Giampiero Lovat - Profile on Academia.edu (original) (raw)

International Journals by Giampiero Lovat

A recent analytical formulation for the low-frequency magnetic shielding effectiveness of a thin ... more A recent analytical formulation for the low-frequency magnetic shielding effectiveness of a thin metal screen against a circular loop source is extended to the case of a screen exhibiting both magnetic and conductive properties. The presence of magnetic properties introduces a series impedance in the equivalent network of the structure, in addition to the shunt admittance associated with the screen conductivity. However, the fields scattered by the two elements do not interact and hence can be treated independently; furthermore, the series impedance gives rise to integrals akin to those stemming from the shunt admittance. A generalized analytical formulation for a combined magnetic and conductive screen is thus obtained, that is favorably compared with both classical approximate formulations and exact results.

Fundamental properties of surface plasmon polariton modes propagating along graphene nanoribbons ... more Fundamental properties of surface plasmon polariton modes propagating along graphene nanoribbons are investigated by means of a full-wave method-of-moments approach adopting a spatially dispersive dyadic model for graphene conductivity. Modal propagation, current profiles, field distributions, and characteristic impedances are studied in detail showing the dramatic effects of spatial dispersion and the need to go beyond the usual weak (low-) spatial dispersion formulation.

A full-wave analysis of the fundamental quasi-TEM modes supported by multiple graphene nanoribbon... more A full-wave analysis of the fundamental quasi-TEM modes supported by multiple graphene nanoribbons above a ground plane is presented, aimed at characterizing crosstalk in graphene multiconductor lines. A method-of-moments discretization of the relevant electric-field integral equation is performed. Assuming first a local scalar conductivity, an efficient spatial-domain approach with subsectional basis functions is assuming first a local scalar conductivity, a spatial-domain approach with subsectional basis functions is developed. This allows for the efficient treatment of nanoribbons with wide transverse separations, and can be expanded to include in the simulation model spatial nonuniformity of the graphene conductivity. This spatial-domain formulation is then extended to treat the case of weakly nonlocal conductivity, via an original integro-differential approach derived by approximating a recent full spectral graphene conductivity model in the limit of low wavenumbers. Numerical results are provided for propagation constants and characteristic impedances of two identical coupled graphene nanoribbons; on this basis, a crosstalk analysis is performed by means of the modal decomposition method.

The 2-D time-domain Green's function of a graphene sheet is here derived, by assuming a local Dru... more The 2-D time-domain Green's function of a graphene sheet is here derived, by assuming a local Drude-like model for the graphene conductivity valid in the absence of biasing magnetic fields and when both spatial-dispersion effects and interband terms are negligible (i.e., up to the low terahertz range). The sought Green's function is derived in a semi-analytical form through a modified Cagniard-De Hoop approach. This allows for deriving simple semi-analytical expressions for the fields radiated by a pulsed line source in the presence of a graphene sheet, which can be computed in a fast and straightforward way. Theoretical and numerical validations are presented by obtaining the known results for nondispersive metallic sheets as limiting cases and through comparisons with results obtained numerically through an exact canonical double inverse Fourier transform.

In this paper, an original closed-form approximate evaluation is performed for the continuous-spe... more In this paper, an original closed-form approximate evaluation is performed for the continuous-spectrum field excited by an infinite line source in a dielectric substrate-superstrate configuration, optimized for leaky-wave radiation. By means of a suitable approximate asymptotic representation obtained via Watson's lemma, the continuous-spectrum field has been expressed as the sum of the contributions of two leaky-pole singularities, each weighted by a transition function that depends on both the frequency and observation distance. The validity of these results is shown in the near and far fields at different frequencies, including the frequency range in which the leaky wave is physical and the entire transition region through the spectral gap. This new closed-form result explicitly shows the nature of the continuous-spectrum field in the transition region, and provides insight into the nature of the fields on more complicated structures in microwave integrated circuits.

In this work, surface-wave propagation in a metamaterial grounded slab is investigated. In partic... more In this work, surface-wave propagation in a metamaterial grounded slab is investigated. In particular, a double-negative (DNG) medium is considered. On the basis of the dispersion equations for TE and TM surface waves supported by such a grounded slab, conditions are presented, which ensure the suppression of a guided-wave regime for both polarizations. In contrast with ordinary grounded slabs, two kinds of surface waves (one evanescent only in air, the other evanescent both in air and inside the slab) have to be taken into account. The possible absence of any surface wave makes the considered structure a promising candidate as a substrate for microstrip antennas with reduced edgediffraction effects and enhanced radiation efficiency.

In this work, an original closed-form evaluation is performed for the continuous-spectrum field e... more In this work, an original closed-form evaluation is performed for the continuous-spectrum field excited by a dipole source in substrate-superstrate configurations, which have been employed in microwave planar antennas. In particular, we derive an analytical representation of the continuous-spectrum field in the form of angular-dependent cylindrical leaky waves of TE or TM type, weighted by an appropriate transition function. The proposed formulation is uniformly valid as frequency is varied, i.e., also in the neighborhood of the transition region between leaky-wave and bound-wave ranges. In this way it is possible to describe in a simple and efficient way the behavioral features of guided and radiated fields excited by practical sources in open planar structures. Numerical results which show the accuracy of this approach are provided in different operating conditions.

1] In this work, an asymptotic analysis is presented for bound-mode and free-space residual-wave ... more 1] In this work, an asymptotic analysis is presented for bound-mode and free-space residual-wave currents, with the aim of obtaining an analytical representation of the continuous-spectrum current excited by a delta-gap source on a microstrip line. The proposed approach is valid in a frequency region which extends through the entire spectral gap of the dominant leaky mode. The analysis proves in a rigorous way the different asymptotic behaviors of the TM 0 bound-mode and free-space residual-wave currents. The relevance of the nature of the involved branch points to these asymptotic behaviors is also discussed. An explicit expression for the numerical coefficient arising in the asymptotic expansion obtained through Watson's Lemma is provided in a simple closed form for both types of residual-wave current. Analytical details are supplied, together with numerical results which confirm the predicted asymptotic behaviors and the accuracy of the closed-form representation also in proximity of the source, if the presence of singularities of the relevant spectral integrands is properly taken into account.

In this paper, dispersive propagation and radiation properties of leaky waves on metamaterial gro... more In this paper, dispersive propagation and radiation properties of leaky waves on metamaterial grounded slabs are investigated. The proper or improper nature of leaky modes supported by such structures is shown to be related to the metamaterial being -negative, -negative, or double-negative, and to field polarization, giving rise to backward or forward radiation depending on the frequency range of operation. These spectral features and the associated frequency scan of the radiated beam are illustrated by considering the field excited by a dipole source in the presence of an infinite metamaterial grounded slab. The possibility to achieve nearly equal values for the phase constants of a TE and a TM leaky mode on a large frequency range is shown; this allows us to obtain a conical radiation pattern and, also, for suitable values of the attenuation constants, the radiation of a pencil beam at broadside. Conditions for achieving maximum power density at broadside are derived, when one constitutive parameter is much smaller than the other. In order to illustrate these novel features, numerical results based on experimentally tested dispersion models for permittivity and permeability of the metamaterial media are provided, concerning leaky-wave modal properties and near and far fields excited by a dipole source.

In this paper, a full-wave analysis is presented of the dispersion properties of modes supported ... more In this paper, a full-wave analysis is presented of the dispersion properties of modes supported by a grounded dielectric slab periodically loaded with metal strips, which represents a canonical configuration employed in planar microwave antennas and arrays and in the realization of artificially hard and soft surfaces. Propagation of surface and leaky modes at arbitrary angles is considered here, without any restrictive assumption on the values of the involved physical and geometrical parameters. Spectral properties of modes are studied, by deriving generalized conditions for establishing the proper or improper nature of the spatial harmonics in the Floquet representation of the fields. The proposed approach, based on a full-wave moment-method discretization of the relevant electric-field integral equation in the spectral domain, is validated through comparisons with the available data in the literature. Novel results are presented which illustrate the continuous evolution of modes as a function of the propagation angle along the grating, both in surface and leaky propagation regimes.

In this paper, we consider a multilayer stripline structure excited by a delta-gap source and we ... more In this paper, we consider a multilayer stripline structure excited by a delta-gap source and we study the continuous-spectrum current, which may be responsible of several spurious transmission effects such as interference with the quasi-TEM mode, undesired radiative effects, and crosstalk. In particular, a uniform asymptotic analysis of such continuous spectrum is presented, based on two alternative closed-form representations. Both representations are accurate also very close to the source and are valid inside a wide frequency range starting from DC. One formulation has a direct implementation, since it needs only knowledge of the spectral Green's function of the background structure. The other formulation requires knowledge of the exact complex (leaky) poles of the stripline Green's function and of their residues, but provides more physical insight since it gives quantitative information on the role of leaky waves in the continuous-spectrum representation in the entire considered frequency range. Numerical results which validate the accuracy and efficiency of the proposed approaches are provided at different frequencies for covered-microstrip structures with different strip widths.

This paper deals with the analysis of surface waves supported by a metamaterial layer on a ground... more This paper deals with the analysis of surface waves supported by a metamaterial layer on a ground plane, and investigates the potentiality of these grounded slabs as substrates for planar antennas. Both double-and single-negative media, either epsilon-or mu-negative, are considered. As is known, such structures may support two kinds of surface waves, i.e., ordinary (transversely attenuating only in air) and evanescent (transversely attenuating also inside the slab) surface waves. A graphical analysis is performed for proper real solutions of the dispersion equation for TE and TM modes, and conditions are presented that ensure the suppression of a guided-wave regime for both polarizations and kinds of wave. In order to demonstrate the feasibility of substrates with such desirable properties, numerical simulations based on experimentally tested dispersion models for the permittivity and permeability of the considered metamaterial media are reported. Moreover, the effects of slab truncation on the field radiated by a dipole source are illustrated by comparing the radiation patterns at different frequencies both in the presence and in the absence of surface waves. The reported results make the considered structures promising candidates as substrates for planar antennas and arrays with reduced edge-diffraction effects and mutual coupling between elements.

1] In this paper it is shown for the first time that a novel transition may occur for dominant (q... more 1] In this paper it is shown for the first time that a novel transition may occur for dominant (quasi transverse electromagnetic (quasi-TEM)) leaky modes on microstrip line, between a leaky mode that leaks into the TM 0 surface wave mode and one that leaks both into the surface wave and into space. The modal evolution of leaky modes as a function of frequency is studied by means of a full-wave spectral domain approach, and it is shown that such transitions occur on microstrip lines that have a critical strip width, which is relatively large. When the strip width is equal to the critical value, a transition frequency will exist at which the attenuation constant of the leaky mode drops exactly to zero and the leaky mode has a real propagation wave number exactly equal to the free-space wave number. At this frequency, the leaky mode transitions from one that leaks into only the surface wave to one that also leaks into space. In a frequency neighborhood of the transition frequency, very large spurious effects may be produced because of interference between the fundamental quasi-TEM mode and the continuous spectrum current. (2005), Direct modal transition from space wave to surface wave leakage on microstrip lines, Radio Sci., 40, RS6017,

In this paper an investigation is presented of metamaterial structures excited by a line source a... more In this paper an investigation is presented of metamaterial structures excited by a line source aimed at producing narrow directive beams. The structure under consideration is a grounded slab made of a homogeneous metamaterial medium with a plasma-like dispersive permittivity; for low values of the slab permittivity an extremely directive beam pointing at broadside can be obtained. Conditions for the maximization of radiation at broadside are given and the narrow-beam effect is shown to be related to the excitation of a leaky mode supported by the slab, with radiation maximization corresponding to small and equal values of the phase and attenuation constants. The frequency bandwidth and directivity are expressed in a simple closed form in terms of the attenuation constant of the leaky mode. By increasing the slab height for a fixed frequency, the leaky mode is analytically shown to give rise to a beam that is scanned from broadside to the critical angle for plane-wave refraction, thus being confined to a narrow angular region around broadside. Numerical results are given that illustrate these features, and full-wave simulations of a metamaterial structure made of an array of metallic cylinders are presented that confirm the results of the analytical study. The case of a line source inside a semi-infinite metamaterial region is also considered and its radiation characteristics compared with those of the metamaterial slab.

A comparative study is made of two types of planar leaky-wave antennas. The first type is a "conv... more A comparative study is made of two types of planar leaky-wave antennas. The first type is a "conventional" planar leaky-wave antenna composed of a grounded slab that is covered with a metallic or dielectric partially-reflecting surface, which acts as a leaky parallel-plate waveguide. The second type is a leaky-wave antenna consisting of a grounded metamaterial layer, having either a very low permittivity or permeability. For either type of structure, directive pencil beams at broadside may be produced when the structure is excited with a simple source such as a horizontal electric or magnetic dipole. A high directivity is obtained by the excitation of weakly-attenuated cylindrical leaky waves that propagate radially outward from the source on the planar structure. The comparison is made for the fundamental antenna properties such as broadside directivity, radiated broadside power density, pattern bandwidth, and the attenuation constants of the relevant leaky modes.

In this paper a review of some recent results concerning guidance and radiation phenomena in a do... more In this paper a review of some recent results concerning guidance and radiation phenomena in a double-negative (DNG) metamaterial grounded slab is presented. The dispersion behaviour of TM and TE modes supported by this structure is studied by means of a graphical analysis of the relevant dispersion equations and conditions are derived which ensure the suppression of surface waves in certain frequency ranges. Particular attention has been devoted to the excitation of such structure by means of dipole sources. With reference to leaky modes, it has been shown that, contrarily to ordinary double-positive (DPS) slabs, proper leaky modes are responsible for backward radiation from DNG slabs: moreover, frequency scan of a circular conical beam and highly-directive beams pointing at broadside may be achieved by properly choosing the physical and geometrical parameters of the structure. Numerical results are presented for specific structures with frequency-dependent constitutive parameters in order to illustrate the effects of both the surface-wave suppression and the peculiar radiation features of leaky waves.

In this paper, radiation at broadside is studied for a general class of leaky-wave antennas (LWAs... more In this paper, radiation at broadside is studied for a general class of leaky-wave antennas (LWAs) comprised of a grounded slab covered with a partially reflecting surface, on the basis of a simple transverse equivalent network model of the structure. The analysis of the one-dimensional (1-D) version of such a LWA excited by a line source shows that a central role in establishing the features of broadside radiation is played by the condition that the phase and attenuation constants of the leaky mode responsible for radiation are equal. When this happens, a beam with a single peak at broadside is on the verge of splitting into two distinct peaks, and maximum power density is radiated at broadside. Design formulas to achieve such an optimized condition, as well as approximate expressions for the frequency bandwidth and pattern beamwidth of the antenna and for the leaky-wave phase and attenuation constants are derived, both in the absence and in the presence of losses; in addition, an optimal-beamwidth condition (which gives the narrowest broadside beam) is derived. Finally, all the results are extended to the practical case of a 2-D LWA excited by a horizontal dipole.

This paper describes a versatile exposure system for the reproduction of static and extremely low... more This paper describes a versatile exposure system for the reproduction of static and extremely low-frequency (ELF) magnetic fields. The system can be used for in vitro and in vivo experiments. The exposure setup consists of six circular loops, each placed on the face of a cubical box and suitably fed. The paper shows, by means of a genetic-algorithm approach to optimize the degrees of freedom of the system, that such a configuration allows a given magnetic field to be reproduced, with great accuracy, in amplitude, phase, and polarization, on a specific surface of interest. The paper provides details of the design procedure and presents extensive numerical simulations that confirm the validity of the proposed design procedure.

A study is presented on the directivity of grounded lowpermittivity metamaterial slab structures ... more A study is presented on the directivity of grounded lowpermittivity metamaterial slab structures that achieve highly directive broadside radiation. Two-dimensional (2D) configurations excited by electric line sources are considered, adopting a scalar plasma-like dispersive permittivity for the metamaterial medium that suitably models a wire medium in the presence of a 2D electromagnetic field, with the electric field directed along the wire axes. The role of leaky waves in producing the high directivity attainable with such structures is illustrated by comparing it with a simple ray-optic model for the radiation mechanism. We show how for increasing substrate heights, the leakywave effect (dominant for "optimal" thicknesses, and responsible for the very high directivity) evolves into a lensing effect purely explained with ray-optics, when losses are present.

Radiation at broadside from leaky waves is studied for one-dimensional periodic structures modele... more Radiation at broadside from leaky waves is studied for one-dimensional periodic structures modeled by means of a transmission line periodically loaded by series or shunt loads. Radiation at broadside occurs via an axially fast spatial harmonic in a neighborhood of the open-stopband frequency; operation in such a frequency range is studied here by means of an approximate asymptotic Bloch analysis of the adopted equivalent network and a simple array-factor calculation of the radiated far field. The condition of equal values for the phase and attenuation constants of the radiating spatial harmonic is shown to give rise to maximum radiation at broadside in the lossless case and to the splitting of a single peak of the radiation pattern at broadside into two distinct peaks in both the lossless and the lossy cases. The 3 dB frequency band for broadside radiation is characterized in terms of the leaky-pole locations in the complex plane and an approximate formula for the antenna bandwidth is provided. Numerical results illustrating these properties are provided, including full-wave simulations of a specific structure through the method of moments.

In this work, one-dimensional and two-dimensional leaky-wave antennas based on planar structures ... more In this work, one-dimensional and two-dimensional leaky-wave antennas based on planar structures are considered, aimed at producing narrow directive beams pointing at broadside. A comparative analysis is presented of different classes of radiators, namely, conventional grounded slabs covered with metallic or dielectric partially-reflecting surfaces, and metamaterial grounded slabs with very low values of permittivity or permeability. The antenna performance is expressed in terms of enhancement of broadside power density with respect to free space, of broadside directivity, and of the relevant frequency bandwidth, as a function of the attenuation constant of the involved leaky modes. The latter is in turn related to the antenna structural parameters, in order to ascertain advantages and drawbacks of the various structures. Numerical full-wave results for antennas excited by printed dipoles are provided.

In this work, radiation at broadside is studied for a general class of one-and two-dimensional un... more In this work, radiation at broadside is studied for a general class of one-and two-dimensional uniform leaky-wave antennas. The analysis, based on a simple transverse equivalent network model of the antenna, reveals that a fundamental condition for optimum antenna operation at broadside is the equality between the phase and attenuation constants of the leaky mode responsible for radiation. This corresponds to having the maximum level of radiation at broadside and the beam being on the verge of splitting into two distinct peaks off broadside. Simple design formulas based on the optimized conditions are provided for both lossless and lossy structures. In addition, the antenna bandwidth for operation at broadside is studied and a useful approximate expression for the fractional bandwidth is given in terms of the physical parameters of the antenna.

In this paper it is shown that a novel transition may occur for dominant leaky modes on microstri... more In this paper it is shown that a novel transition may occur for dominant leaky modes on microstrip line, between a leaky mode that leaks into the TM 0 surface-wave and one that leaks both into the TM 0 surface wave and into space. The evolution of these leaky modes is studied as a function of frequency by means of a full-wave spectral domain approach, and it is shown that such a transition may occur on microstrip lines that have a critical strip width, which is relatively large. The continuous-spectrum current excited by a finite source can be significantly enhanced when the frequency is near the transition frequency, resulting in very high spurious effects.

Very recently, the development of highly-directive antennas for broadside radiation has attracted... more Very recently, the development of highly-directive antennas for broadside radiation has attracted the interest of many researchers, and several structures have been proposed and realized. With respect to other highly-directive structures, planar leaky-wave antennas (LWAs) have the typical advantages of being simple, low-cost, and compatible with planar integration. Conventional planar LWAs may be built in several ways, with dielectric and/or metallic layers. Most of these structures may be viewed as a grounded dielectric slab covered with a "partially reflecting surface" (PRS). A simple source is used to launch a cylindrically propagating set of TM and TE leaky waves. The source can be a horizontal infinitesimal electric or magnetic dipole (representing a finite printed dipole or a narrow slot on the ground plane, respectively), although the radiated pattern and the high-directivity effect mainly depend on the structure and not on the particular source. Although not immediately recognized, recently proposed one-dimensional electromagnetic bandgap structures also fall into this class of antennas.

Electromagnetic (EM) scattering is effectively used in the detection of buried objects. However, ... more Electromagnetic (EM) scattering is effectively used in the detection of buried objects. However, the most challenging problem is represented by the discrimination of targets and clutters. The electromagnetic signature of known objects is often used to this purpose, but generally it is limited either to the r.m.s. value of the scattered field or to one component. In this work a technique based on the analysis of all the components of the EM field is presented and applied to the detection of landmines.

Radiation features of an elemental electric dipole source embedded inside a grounded wire-medium ... more Radiation features of an elemental electric dipole source embedded inside a grounded wire-medium slab are investigated. A homogenous model is adopted which takes into account both anisotropy and spatial dispersion of the metamaterial medium in the long wavelength limit. An equivalent transverse network is derived for plane waves TE and TM with respect to the wires' axis, coupled by a four-port network associated with the air-slab interface, and the farfield pattern is then derived via the reciprocity theorem. Numerical results are provided which show the possibility of achieving a narrow azimuthally symmetric pencil beam at broadside by properly choosing the physical and geometrical parameters of the structure. MoM results for the wire medium, not shown in this summary, confirm the phenomenology observed with the homogenous model.

In this work, bandwidth properties of planar leaky-wave antennas made of a grounded dielectric sl... more In this work, bandwidth properties of planar leaky-wave antennas made of a grounded dielectric slab covered with a partially-reflecting surface are investigated. The partially-reflecting surface is described by an equivalent shunt susceptance. It is shown that, with respect to a constant frequency-independent susceptance, the antenna bandwidth is not reduced using a frequency-dependent model, except for very specific cases. More generally, it is concluded that passive lossless realizations of the partially-reflecting surface cannot lead to bandwidth improvements with respect to the constant-susceptance case because of Foster reactance theorem. With reference to passive lossless structures, numerical examples are provided that illustrate the attainable performance and confirm the theoretical analysis.

Radiation features of horizontal electric dipole sources embedded in a grounded wire-medium slab ... more Radiation features of horizontal electric dipole sources embedded in a grounded wire-medium slab are numerically investigated, first with a homogenized model and then with a full-wave approach. The homogenized model, which takes into account anisotropy and spatial dispersion of the medium, demonstrates the possibility to excite a dominant leaky wave propagating isotropically along the slab, which radiates a directive omnidirectional pencil beam pointing at broadside. A two-element array of such dipoles can be used to obtain nearly-omnidirectional conical scanned beams by varying the operating frequency. These features are confirmed by full-wave calculations of the far field based on the use of reciprocity theorem in conjunction with the method of moments employing a suitably-accelerated periodic Green's function.

Dispersion properties are studied for leaky modes propagating along a grounded wire-medium slab. ... more Dispersion properties are studied for leaky modes propagating along a grounded wire-medium slab. The medium is constituted by periodic layers of metal wires in air and the modes are allowed to propagate at an arbitrary angle with respect to the wire axis. Only modes TM-polarized with respect to the wire axis may exist. An homogenized model of the structure, which takes into account both anisotropy and spatial dispersion of the wire medium, shows that the wavenumber of such complex TM modes is independent of their propagation direction. This peculiar property, which has been shown to give rise to omnidirectional radiation from simple sources, crucially depends on the spatially-dispersive nature of the homogenized medium, recently described in the literature. A full-wave analysis of the actual periodic structure based on the method of moments confirms this feature, which turns out to be exact in the asymptotic limit of wavelengths large with respect to the wire spacing. Numerical results from both the approximate homogenized model and the rigorous full-wave approach are presented.

Leaky modes supported by a grounded wiremedium slab are studied here in detail for the first time... more Leaky modes supported by a grounded wiremedium slab are studied here in detail for the first time. Such leaky modes are responsible for highly-directive radiation when the structure is excited by a dipole source. The analysis is carried out by means of a rigorous full-wave numerical approach based on the method-of-moments and also by using an approximate homogenized model. The use of a homogenized model that properly accounts for anisotropy and spatial dispersion leads to a remarkable and surprising result: the wavenumber of the leaky mode is independent of the azimuth angle of propagation on the surface. This result, which has not previously been observed in any other leaky-wave structure, allows for the creation of narrow conical beams that are azimuthally symmetric, as well as symmetric broadside beams. This conclusion is confirmed by the full-wave method-of-moments analysis.

A study of the shielding properties of a metamaterial slab is presented. The metamaterial nature ... more A study of the shielding properties of a metamaterial slab is presented. The metamaterial nature of the slab arises from the negative values of its effective scalar dielectric permittivity in the considered lowfrequency region of operation. Such an effective permittivity is the result of a homogenization process of an actual periodic structure which consists of a finite number of periodic layers of thin conducting cylinders embedded in a dielectric matrix, screening an E-polarized plane wave. When the incident plane wave is arbitrarily polarized, a more sophisticated model for the effective permittivity has to be employed which takes into account anisotropy and spatial dispersion. Comparisons with conventional planar metal screens show that the proposed structure can be designed to be advantageous in terms of low density and weight savings.

The effects of apertures with finite thickness on the shielding effectiveness of rectangular meta... more The effects of apertures with finite thickness on the shielding effectiveness of rectangular metallic enclosures is investigated through an approximate but efficient formulation based on the method of moments. As expected, the analysis shows that the finite aperture thickness can help to increase the shielding effectiveness of the structure, especially in the highfrequency region. Comparisons with rigorous full-wave results confirm the validity of the proposed technique.

Leaky-wave propagation and radiation is studied for periodically modulated silver films at optica... more Leaky-wave propagation and radiation is studied for periodically modulated silver films at optical frequencies. For these structures surface-plasmon modes are responsible for enhanced transmission through subwavelength apertures in the silver film, as well as directive beaming of light that is emitted from an aperture on the surface of the silver film. These effects are explained in terms of leaky-mode propagation and radiation.

A study of directive radiation from a horizontal electric dipole embedded in a grounded wire-medi... more A study of directive radiation from a horizontal electric dipole embedded in a grounded wire-medium slab covered with a partially reflecting surface in the form of a metal strip grating is presented. The combined effect of the volumetric metamaterial and the partially reflective surface allows for very directive beams. The performance is compared with antennas made from a wire medium slab alone and from an air substrate covered with a metal strip grating. In all cases omnidirectional beams are obtained.

The dispersion behaviour of metamaterial structures that support backward waves is studied by ana... more The dispersion behaviour of metamaterial structures that support backward waves is studied by analyzing the modal spectrum of homogenized dispersive double-negative slab waveguides with magnetic losses. The evolution of complex wavenumbers by varying frequency is investigated for TE modes in the frequency range where the metamaterial changes from epsilon negative to double negative, identifying modeinteraction regimes that are interpreted in terms of migration of branch-point singularities in the complex frequency plane parameterized by the value of magnetic loss. 5 4 3 2

Artificial Magnetic Materials (AMMs or metamaterials) are artificial engineered materials compose... more Artificial Magnetic Materials (AMMs or metamaterials) are artificial engineered materials composed of periodic metallo-dielectric inclusions which may offer unusual and interesting properties. In particular, the same structure can present different electromagnetic behaviors depending on the frequency range of operation due to its peculiar dispersion characteristics. One remarkable property is the creation of highly directive beams from simple sources, such as dipoles or slots, placed inside such metamaterial substrates.

The effects of conducting loads on the shielding effectiveness of rectangular metallic enclosures... more The effects of conducting loads on the shielding effectiveness of rectangular metallic enclosures is investigated through an efficient formulation based on the method of moments. As expected, the analysis shows that the classical shielding effectiveness is a very tricky figure of merit which is seriously affected by the possible presence of loads inside the enclosure, especially in the frequency range above the first resonant frequency of the enclosure. Comparisons with rigorous full-wave results obtained through commercial software confirm the validity of the proposed technique and its high efficiency.

Modal Properties of Layered Metamaterials

Theory and Phenomena of Metamaterials, 2009