Alessandro Toscano

Design of Spiral and Multiple Split-Ring Resonators for the Realization of Miniaturized Metamaterial Samples

We present the design of miniaturized resonant inclusions to be employed in the practical realization of metamaterial samples with anomalous values of the real part of the permeability. Such inclusions, in fact, can be employed in the design of both mu-negative (MNG) materials and artificial magnetodielectrics (with negative and high-positive values of the real part of the permeability, respectively). The inclusions here considered are the multiple split-ring resonators (MSRRs), that represent a straightforward extension of the commonly used split-ring resonators (SRRs), and the spiral resonators (SRs), that enable a greater miniaturization rate. Some physical insights on the resonance mechanism and on the inherent saturation of the resonant frequency when increasing the number of the rings of the MSRRs and the number of the turns of the SRs are given in the paper. New and accurate analytical design formulas, based on a quasi-static model, for both MSRRs and SRs are derived and tested through a proper comparison with the existing formulas and full-wave numerical results. Both MSRRs and SRs are shown to be useful to reduce the electrical dimensions of the resonant inclusions when synthesizing artificial metamaterials.

Equivalent-Circuit Models for the Design of Metamaterials Based on Artificial Magnetic Inclusions

In this paper, we derive quasi-static equivalent-circuit models for the analysis and design of different types of artificial magnetic resonators-i.e., the multiple split-ring resonator, spiral resonator, and labyrinth resonator-which represent popular inclusions to synthesize artificial materials and metamaterials with anomalous values of the permeability in the microwave and millimeter-wave frequency ranges. The proposed models, derived in terms of equivalent circuits, represent an extension of the models presented in a recent publication. In particular, the extended models take into account the presence of a dielectric substrate hosting the metallic inclusions and the losses due to the finite conductivity of the conductors and the finite resistivity of the dielectrics. Exploiting these circuit models, it is possible to accurately predict not only the resonant frequency of the individual inclusions, but also their quality factor and the relative permeability of metamaterial samples made by given arrangements of such inclusions. Finally, the three models have been tested against full-wave simulations and measurements, showing a good accuracy. This result opens the door to a quick and accurate design of the artificial magnetic inclusions to fabricate real-life metamaterial samples with anomalous values of the permeability.

Design of Miniaturized Narrowband Absorbers Based on Resonant-Magnetic Inclusions

In this paper, we present the design of miniaturized narrowband-microwave absorbers based on different kinds of magnetic inclusions. The operation of the proposed components originates from the resonance of a planar array of inclusions excited by an incoming wave with a given polarization. As in common absorber layouts, a 377 Ω resistive sheet is also used to absorb the electromagnetic energy of the impinging field. Since the planar array of magnetic inclusions behaves at its resonance as a perfect magnetic conductor, the resistive sheet is placed in close proximity of the resonating inclusions, without perturbing their resonance condition. In contrast to other typical absorber configurations presented in the literature, the absorber proposed in this paper is not backed by a metallic plate. This feature may be useful for stealth applications, as discussed thoroughly in the paper. The other interesting characteristic of the proposed absorbers is the subwavelength thickness, which has shown to depend only on the geometry of the basic resonant inclusions employed. At first, regular split-ring resonators (SSRs) disposed in an array configuration are considered and some application examples are presented. Absorbers based on SRRs are shown to reach thickness of the order of λ0/20. In order to further squeeze the electrical thickness of the absorbers, multiple SRRs and spiral resonators are also used. The employment of such inclusions leads to the design of extremely thin microwave absorbers, whose thickness may even be close to λ0/100. Finally, some examples of miniaturized absorbers suitable for a practical realization are proposed.

Broadband Compact Horn Antennas by Using EPS-ENZ Metamaterial Lens

We present the design of a flat lens, made by a conventional material and an epsilon near-zero metamaterial, to plug up the aperture of a short horn antenna, in order to achieve radiation performances similar to the ones of the corresponding optimum horn over a broad frequency range. Lens operation is based on the phase-compensation concept: phase-fronts of the field propagating along the short flare of the horn propagate with different phase velocities in the two lens materials, resulting in an uniform phase distribution on the aperture. Starting from the theoretical study of the transmission properties of a bulk epsilon near-zero slab, we derive the analytical formulas for the design of the flat lens and validate them through full-wave numerical simulations. Then, a realistic version of the lens, realized with a wire-medium and exhibiting a near-zero real part of the effective permittivity in the frequency range of interest, is presented. Considering two examples working in the C-band, we show that the lens can be designed for both conical and pyramidal horn antennas. In both cases, the length of the horns is half the one of the corresponding optimum versions, while the obtained radiation performances are similar to those of the optimum horns over a broad frequency band. This result may open the door to several interesting applications in satellite and radar systems.

Circular Polarized Patch Antenna Generating Orbital Angular Momentum

The recent extension of the orbital angular momentum (OAM) concept from optical to microwave frequencies has led some researchers to explore how well established antenna techniques can be used to radiate a non-zero OAM electromagnetic fleld. In this frame, the aim of the present paper is to propose a new approach to generate a non-zero OAM fleld through a single patch antenna. Using the cavity model, we flrst analyze the radiated fleld by a standard circular patch and show that a circular polarized (CP) TMnm mode excited by using two coaxial cables generates an electromagnetic fleld with an OAM of order §(ni1). Then, in order to obtain a simpler structure with a single feed, we design an elliptical patch antenna working on the right-handed (RH) CP TM21 mode. Using full-wave simulations and experiments on a fabricated prototype, we show that the proposed antenna efiectively radiates an electromagnetic fleld with a flrst order OAM. Such results prove that properly designed patch antennas can be used as compact and low-cost generators of electromagnetic flelds carrying OAM.

Dynamic LOS/NLOS Statistical Discrimination of Wireless Mobile Channels

This paper presents the problem of identifying whether a received signal at a base station is due to a line-of-sight (LOS) transmission or not (NLOS). This is a first step towards estimating the mobile stations location. We formulate the NLOS identification problem as a binary hypothesis test by exploiting the Rician factor estimation. In particular, results from wireless environments with simulated geometry showed that the new test can decide for a LOS or NLOS condition using a small amount of samples. Therefore, changes from NLOS to LOS propagation can also be tracked quickly. This information is of high significance for location purposes in a wireless cellular network since time-of-arrival (TOA) and time-difference-of-arrival (TDOA) information based on LOS connections can be weighted stronger in a location computing algorithm and hence can lead to higher positioning accuracy. Otherwise, if the connection is identified as NLOS it can be useful to adopt a 2-D signal processing (space-time processing) strategy with an antenna array, instead of using the high complexity of the TOA and TDOA methods.

Design of a Non-Foster Actively Loaded SRR and Application in Metamaterial-Inspired Components

In this paper, we investigate on the use of non-Foster active elements to increase the operation bandwidth of a split-ring resonator (SRR) for possible application in metamaterial-inspired components. First, we design the circuit topology of the active load required to compensate the intrinsic reactance of the SRR and get a broadband response. Then, we show that the same procedure can be successfully applied to the case of a SRR-based monopole antenna and, in principle, to any metamaterial-inspired device employing SRRs. Finally, integrating an electromagnetic and a circuit simulator, we propose a possible realistic implementation of the active load, based on the employment of commercially available circuit elements. The obtained results (seven times improvement of the impedance bandwidth of the SRR-based monopole antenna) prove that non-Foster active loads can be successfully used to overcome the inherent narrow-band operation of SRR-based passive metamaterials and metamaterial-inspired components. The implementation issues related to circuit element dispersion, parasitic effects, and stability of the active circuit are fully considered in the proposed design.

Controlling scattering and absorption with metamaterial covers

We discuss the use of metasurfaces and plasmonic metamaterials to minimize the scattering from receiving antennas and sensors, with the goal of maximizing their absorption efficiency. We first analytically study and highlight the potential of these approaches to realize optimized sensors with the desired level of efficiency, being able to minimize the electrical presence of a receiving antenna for a chosen level of overall absorption. Realistic cloak designs, investigated using full-wave simulations, verify the behavior analytically predicted by Mie theory. These optimized cloaks offer a practical way to flexibly tailor the scattering of receiving antennas, with great benefits in the design and optimization of near-field sensors, remote communication systems, spoof targets and improved antenna blockage resiliency. Optimized covers may also provide other interesting features for the same receiving antenna by just tuning its resistive load, such as optimal wireless power harvesting or high-to-low tunable absorption efficiency.

Optical cloaking of cylindrical objects by using covers made of core-shell nanoparticles.

In this Letter, we propose an engineered design of optical cloaks based on the scattering cancellation technique and intended to reduce the observability of cylindrical objects. The cover, consisting of a periodic arrangement of core-shell nanospheres, is designed in such a way to exhibit near-zero values of the real part of the homogenized effective permittivity at optical frequencies. Full-wave numerical simulations, considering the measured data of the dielectric function of the plasmonic material composing the shell, show that the cloak is able to reduce by about 6 dB the scattering cross section of a finite-length cylinder at around 740 THz with a -3 dB fractional bandwidth of about 7%. We show also that this result is not significantly affected by the perturbation of the periodic alignment of the core-shell nanospheres, due to possible fabrication issues or to an amorphous arrangement.

A novel design method for Blass matrix beam-forming networks

A novel design method for lossy Blass matrix beam-forming networks (LBNMFNs) is presented. Compared to those formerly developed, the new method allows the design of an LBMBFN in order to generate not only two simultaneous beams but also an arbitrary number of them. This skill is obtained by means of a new approach to minimize losses that allows one to transform a nonlinear multivariable programming problem into a linear one-variable problem. The solution of such a design problem, then, can be carried out in a very straightforward way by applying Gram-Schmidt orthogonalization. Such a design method takes into account also the limited availability of coupling values of directional couplers. Numerical results obtained through the application of such a design method are then presented. The ease, accuracy, and efficiency of this novel method for the design of LBMBFN make it very useful in modern applications of multibeam antenna arrays.