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Stefano Maci

StefanoMaci

Prof. Stefano Maci
Professor of Electromagnetics and Antennas
Department of Information Engineering
University of Siena
Via Roma 56, 53100, Sienna Italy
390 577 2346235
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Stefano Maci is a Professor the University of Siena (UNISI), with scientific responsibility of a group of 15 researchers (http://www.dii.unisi.it/~lea/).  He is the Director of the UNISI PhD School of Information Engineering and Science, which presently includes about 60 PhD students. His research interests include high-frequency and beam representation methods, computational electromagnetics, large phased arrays, planar antennas, reflector antennas and feeds, metamaterials and metasurfaces.

Since 2000, he was responsible of 5 projects funded by the European Union (EU); in particular, in 2004-2007 he was WP leader of the Antenna Center of Excellence (ACE, FP6-EU) and in 2007-2010 he was International Coordinator of a 24-institution consortium of a Marie Curie Action (FP6). He also carried out several projects supported by the European Space Agency (ESA-ESTEC), the European Defense Agency (EDA), the US-Army Research Laboratory (ARL), and by various industries and research institutions: EADS-MATRA, (Tolosa, Francia), IDS (Pisa, Italia), TICRA (Copenhagen), ALENIA MARCONI SYSTEM (Rome, Italy), SAAB-ERICSON SPACE (Gotheborg, Svezia), THALES (Paris, France), TNO (L'Aia, Olanda), OTO MELARA (La Spezia, Italia), OFFICINE GALILEO (Florence, Italy), SELEX Communication (Rome), Thales Alenia Space (Rome).

Since 2001 he was a member the Technical Advisory Board of 11 international conferences, member of the Review Board of 6 International Journals; in the same period, he organized 23 special sessions in international conferences, and he held 10 short courses in the IEEE Antennas and Propagation Society (AP-S) Symposia about metamaterials, antennas and computational electromagnetics. He was an Associate Editor of IEEE Trans on EMC and of IEEE Trans. on AP and two times Guest Editor of special issues of the latter journal. In 2003 he was elected Fellow of IEEE.

In 2004 he founded the European School of Antennas (ESoA), a PhD school that presently comprises 30 courses on Antennas, Propagation, Electromagnetic Theory, and Computational Electromagnetics. ESoA counts about 150 among the best teachers of Europe (which include eleven IEEE Fellows) and it is frequented by an average of 220 students per year. The ESoA consortium presently comprises 33 European research centers and offers 12 one-week courses per year.

In 2005-2007, he was Italian National representative of the NATO SET-TG 084 "Emerging Technology for Sensor and Front-ends”, and he is presently involved as co-representative in the NATO SET-181 RTG on "Metamaterials for Defense and Security Applications".
Stefano Maci was co-founder of two spinoff-companies and since 2008 he is honorary President of LEAntenne e Progetti SPA (Valeggio sul Mincio, VE).

Stefano Maci is presently Director of ESoA, a member of the Board of Directors of the European Association on Antennas and Propagation (EuRAAP), a member of the Technical Advisory Board of the URSI Commission B, a member elected of the AdCom of IEEE Antennas and Propagation Society, a member of the Governing Board of the European Science Foundation (ESF) Project “NewFocus”, a member of the Governing board of the FP7 coordination action “CARE” (Coordinating the Antenna Research in Europe), a member of the Award Committee of the IEEE Antennas and Propagation Society (AP-S), a member of the Antennas and Propagation Executive Board of the Institution of Engineering and Technology (IET, UK), and a member of the Focus Group on METAMATERIALS in the Finmeccanica project “Mind-Share”.  

His research activity is documented in 10 book chapters, 110 papers published in international journals, (among which 76 on IEEE journals), and about 300 papers in proceedings of international conferences. His h index is 24, with a record of more than 2000 citations (source Google Scholar).

Metasurfing Wave Antennas

Metasurfaces constitute a class of thin metamaterials, which can be used from microwave to optical frequencies to create new electromagnetic engineering devices. They are obtained by a dense periodic texture of small elements printed on a grounded slab without or with shorting vias. These have been used in the past for realizing electromagnetic bandgaps (EBG) or equivalent magnetic-walls. Changing the dimension of the elements, being the sub-wavelength 2D-periodicity equal, gives the visual effect of a pixelated image and the electromagnetic effect of a modulation of the equivalent local reactance. The modulated metasurface reactance (MMR) so obtained is able to transform surface or guided waves into different wavefield configurations with required properties. This MMR-driven wavefield transformation is referred to as “Metasurfing”. The MMR allows in fact a local modification of the dispersion equation and, at constant operating frequency, of the local wavevector. Therefore, the general effects of metasurface modulation are similar to those obtained in solid (volumetric) inhomogeneous metamaterial as predicted by the Transformation Optics; namely, re-addressing the propagation path of an incident wave. However, significant technological simplicity is gained.

When the MMR is covered by a top ground plane (Parallel-plate waveguide Metasurfing) the real part of the Poynting vector follows a generalized Fermat principle as happen in ray-field propagation in inhomogeneous solid medium. This may serve for designing lenses or point-source driven beam-forming networks. When the MMR is uncovered, wave propagation is accompanied by leakage; i.e., a surface wave is transformed into a leaky-wave, and the structure itself becomes an extremely flat antenna. In every case, introducing slots in the printed elements allows a polarization control. In such cases, the metasurface associated with can be described by an anisotropic surface impedance.

In this lecture, after illustration of the design method of metasurfing-wave antennas, various examples are presented and discussed, including Luneburg lenses, Maxwell’s Fish-eyes, isoflux antennas, Doppler-guide antennas and new transmission lines.        

Retrieval of Constitutive Parameters in Metamaterials

The amazing interest on metamaterials, which has been growing for a decade, has in parallel posed a lot of questions about the most reliable and accurate approach to the characterization of their electromagnetic behavior.  Most of the metamaterials can be described as a periodic repetition of some inclusions in a host medium; the inclusions can be made resonant despite their small dimension in terms of a wavelength. It is well accepted in various scientific communities to consider this type of artificial structures as effective media, described by a set of equivalent constitutive parameters. These parameters can be obtained by using analytical models, measurements or full-wave simulations in conjunction with numerical retrieval algorithms. However, the procedure for the definition of the equivalent parameters is not univocally defined, and different approaches may be used depending on the metamaterial characteristics and on the goal of the homogenization process.

The analysis methodologies for retrieval of constitutive parameters include those based on a microscopic-equivalence and those based on a macroscopic-equivalence. The former set up a model relating each element of the periodic arrangement to an equivalent particle with electric and/or magnetic dipolar moment. This requires constituent particles small in terms of a wavelength, since the response of the single particle is approximated by the dominant term of a multi-polar expansion. On the other hand, macroscopic-equivalence approaches identify equivalent wave impedance and propagation constant for the field propagating inside the metamaterial, and they look for a homogeneous medium supporting the same modal structure. In this case the restrictions on the electrical size of the inclusion may be less severe, and it is possible to correctly retrieve the scattering parameters from a metamaterial sample. However, there could be some intrinsic ambiguities in the extraction of the equivalent parameters.

In this lecture, after the review of the literature, an original method of retrieval of the metamaterial constituent parameters is described, with emphasis on removing ambiguity and on the role played by the spatial dispersivity of the constituent parameters. The special class of metamaterials formed by periodic multilayer arrangements of 2D printed structures will be considered with much attention. Anisotropy and special case of gyrotropy, as well as bianisotropy and the special case of chirality will be considered in the parameter retrieval.

Scattering Matrix Domain Decomposition Method Formalized with Different Wave Propagators

In most of the real applications, antennas need to be located in a complex operative environment;  accurate analysis is needed to take into account interaction with antenna platform or other surrounding antennas. A rigorous numerical analysis of these large problems is a very complex task, due to the prohibitive number of unknowns; furthermore, the simultaneous presence of electrically large structures and small features may lead to ill conditioning.

The Domain Decomposition Method is a general approach for the solution of complex multiscale problems, which allows one to overcome the above mentioned impairments; it consists in dividing the original problem into simpler, more tractable non-overlapped subdomains that are solved separately, and then obtaining the overall solution by imposing proper connections among different subdomains. In particular, if the sub-domains boundaries are associated with ports through which a proper set of modes flow, the interactions among different sub-domains can be rigorously described through an equivalent network representation. Depending on the propagation mechanism within each sub-domain, different types of modes or ”wave objects” can be used. In particular, beam-type fields or radiated modes are conveniently used when dealing with radiation and scattering problems, while waveguide modes are well suited for representing guided waves.

Different choices of the wave objects used for field representation lead to different implementations of the generalized network formulation. The optimal choice is the one maximizing the efficiency of the overall numerical analysis, and depends on the problem under consideration. In this talk, two particular implementations are considered: the first one uses complex point source (CPS) beams  as wave objects, while the second one uses spherical waves (SW) for the representation of radiated field and waveguide modes at the antenna input port. In the first case, thanks to the angular selectivity of the CPS beams, only a small fraction of the beams contribute to the subdomain interactions, thus, leading to an efficient numerical procedure. In the second case, the choice of spherical waves offers the advantage of direct interfacing with the output of spherical near-field measurements or numerical simulations, while the inclusion of waveguide modes provides the information about the reflection coefficient at the antenna input port.