Agostino Monorchio

Analysis and Design of Ultra Thin Electromagnetic Absorbers Comprising Resistively Loaded High Impedance Surfaces

High-impedance surfaces (HIS) comprising lossy frequency selective surfaces (FSS) are employed to design thin electromagnetic absorbers. The structure, despite its typical resonant behavior, is able to perform a very wideband absorption in a reduced thickness. Losses in the frequency selective surface are introduced by printing the periodic pattern through resistive inks and hence avoiding the typical soldering of a large number of lumped resistors. The effect of the surface resistance of the FSS and dielectric substrate characteristics on the input impedance of the absorber is discussed by means of a circuital model. It is shown that the optimum value of surface resistance is affected both by substrate parameters (thickness and permittivity) and by FSS element shape. The equivalent circuit model is then used to introduce the working principles of the narrowband and the wideband absorbing structure and to derive the best-suited element for wideband absorption.

The design synthesis of multiband artificial magnetic conductors using high impedance frequency selective surfaces

This paper introduces several different design methodologies for multiband artificial magnetic conducting (AMC) surfaces. The paper begins by investigating the multiband properties exhibited by a conventional electromagnetic bandgap (EBG) AMC that consists of a frequency selective surface (FSS) on top of a thin dielectric substrate with a PEC back plane. The higher-order resonances associated with these surfaces have not been discussed in detail to date, as previous research has been concerned only with exploiting the primary resonant frequency. However, it will be shown that by understanding and making appropriate use of these higher order resonances, it is possible to design multiband AMC surfaces that work for nearly any desired combination of operating frequencies. The first multiband AMC design approach that will be considered is based on the introduction of FSS screens that have fractal or nearly fractal unit cell geometries. This is followed by a more general and robust genetic algorithm (GA) technique for the synthesis of optimal multiband AMC surfaces. In this case, a GA is used to evolve multiband AMC surface designs by simultaneously optimizing the geometry and size of the FSS unit cell as well as the thickness and dielectric constant of the substrate material. Finally, several examples of multiband AMC surfaces are presented, including some practical dual-band and tri-band designs genetically evolved for operation at GPS and cellular frequencies, as well as an example illustrating the success in creating a multiband AMC surface with angular stability.

A Frequency Selective Radome With Wideband Absorbing Properties

A frequency selective radome is presented, acting as a pass band filter at a given frequency band, while behaving as an absorber above the transmission band. The pass band behavior is obtained by a metallic FSS realized through a compact interdigitated Jerusalem cross element characterized by a very large rejection band. The metallic FSS is used as the ground plane of a thin wideband absorber based on resistive high-impedance surfaces within the total reflection band. The outer absorber reduces the signature of the antenna system when the radome is illuminated by out of band signals. The resistive FSS which comprises the absorber is designed so to minimize losses within the transmitting band of the radome. The composite structure is thoroughly analyzed by an efficient equivalent circuit approach and by full-wave numerical simulations.

A Thin Electromagnetic Absorber for Wide Incidence Angles and Both Polarizations

A design for planar electromagnetic absorbers is presented. The performance of this absorber is maintained over a wide incidence angles and for both TE and TM polarization. The absorber is composed of a high-impedance surface comprising an array of patches over a grounded lossy dielectric slab perforated with metallic vias (wire medium). The main contribution of the paper is to demonstrate and practically use the presence of an additional resonance when the plasma frequency of the wire medium in the dielectric substrate is close to the original resonance of the high-impedance surface. The presence of the vias between FSS and the ground plane is discussed both for the case of a high-permittivity absorber and for a low permittivity one, through the derivation of simple and efficient analytical expressions, specifically derived for the problem at hand. We show that the presence of the vias influences the oblique incidence TM absorption, and when properly designed, the insertion of the vias into the absorber structure results in a bandwidth enlargement and higher absorption performance.

An Iteration-Free MoM Approach Based on Excitation Independent Characteristic Basis Functions for Solving Large Multiscale Electromagnetic Scattering Problems

We describe a numerically efficient strategy for solving a linear system of equations arising in the Method of Moments for solving electromagnetic scattering problems. This novel approach, termed as the characteristic basis function method (CBFM), is based on utilizing characteristic basis functions (CBFs)-special functions defined on macro domains (blocks)-that include a relatively large number of conventional sub-domains discretized by using triangular or rectangular patches. Use of these basis functions leads to a significant reduction in the number of unknowns, and results in a substantial size reduction of the MoM matrix; this, in turn, enables us to handle the reduced matrix by using a direct solver, without the need to iterate. In addition, the paper shows that the CBFs can be generated by using a sparse representation of the impedance matrix-resulting in lower computational cost-and that, in contrast to the iterative techniques, multiple excitations can be handled with only a small overhead. Another important attribute of the CBFM is that it is readily parallelized. Numerical results that demonstrate the accuracy and time efficiency of the CBFM for several representative scattering problems are included in the paper.

Efficient Analysis of Frequency-Selective Surfaces by a Simple Equivalent-Circuit Model

The transmission and reflection properties of frequency-selective surfaces (FSSs) are evaluated through a simple and accurate first-order circuit approach. The approximate analysis, based on the parallel between real structure and a lumped-LC-network counterpart, is also useful for acquiring physical insights into the working principles of frequency-selective surfaces. The first part of the paper describes a technique for computing lumped parameters of the most common frequency-selective-surface elements. The L and C parameters representing a given frequency-selective-surface element are derived only one time, at normal incidence, and stored, so as to form a database. The second part of the paper deals with the derivation of simple relations allowing the generalization of the stored LC couples in the case where the frequency-selective surface is printed or embedded in arbitrarily thick dielectric slabs, when the incident angle is varied from normal incidence, or if a different periodicity with respect to the reference periodicity is adopted. The generalized lumped parameters are included in an equivalent transmission line for computing the response of generic frequency-selective-surface configurations with no additional computational effort. The results obtained through the simplified model presented here are verified by a careful comparison with MoM simulations.

A Circuit-Based Model for the Interpretation of Perfect Metamaterial Absorbers

A popular absorbing structure, often referred to as Perfect Metamaterial Absorber, comprising metallic periodic pattern over a thin low-loss grounded substrate is studied by resorting to an efficient transmission line model. This approach allows the derivation of simple and reliable closed formulas describing the absorption mechanism of the subwavelength structure. The analytic form of the real part of the input impedance is explicitly derived in order to explain why moderate losses of the substrate is sufficient to achieve matching with free space, that is, perfect absorption. The effect of the constituent parameters for tuning the working frequency and tailoring the absorption bandwidth is addressed. It is also shown that the choice of highly capacitive coupled elements allows obtaining the largest possible bandwidth whereas a highly frequency selective design is achieved with low capacitive elements like a cross array. Finally, the angular stability of the absorbing structure is investigated.

On the Bandwidth of High-Impedance Frequency Selective Surfaces

In this letter, the bandwidth of high-impedance surfaces (HISs) is discussed by an equivalent circuit approach. Even if these surfaces have been employed for almost 10 years, it is sometimes unclear how to choose the shape of the frequency selective surface (FSS) on the top of the grounded slab in order to achieve the largest possible bandwidth. Here, we will show that the conventional approach describing the HIS as a parallel connection between the inductance given by the grounded dielectric substrate and the capacitance of the FSS may induce inaccurate results in the determination of the operating bandwidth of the structure. Indeed, in order to derive a more complete model and to provide a more accurate estimate of the operating bandwidth, it is also necessary to introduce the series inductance of the FSS. We will present the explicit expression for defining the bandwidth of a HIS, and we will show that the reduction of the FSS inductance results in the best choice for achieving wide operating bandwidth in correspondence with a given frequency.

An Active High-Impedance Surface for Low-Profile Tunable and Steerable Antennas

In this letter, an approach for designing a tunable and steerable antenna is presented. The antenna model is based on a wideband bow-tie radiating element mounted above an active artificial magnetic conductor (AMC). The AMC geometry consists of a frequency selective surface (FSS) printed on a thin grounded dielectric slab in which some chip-set varactor diodes are placed between the metallic elements and the backing plane through vias. The resulting antenna can be tuned over the S-Band by simply changing all varactor capacitances through an appropriate biasing voltage. Moreover, this structure can operate a beam scanning over each working frequency by applying an appropriate biasing voltage to the active elements of the AMC surface in accordance to leaky radiation principles. The low-profile active antenna is characterized by an overall thickness of 5.32 mm, which corresponds to approximately lambda/24 at the center of the operating band.

Time-domain (FE/FDTD) technique for solving complex electromagnetic problems

A hybrid finite element finite-difference time-domain (FE/FDTD) technique for solving complex electromagnetic problems is presented. The method combines the computational simplicity of the structured FDTD scheme with the versatility as well as flexibility of the finite-element method (FEM) and enables us to accurately model curved geometries and those with fine features. Numerical results that illustrate the accuracy of the method are included.