1,720,972 research outputs found
Overlapped and sequential metasurface modulations for Bi-Chromatic beams generation
No full textThis paper describes the generation of directive beams at two different frequencies with the same circular metasurface (MTS) antenna based on a surface wave (SW) excitation. Two methods are presented to achieve the desired bi-chromatic beam operation. The first one consists in mathematically superimposing two MTS modulations, each one matched to the SW wavelength at a different frequency. In the second method, the period of the MTS modulation matches the SW wavelengths at two different frequencies in two different regions of the antenna, i.e., close to the center for the high frequency band and close to the periphery for the low frequency band. The first method allows one to have different feed points for each beam in a self-diplexed structure, although it is also possible to use a single feed configuration and a diplexer. The second approach only holds for a single physical feed-point. Numerical and experimental results are shown
Modulated Metasurface Antennas with Enhanced Broadband Response
No full textThis paper deals with the crucial need of enhancing the gain-bandwidth of typically narrowband modulated metasurface (MTS) antennas. Despite their low- profile and low-mass, modulated MTSs have typically been limited in terms of bandwidth. In broadside MTS antennas, this shortcoming stems from the progressive mismatch between the periodicity of the modulation (usually constant) and the wavenumber of the dispersive surface wave (SW). Here, an optimization scheme is introduced for the periodicity function, which is described as a piecewise monotonically increasing function along the antenna radius. The variable period leads to the generation of annular active regions, where the SW wavenumber matches the local periodicity, and a strong radiation occurs at the prescribed frequency. On the other hand, outside the annular region the SW weakly interacts with the MTS modulation. This works shows that the proposed method can considerably extend the bandwidth of MTS antennas
Design Methodologies for Dual-band Modulated Metasurface Antennas
No full textThis paper presents two different design methodologies to cope with the somehow unexplored design of dual-band modulated metasurface (MTS) antennas with circular shape. In the first approach, one overlays two different modulations. Each modulation is appropriately chosen to provide a broadside beam for one of the bands and a very weak radiation for the other frequencies. The second approach builds on the active region modulated MTS concept, recently applied to broadband designs. In this case, the periodicity of the modulation at the central region of the aperture is selected to radiate at the high frequency band. In turn, a larger periodicity in the outer annular region provides the broadside beam at the low frequency band. Both approaches are compared and their advantages and drawbacks, discussed
Perfect non-specular reflection with polarization control by using a locally passive metasurface sheet on a grounded dielectric slab
No full textThis paper investigates the conditions for a perfect anomalous reflection through a modulated metasurface consisting of a metallic cladding printed over a grounded slab. Differently to what has been previously published, the problem is rigorously addressed by modeling the metallic cladding through an equivalent penetrable impedance and accounting for the grounded slab through the problem's Green's function. It is shown that without polarization transformation, the exact solution exists only for the special case of retroreflection, and, in that case, it can be done simultaneously for the two orthogonal polarizations, with an arbitrary phase shift among the two. On the other hand, changing the polarization of the reflected wave allows one to find an exact solution for arbitrary combinations of incidence and reflection angles. The exact solution is found by imposing that the induced currents radiating with the Green's function of the background problem simultaneously create the desired reflected beam and cancel the specular reflection from the grounded slab. This approach leads to the derivation of a closed-form expression for the homogenized penetrable impedance profile providing perfect anomalous reflection, i.e., ensuring the vanishing of all the coefficients of the waves associated with unwanted diffraction orders, including the specular reflected wave and the evanescent waves. This result is of great practical interest, since the derived penetrable impedance profile can be readily implemented through a simple distribution of metallic patches. The feasibility of this approach is verified through full wave simulations of both the ideal impedance and the patch-based structure, which confirm the effectiveness of the proposed solution
Design Methods for Dual Polarized Metasurface Antennas: Three Simple Approaches
No full textMetasurface (MTS) antennas are based on the transformation of a cylindrical-wavefront surface wave (SW) into a general wavefront leaky wave (LW). The MTS aperture is constituted by a grounded dielectric slab printed with an electrically dense distribution of subwavelength patches realizing space-variable, homogenized tensor impedance boundary conditions (IBCs). One of the challenges in this type of antenna is related to obtaining dual polarization operations by using the same impedance modulation. In this article, we explore and compare three simple approaches to obtain two beams with orthogonal polarizations by feeding two ports. A first known method is based on exciting both a transverse electric (TE) and a transverse magnetic (TM) SW mode on the same modulated impedance. A second method exploits the concept of impedance modulation sharing, according to which two distinct modulations, designed to radiate different polarizations when properly illuminated by distinct offset feeding points, are superimposed on the same aperture. A third method consists in duplexing an outward (radially diverging) and an inward (converging to the center) SW. Simple analytical formulas are presented for the synthesis of the impedance that allows for the control of the inward/outward waves to ensure balanced radiation performances in terms of aperture efficiency for the two polarizations. A comparison in terms of performances between the latter two approaches is presented
Synthesis of passive lossless metasurfaces for perfect anomalous reflection
No full textIn this paper, practical implementation of metasurfaces providing perfect anomalous reflection (i.e. reflection to a non specular direction with no higher order modes excited) is discussed with reference to metasurfaces consisting of a metallic cladding over a grounded dielectric slab. The metallic cladding is modeled through a continuous penetrable impedance sheet, while the grounded slab is accounted for through the Green's function. It is shown that an exact solution without polarization transformation only exists for the case of retroreflection, while a solution with polarization conversion can be found for arbitrary incidence and reflection angles. The derived solutions are implemented through a distribution of patches and full wave simulations are performed to confirm their effectiveness
Numerical issues in the analysis of large BoR antennas involving dielectric and metallic parts
No full textThe aim of this paper is to investigate some numerical issues involved in the Moment Method solution of the radiated field by axially symmetric antennas (Body of Revolution antennas) that are very large in terms of the wavelength. The approaches found in the literature are reviewed and an improvement is introduced for decreasing the computational cost when the integrands are singular and oscillatory
Dual band isoflux ultraflat meta antennas
No full textThis paper presents an approach for the design of a dual-band planar antenna radiating an isoflux-shaped beam suitable for earth observation missions. The antenna is based on a modulated metasurface excited by a central feed. Preliminary numerical results are presented to validate the concept
Evidenza di pneumovirus aviare sottotipo A in corso di un focolaio di TRT in tacchini da carne in Italia
No full textAn outbreak of Turkey rhinotracheitis caused by an Avian pneumovirus (APV) subtype A is described in one Italian meat turkey farm. The flocks were sited in Perugia province. Virus isolation was performed in chicken tracheal organ coltures and the strain was typed by a subtype specific nested RT-PCR. This is the first evidence that APV subtype A is present in Italy
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