number of incoming incident direction or view for which the object appears perfect matched to the background (unidirectional cloaking) or quasiinvisible (partial invisibility). Applications of such theoretical findings span from the reduction of mutual coupling between nearby antennas to the ultimate routing of electromagnetic wave propagation.

In this contribution, the integral equations underlying the two-dimensional electromagnetic inverse scattering are conveniently rewritten in such a way to exhibit a lower degree of nonlinearity with respect to parameters embedding dielectric characteristics as compared to the traditional model. The proposed inverse scattering model is based on an original decomposition of the adopted Green’s function and the use of the ‘reduced scattered field’, which is underlying a recently introduced qualitative method. Its adoption could allow to simply the solution of inverse scattering problems.

A new approach to the design of graded artificial materials (GAMs)-based devices is proposed by exploiting the inverse scattering framework as a synthesis tool. The introduced general methodology can be applied to arbitrary far-field specifications, thus allowing the design of noncanonical devices. In particular, two different strategies are developed for GAMs based on either graded refractive index (GAM $_{R}$ ) or graded filling factor (GAM $_{F}$ ). In both strategies, the inverse scattering problem is solved by a proper reformulation of the contrast source inversion method, wherein a proper rescaling of the amplitude of the primary sources is also used. In particular, in the first strategy, the GAM $_{F}$ is obtained by exploiting homogenization theories. In the second strategy, the GAM $_{R}$ is synthesized by exploiting a suitable representation basis for the unknown contrast function, and then simple analytical formulas are used to determine the corresponding GAM $_{F}$ . The proposed approach is assessed through the synthesis of an antenna generating a $\Sigma /\Delta $ reconfigurable pattern.

This letter addresses the problem of reconstructing the geometrical features, i.e., location, shape, and size of targets embedded into a nonaccessible region. In particular, an approach recently introduced for the case of free space and full aspect measurements is extended, discussed, and validated for the more realistic and challenging case of data collected under “aspect limited” measurement configurations, which include subsurface sensing, cross-borehole imaging, and many other cases. Results obtained by considering 2-D scenario and noise affected simulated data confirm the potentialities of the proposed method in dealing with realistic applications.

Due to the plethora of possible applicative fallouts, in the recent years several efforts have been made to develop three-dimensional (3-D) field intensity shaping methods. While many approaches able to focus a wave-field into a target point are available, few strategies have been instead developed to generate a field having a given intensity spatial distribution. This is probably due to the higher challenging nature of such a problem. In this letter, with reference to scalar fields, we present an innovative 3-D shaping procedure, which casts the problem as the solution of a finite number of convex programming problems. An example of the performance of the proposed procedure in a 3-D inhomogeneous scenario is given to show the capability of ensuring uniform field intensity within a target area while keeping it arbitrarily bounded elsewhere.

In hyperthermia treatments, the possibility of generating a specific absorption rate distribution peaked into the tumor and bounded elsewhere represents the main clinical need. As multi-frequency applicators can be exploited to reduce the occurrence of undesired hot-spots, an optimal multi-frequency approach is proposed and discussed. Being formulated in terms of a convex programming problem, the globally optimal solution can be determined (only for very special cases). The procedure, presented for the case of scalar fields, can be extended both to the case of vector fields and to the more interesting problem of shaping (rather than just focusing) the specific absorption rate distribution.

The topic of antenna array diagnostics, and in particular of phase faults detection, is faced within the framework of solution algorithms based on phaseless data and sparsity promotion approaches. To this aim, a smart procedure is proposed to make compliant the problem at hand with both the requirements underlying the compressive sensing paradigm.

This contribution deals with a spectral analysis of radiating components of a generic source which radiates a given field. This analysis, even if general, has a great relevance to inverse scattering problems, wherein some currents are induced in unknown obstacles due to the interaction with a known electromagnetic incident field and irradiate in turn another field, known as scattered field. The analysis has been detailed to both one-dimensional and two-dimensional geometries and allows to understand how the spectral contents can change depending on the dimension of the source support (which is exactly equal to the target support). Note that the understanding of spectral properties of radiating currents can be relevant not only to microwave imaging but also to antenna synthesis and diagnosis.

The synthesis of a shaped beams by means of given fixed-geometry arrays is a canonical open problem relevant in many engineering areas. In fact, several approaches have been proposed for optimizing as much as possible some performance parameters or the dimensions of the arrays, but they all suffer from several drawbacks which are effectively tackled by the proposed approach. In this contribution, we propose an innovative approach to the mask-constrained power synthesis of shaped beams through fixed-geometry antenna arrays having whatever layout and element patterns. In particular, we are able to formulate the problem as the global optimization of a very small number of parameters, and we propose and discuss two different solution procedures.

A new model for two-dimensional electromagnetic inverse scattering is introduced by means of a convenient rewriting of the pertinent integral equation and by exploiting the ‘reduced scattered field’, which is underlying a recently introduced qualitative method. The proposed inverse scattering model exhibits a lower degree of nonlinearity with respect to parameters embedding dielectric characteristics as compared to the traditional model, thus suggesting its exploitation in the solution of the inverse problem.

Orbital angular momentum (OAM) is gaining great attention in physics and electromagnetic community owing to an intriguing debate concerning its suitability for widening the channel capacity in next generation wireless communications. While such a debate is still a matter of controversy, in this paper we exploit OAM for microwave imaging within the framework known as Virtual Experiments (VE). VE tackles the inverse scattering problem introducing an effective rearrangement of the original scattering experiments into new “virtual” ones, by means of a proper “design equation”, in order to condition time-harmonic fields to assume a specific spatial distribution in the imaging domain. In particular, in this paper we exploit OAM fields to perform linear quantitative imaging via linear of strong targets, i.e. targets electrically large and with high contrast, which in general require the use of non linear approaches to be carefully imaged.

An innovative and effective approach to the diagnostics of faulty elements in planar array antennas is presented and assessed. The introduction and exploitation of a new field expansion allow casting the faulty elements' recovery problem as the determination of the actual aperture-field distribution. Notably, a number of remarkable features is provided, including the exploitation of just phaseless far-field measurements, the profitable use of the Compressive Sensing theory, and the capability of dealing with whatever kind of planar arrays (e.g., realistic radiating systems where mutual coupling and mounting-platform effects come into play). The given theory is supported by numerical experiments involving full-wave data relative to a realistic 100-elements waveguide array.

We propose a new approach to the phase retrieval of 2-D radiated scalar fields. The technique takes advantage of existing knowledge and procedures for the recovery of a 1-D discrete complex signal from the intensity of its Fourier Transform and then uses an effective crosswords-like procedure for the solution of the corresponding 2-D problem. By exploiting only one measurement surface and without recurring to global-optimization algorithms the approach is able to find in a deterministic fashion all the different solutions of the problem.