Davide Ramaccia

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.

Self-Filtering Low-Noise Horn Antenna for Satellite Applications

In this letter, we present the design and the experimental realization of an innovative self-filtering low-noise horn antenna. The proposed radiator consists of the following: a regular WR-62 waveguide connected to a horn antenna, a metallic screen with a vertical slit placed at the section connecting the waveguide and the horn, and a dielectric slab, with metallic omega shapes printed on both faces, placed across the slit. The goal of this design is to reduce the bandwidth of operation of the regular WR-62 horn in order to self-filter the noise captured within the antenna operation band. In the receiving mode, the metallic omega shapes placed across the slit allow transmission only in a narrow frequency band centered around the resonant frequency of the omegas. In such a frequency band, the radiating performances of the proposed antenna are comparable to the ones of a regular WR-62 horn radiator. A proper set of numerical simulations and measurements confirm the effectiveness of the proposed design, which can be successfully used in receiving satellite communication systems.

A New Accurate Model of High-Impedance Surfaces Consisting of Circular Patches

In this paper, we consider a dense array of metallic circular patches printed on a electrically thin metal-backed dielectric substrate. Since the sub-wavelength dimensions, the array and the metal-backed substrate can be described in terms of a lumped capacitance and a lumped inductance, respectively. Around the resonant frequency, the structure, known as high-impedance surface, re∞ects totally an incident electromagnetic wave with zero shift in phase. Due to this property, it is widely employed in antenna systems as compact back re∞ector with improved performances with respect to typical metal re∞ector. Starting from the concept of the grid capacitive reactance of a planar array of squared patches and its related formulas, we investigate on the fleld distribution on the array plane and properly modify the formulas for the case of the circular patches. We present two new analytical formulas which can be efiectively used for the fast design of 2D-isotropic circular HISs. In order to validate the models, we compare the resonant frequency of the array obtained through our approaches to the one resulting from full-wave numerical simulations and from other analytical methods available in the open technical literature.

Analytical Model of Connected Bi-Omega: Robust Particle for the Selective Power Transmission Through Sub-Wavelength Apertures

In this paper, we present a new analytical model of the connected bi-omega structure consisting of two bi-omega particles connected together through their arms. A single bi-omega particle consists of a pair of regular equal omegas with mirror symmetry. Assuming the individual bi-omega particle electrically small, the equivalent circuit is derived, in order to predict its resonant frequency. Then, two bi-omega particles are connected together, obtaining a symmetric structure that supports two fundamental modes, with even and odd symmetries, respectively. The proposed analytical model, then, is used to develop a procedure allowing the design of the particle for a desired resonant frequency. The effectiveness of the proposed analytical model and design guidelines is confirmed by proper comparisons to full-wave numerical and experimental results. We also demonstrate through a proper set of experiments that the resonant frequencies of the connected bi-omega particle depend only on the geometrical and electrical parameters of the omegas and are rather insensitive to the practical scenario where the particle itself is actually used, e.g. in free-space, rectangular waveguide or across an aperture in a metallic screen.

Efficient and wideband horn nanoantenna

In this Letter, we present the design of a horn nanoantenna working at near-IR frequencies. The proposed layout consists of an Ag-air-Ag nanotransmission line terminated in a tapered horn. The antenna design is validated through proper full-wave numerical simulations, taking into account actual dispersion and losses of the involved materials. The numerical results show that the designed nanohorn is matched over a broad range of frequencies (more than 50% of fractional bandwidth) and radiates efficiently in the same frequency band (the realized gain is greater than 10  dBi). Such promising results may find application in different technical and scientific fields, ranging from smart lighting to optical wireless communications.

Mantle cloaking for co-site radio-frequency antennas

We show that properly designed mantle cloaks, consisting of patterned metallic sheets placed around cylindrical monopoles, allow tightly packing the same antennas together in a highly dense telecommunication platform. Our experimental demonstration is applied to the relevant example of two cylindrical monopole radiators operating for 3G and 4G mobile communications. The two antennas are placed in close proximity, separated by 1/10 of the shorter operational wavelength, and, after cloaking, are shown to remarkably operate as if isolated in free-space. This result paves the way to unprecedented co-siting strategies for multiple antennas handling different services and installed in overcrowded platforms, such as communication towers, satellite payloads, aircrafts, or ship trees. More broadly, this work presents a significant application of cloaking technology to improve the efficiency of modern communication systems.

INDUCTIVE TRI-BAND DOUBLE ELEMENT FSS FOR SPACE APPLICATIONS

In this contribution we propose the design of an inductive Frequency Selective Surface (FSS) with double resonant elements aimed at the achievement of a simple well-performing, dielectric-free, space fllter screen able to separate the Ku band from the Ka band. The FSS performance is compared to that of a typical double ring FSS which major drawback is the use of a dielectric substrate that leads to unavoidable additional transmission losses and makes the dichroic mirror more complex with respect to a simple single perforated screen. For all applications in which the FSS is asked to be as simple as possible and the transmission losses speciflcations are severe, the Inductive FSS Double Resonant Elements here proposed turns out to be an interesting alternative to typical Double Ring FSS.

Nonreciprocal Horn Antennas Using Angular Momentum-Biased Metamaterial Inclusions

In this work, we apply angular momentum-biased metamaterials to break the intrinsic reciprocity of antenna systems, with potential applications to satellite communications. We focus our attention on a conical horn antenna exhibiting nonreciprocal response for left-handed and right-handed circularly polarized fields. A metallic screen with an annular aperture is placed between the feeding waveguide and the horn. This inclusion is loaded with modulated capacitors to achieve a nonreciprocal transmittance for circularly polarized fields impinging from opposite sides. We present the analysis of the nonreciprocal annular inclusion and explain its nonreciprocal response, obtained through proper spatiotemporal modulation, with the help of an equivalent transmission-line representation. A possible practical implementation of the modulation circuit based on band-pass and band-stop filters is also discussed. The electrical and radiating performance of the conical horn is numerically evaluated, showing the ability of the proposed antenna to filter signals with same polarization in two separate bands, e.g., uplink and downlink bands of a satellite link, depending on the propagation direction of the signal. These results may pave the way to several interesting applications of nonreciprocal radiators in satellite and radar systems.

Analytical Model of a Metasurface Consisting of a Regular Array of Sub-Wavelength Circular Holes in a Metal Sheet

In this work, a metasurface consisting of an array of circular holes in a metal conducting sheet with a sub-wavelength periodicity is considered. The surface partially re∞ects the incident fleld according to the shape and geometrical dimensions of the inclusions and, due to this property, is widely employed in antenna systems to improve the radiation pattern of regular radiators. Since the re∞ection properties of the metasurface are determined by the current density distribution on the metal, we inspect this distribution and coherently develop a new, easy, and accurate analytical model to describe the grid impedance of the metasurface. In order to validate the model, we compare the re∞ection coe-cient of the array obtained through our approach to the one resulting from full-wave numerical simulations and to other accurate analytical methods available in the open technical literature.

Exploiting Intrinsic Dispersion of Metamaterials for Designing Broadband Aperture Antennas: Theory and Experimental Verification

The intrinsic frequency dispersion has usually been considered as a relevant drawback of metamaterials, limiting their application to narrowband microwave components. However, we have recently demonstrated that a particular metamaterial-based lens, consisting of a combination of an epsilon-near-zero (ENZ) material and air, is able to enhance the gain of short horn antennas over a broad frequency range. This result was achieved by exploiting the dispersive nature of ENZ metamaterials. In this communication, we further discuss the latter aspect, giving more details about the physical mechanism enabling broadband operation of shortened horns loaded with ENZ-air lenses. The discussion is supported by a set of experimental results obtained on a fabricated prototype.