S. Maci

University of Siena / Università di Siena, Dipartimento di Ingegneria dell'Informazione e Scienze Matematiche, Faculty Member

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Thomas Jaki

University of Cambridge

Maria Huete-Ortega

The University of Sheffield

Aslı SUNER (KARAKÜLAH)

Ege University

Jayjit Majumdar

University of Kalyani

Armando Marques-Guedes

UNL - New University of Lisbon

Meredith Carpenter

Stanford University

Eszter Banffy

Hungarian Academy of Sciences

Alissa Mittnik

Max Planck Institute for Evolutionary Anthropology

Jakob Sedig

Harvard Medical School

Luis Troccoli

Universidad de Oriente, Venezuela

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Papers by S. Maci

High-frequency analysis of an array of line sources on a truncated ground plane

IEEE Transactions on Antennas and Propagation, 1998

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$Research paper thumbnail of Incremental diffraction coefficients for source and observation at finite distances from an edge$

Incremental diffraction coefficients for source and observation at finite distances from an edge

IEEE Transactions on Antennas and Propagation, 1996

An incremental theory of diffraction (ITD) has previously been introduced which provides a self-c... more An incremental theory of diffraction (ITD) has previously been introduced which provides a self-consistent, high-frequency description of a wide class of scattering phenomena within a unified framework. Explicit expressions of the incremental diffracted field contributions have been obtained for a plane-wave illumination of wedge-shaped configurations and observation points at a finite distance from the incremental point. In this paper, this

A Metasurface Antenna for Space Application

IEICE Proceeding Series

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$Research paper thumbnail of Propagating and evanescent edge diffracted waves for a semi-infinite periodic dipole array$

Propagating and evanescent edge diffracted waves for a semi-infinite periodic dipole array

IEEE Antennas and Propagation Society International Symposium. 1998 Digest. Antennas: Gateways to the Global Network. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.98CH36194)

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$Research paper thumbnail of The Truncated Floquet Wave Diffraction Theory for Planar Phased Arrays: An Overview$

The Truncated Floquet Wave Diffraction Theory for Planar Phased Arrays: An Overview

Springer Proceedings in Physics, 2004

This paper summarizes the research activities conducted at the University of Siena during the pas... more This paper summarizes the research activities conducted at the University of Siena during the past six years on the truncated Floquet wave diffraction theory under the continuous guidance of Professor Felsen. During that period, we have analyzed and validated the diffraction phenomena in both the frequency and time domains pertaining to large planar phased arrays of different topologies and with quite general excitation profiles. The associated canonical dipole array Green's functions (AGFs) have been expressed in terms of constituents in a generalized high frequency asymptotic, periodicity-adapted, Floquet wave (FW)-modulated uniform geometrical theory of diffraction (UTD), which highlights the relevant wave physics. The present paper summarizes the essentials of these AGFs for large planar rectangular phased dipole arrays in free space, escalating sequentially from free-space geometries to multilayered media, with the eventual utilization of the Method of Moments (MoM) for practical applications. Particular emphasis is given to those aspects which are in progress, and have not yet appeared in print.

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Metadispersion in anisotropic and bianisotropic media

2011 XXXth URSI General Assembly and Scientific Symposium, 2011

In this work an innovative method for the characterization of dispersion in metamaterials is pres... more In this work an innovative method for the characterization of dispersion in metamaterials is presented. The attention is focused on structures realized by stacks of planar periodic surfaces. The analysis procedure subdivides the artificial material into its constituent planes, performing a full wave analysis of the individual planar periodic structures, and uses analytic formulas from Bloch theory to determine the dispersion characteristics of the overall artificial medium. Physical properties of the equivalent admittance matrix modeling the single planar sheets are exploited to analytically describe the dispersion properties of the metamaterial in the whole first Brillouin zone, starting from a limited number of full-wave simulations.

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Numerical modelling of MCFC cathode degradation in terms of morphological variations

International Journal of Hydrogen Energy, 2011

This paper describes the numerical modelling of a key material-stability issue within the realm o... more This paper describes the numerical modelling of a key material-stability issue within the realm of Molten Carbonate Fuel Cells (MCFC). Differential models have been developed for the 2D and 3D distributions of current density as well as peroxide and carbon dioxide concentrations. By suitable variations of the integration domain e based on the agglomerate concept e one can describe the morphological and attending electrocatalytic evolution of porous NiO electrodes. On the basis of electrochemical data recorded during the operation of a laboratory MCFC, we have shown that this model is able to rationalise the evolution of cathode conditions leading to both improvements of electrocatalytic performance e such as lithiation e and degradation e such as agglomeration e.

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Reflections inside an elliptical dielectric lens antenna

IEE Proceedings - Microwaves, Antennas and Propagation, 1998

The reflections inside the elliptical lens of a typical millimetre wave antenna are investigated.... more The reflections inside the elliptical lens of a typical millimetre wave antenna are investigated. In particular, it is seen that these reflections may significantly affect the input impedance of a radiating element which is located at the lens focus, owing to the presence there of a point caustic of doubly reflected rays. The presence of a caustic prevents the field prediction inside the lens by a direct application of geometrical optics. Consequently, the field inside the lens is calculated by means of the physical optics currents associated with singly and doubly reflected rays.

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A Cloaking Metamaterial Based on an Inhomogeneous Linear Field Transformation

IEEE Transactions on Antennas and Propagation, 2010

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Dispersion Characteristics of a Metamaterial-Based Parallel-Plate Ridge Gap Waveguide Realized by Bed of Nails

IEEE Transactions on Antennas and Propagation, 2011

The newly introduced parallel-plate ridge gap waveguide consists of a metal ridge in a metamateri... more The newly introduced parallel-plate ridge gap waveguide consists of a metal ridge in a metamaterial surface, covered by a metallic plate at a small height above it. The gap waveguide is simple to manufacture, especially at millimetre and sub-millimetre wave frequencies. The metamaterial surface is designed to provide a frequency band where normal global parallel-plate modes are in cutoff , thereby allowing a confined gap wave to propagate along the ridge. This paper presents an approximate analytical solution for this confined quasi-TEM dominant mode of the ridge gap waveguide, when the metamaterial surface is an artificial magnetic conductor in the form of a bed of nails. The modal solution is found by dividing the field problem in three regions, the central region above the ridge and the two surrounding side regions above the nails. The fields within the side regions are expressed in terms of two evanescent TE and TM modes obtained by treating the bed of nails as an isotropic impedance surface, and the field in the central ridge region is expanded as a fundamental TEM parallel-plate mode with unknown longitudinal propagation constant. The field solutions are linked together by equalizing longitudinal propagation constants and imposing point-continuity of fields across the region interfaces, resulting in a transcendental dispersion equation. This is solved and presented in a dispersion diagram, showing good agreement with a numerical solution using a general electromagnetic solver. Both the lower and upper cutoff frequencies of the normal global parallel-plate modes are predicted, as well as the quasi-TEM nature of the gap mode between these frequencies. The evanescent fields in the two side regions decay very rapidly away from the ridge, being in the order of 100dB per lower cutoff wavelengths over most of the single-mode bandwidth.

$Fig. 12. Transverse attenuation constant in x-direction plotted as dB/Xo versus frequency compared with the one obtained through a CST simulation where the field decaying is captured by a couple of 6-spaced probes located as shown in the inset. a “a x Ae = \/k2+ki —k? anda, = \/k? +k, — k? are plotted in Fig. 12 in the stop band of the parallel-plate region, corre- sponding to the working bandwidth of the ridge gap waveguide. While a, is strongly varying with frequency and has a value greater than 100dB/Xo almost over the entire bandwidth, On is almost a constant in the same bandwidth. Indeed, since k, ~ k. Ay ky = 1/(h + d), and the resulting value is smaller over the bandwidth. This implies that H, component has a weaket decay, dictated by the attenuation factor @,. Secondly, regard- ing the CST H,, component, we see oscillations in the nails re- gion which are due to the actual periodicity of the pins, that we neglect in the surface impedance model. It is evident from Fig. 11 that the CST EH, component does not show the same ripples. Indeed, E,, has a stronger decay along x than H,, and this prevents E,, from being influenced by the periodicity of the pins. The results are given in dB/Xg, where Ao is calculated at$

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Analysis of dispersion in dipole-FSS loaded hard rectangular waveguide

IEEE Transactions on Antennas and Propagation, 2006

We investigate the properties of the dipole frequency selective surfaces-loaded rectangular waveg... more We investigate the properties of the dipole frequency selective surfaces-loaded rectangular waveguide. The analysis is carried out by using the method of moments together with an algorithm that solves for a multilayer structure in the spectral domain. The eigenmodes are determined by searching for resonant solutions. Dispersion diagrams obtained from this approach are compared with those generated by commercial software

Frequency-domain Green's function for a planar periodic semi-infinite phased array .I. Truncated floquet wave formulation

IEEE Transactions on Antennas and Propagation, 2000

This second part of a two-paper sequence deals with the physical interpretation of the rigorously... more This second part of a two-paper sequence deals with the physical interpretation of the rigorously derived high-frequency asymptotic wave-field solution in Part I, pertaining to a semi-infinite phased array of parallel dipole radiators. The asymptotic solution contains two parts that represent contributions due to truncated Floquet waves (FW's) and to the corresponding edge diffractions, respectively. The phenomenology of the FW-excited diffracted fields is discussed in detail. All possible combinations of propagating (radiating) and evanescent (nonradiating) FW and diffracted contributions are considered. The format is a generalization of the conventional geometrical theory of diffraction (GTD) for smooth truncated aperture distributions to the truncated periodicity-induced FW distributions with their corresponding FW-modulated edge diffractions. Ray paths for propagating diffracted waves are defined according to a generalized Fermat principle, which is also valid by analytic continuation for evanescent diffracted ray fields. The mechanism of uniform compensation for the FW-field discontinuities (across their truncation shadow boundaries) by the diffracted waves is explored for propagating and evanescent FW's, including the cutoff transition from the propagating to the evanescent regime for both the FW and diffracted constituents. Illustrative examples demonstrate: 1) the accuracy and efficiency of the high-frequency algorithm under conditions that involve the various wave processes outlined above and 2) the cogent interpretation of the results in terms of the uniform FW-modulated GTD.

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High-frequency Green's function for a semi-infinite array of electric dipoles on an infinite grounded stratified dielectric slab: part III $phase-matched wave interactions and numerical results

IEEE Transactions on Antennas and Propagation, 2005

This second part of a three-paper sequence deals with the spatial domain parametrization and phys... more This second part of a three-paper sequence deals with the spatial domain parametrization and physical interpretation of the relevant asymptotic high-frequency Green's function for a semi-infinite phased array of parallel dipoles on an infinite stratified grounded dielectric slab. This array Green's function (AGF) has been previously derived using a spectral domain formulation; the relevant asymptotic solution contains contributions associated with Floquet waves (FWs), and corresponding surface, leaky and diffracted waves excited at the array edge. Both the truncated-FW series and the series of corresponding diffracted field contributions exhibit excellent convergence properties. In the present paper, through application of the Poisson summation, the AGF for a plane-stratified grounded dielectric slab is developed in terms of space domain FW-dependent Kirchhoff radiation integrals which are synthesized by superposition of periodicity-modulated phased line sources oriented parallel to the edge. The asymptotic evaluation of each Kirchhoff radiation integral leads to a grouping of various asymptotic terms, which provide physically appealing interpretations of a variety of wave processes, encompassing slab-modulated propagating (radiating) and evanescent (nonradiating) FWs, slab-guided surface waves (SWs) or leaky waves (LWs), and their edge-coupled phenomenologies. The present space domain parametrization leads to the same asymptotics as that from the spectral domain parametrization, but allows a clear description of the spatial wave interaction processes. Index Terms-Diffraction, high frequency, patch array antennas. I. INTRODUCTION T HE present paper and the following Part III [2] are concerned with the identification of the multiplicity of periodicity-induced wave interactions processes that are embedded within the high-frequency solution presented in [1] for the Green's function of a linearly phased semi-infinite array of

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An Equivalence Theorem Based on the Use of Electric Currents Radiating in Free Space

IEEE Antennas and Wireless Propagation Letters, 2008

The most common formulations of the equivalence principle use both electric and magnetic currents... more The most common formulations of the equivalence principle use both electric and magnetic currents radiating in free space or only electric (magnetic) currents radiating in presence of a perfectly magnetic (electric) conductor. In this paper, a formulation is provided for equivalent currents of only electric (magnetic) type as a function of the tangential total fields of the original problem over the equivalence surface. The formulation of these equivalent currents becomes simple for some canonical surfaces, like a plane, a cylinder and a sphere. Explicit expressions of the equivalent electric currents on a spherical surface are provided in terms of spherical wave functions.

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Near-field line-integral representation of the Kirchhoff-type aperture radiation for a parabolic reflector

IEEE Antennas and Wireless Propagation Letters, 2003

In this letter, a line integral representation is presented for a linearly polarized Kirchhoff-ty... more In this letter, a line integral representation is presented for a linearly polarized Kirchhoff-type aperture radiation from a parabolic reflector antenna. The main purpose of this result is concerned with the acceleration of the numerical integration for calculating the near field of large reflector antennas. The formulation, which is rigorous for uniform aperture field, is based on the application of the equivalence principle to a projecting surface, which allows the analytical evaluation in a closed form of one of the twofold surface integral which defines the radiated field at any space point; the extension to slowly varying primary feed pattern is based on an asymptotic approximation, which is proved to be accurate in proximity of the aperture till to 30 dB of amplitude edge illumination. The present formulation is well suited to be improved by fringe diffraction contributions in the framework of edge-wave theories such as physical theory of diffraction (PTD) and incremental theory of diffraction (ITD).

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A metamaterial surface for compact cavity resonators

IEEE Antennas and Wireless Propagation Letters, 2004

We suggest an idea for miniaturization of cavities by utilizing a properly designed metamaterial ... more We suggest an idea for miniaturization of cavities by utilizing a properly designed metamaterial thin surface inserted inside the cavities. This metamaterial surface is constituted by a thin dielectric slab on both sides of which "gangbuster" dipoles are printed. Inserting the thin slab inside a parallel-plate one-dimensional (1-D) cavity resonator has the effect of decreasing the resonant frequency. Placing the metamaterial slab at the center of a rectangular waveguide also lowers the cutoff frequency of the dominant mode of the waveguide. The corresponding dispersion curve exhibits a smooth transition from a fast-wave to a slow-wave regime and then asymptotically tends to the dispersion curve of the first TE surface-wave mode of the metamaterial slab. This suggests a natural way to conceive a 3-D compact cavity resonator by placing two perfectly electric conducting walls, a half of the wavelength of the slow-wave mode apart, inside the above rectangular waveguide. The analysis, performed by a circuit network theory and validated by a full-wave numerical analysis, provides simple formulas to predict the resonant frequency and the dispersion diagrams of these structures.

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Reduction of Patch Antenna Coupling by Using a Compact EBG Formed by Shorted Strips With Interlocked Branch-Stubs

IEEE Antennas and Wireless Propagation Letters, 2009

... Maci are with the Department of Information Engi-neering, University of Siena, 53100 Siena, I... more

Spherical near field measurements with truncated scan area

This paper presents a technique for the determination of the full sphere pattern of an antenna fr... more This paper presents a technique for the determination of the full sphere pattern of an antenna from truncated spherical near field measurements. The approach is based on an iterative procedure which exploits the band-limitedness properties of the radiated field to correctly reconstruct the full sphere data. Tests performed on synthetic as well as measured near-field data demonstrate the effectiveness of

EBG-based gap waveguide for applications up to THz

Chalmers Publication Library (CPL). Forskningspublikationer från Chalmers Tekniska Högskola.

The European School of Antennas: results and perspectives

The European School of Antennas (ESoA) is a new model of geographically-distributed school that h... more The European School of Antennas (ESoA) is a new model of geographically-distributed school that has the objective to reinforce the European training and research in antennas and relevant applications. This paper presents the objective and perspectives of ESoA and the results obtained in 2006 and the first part of 2007 in terms of flow of students and teachers.

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High-frequency analysis of an array of line sources on a truncated ground plane

IEEE Transactions on Antennas and Propagation, 1998

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$Research paper thumbnail of Incremental diffraction coefficients for source and observation at finite distances from an edge$

Incremental diffraction coefficients for source and observation at finite distances from an edge

IEEE Transactions on Antennas and Propagation, 1996