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In this letter, the scattering of a nondiffractive Bessel beam (BB) incident field by a perfect electric conducting circular cylinder is investigated and compared with the standard scattering solution by a plane wave. It is proven that the BB incident field improves the validity of standard physical optics approximation and possibly extends its range of applicability to cylinders, or more general scatterers, whose curvature radius is not much larger than the operating wavelength. This offers new effective possibilities in the solutions of different electromagnetic scattering problems.

International audience; In this work, we study the use of Bessel-Gauss beam launchers for wireless power transfer (WPT) applications. The power transfer efficiency between two radiating apertures is investigated in the radiative near field. A spectral Green's function approach is used to derive the power efficiency under a simultaneous conjugate impedance match. The transverse electromagnetic (TEM) mode of a coaxial cable, as well as Bessel beam (BB) and Bessel-Gauss beam (BGB) field distributions, are considered as aperture distributions. Numerical results demonstrate that a BGB field distribution is the optimal choice for WPT due to its limited spectrum and reduced diffraction of the radiated beam. We synthesized a Bessel-Gauss launcher that exhibits power efficiency exceeding 50% for distances larger than 30 lambda. These results show that non-diffracting beams can pave the way toward efficient near-field WPT systems with extended operating ranges.

In this article, we present a ray optics-based iterative procedure for the design and optimization of a shaped reflector able to achieve a high roll-off sectoral pattern in the azimuthal plane and a very narrow beam in the elevation plane, as required in a surveillance application at 76–77 GHz. Three prototypes of such kind of reflector with different azimuthal half-power beamwidths were fabricated and measured, showing a fair agreement with numerical simulations.

In this paper, we propose the design of a dual circular-polarized antenna for L-band applications (1.1-1.6 GHz). The designed antenna has been developed by considering crossed-dipoles on top of which a circular patch has been placed to properly act on the mutual coupling between the dipoles. The dual circular polarization is achieved by feeding dipoles in quadrature. Moreover, to increase the antenna gain and polarization purity at high elevation angles, a fence of passive monopoles has been added. The proposed antenna can profitably used as primary feed of a low-frequency parabolic reflector.

International audience; In this paper, we present a theoretical analysis, one design and fabrication of a limited-diffractive planar Bessel beam launcher, that exhibits a zeroth order Bessel profile in the transverse electric field component with respect to the z-propagation axis. The launcher is designed by synthesizing a finite zeroth order, first kind Hankel aperture distribution, polarized along a fixed polarization unit vector. The field radiated by such an aperture distribution is derived by following an approximate model based on geometric theory of diffraction, thus allowing to highlight the relevant wave constituents involved in transverse Bessel beam generation, also including the effect of aperture truncation on the radiated beam. Moreover, the theoretical analysis has been profitably applied to the design of a circularpolarized planar transverse Bessel beam launcher by means of a slotted radial waveguide. A prototype of right-handed circular polarized (RHCP) transverse Bessel beam launcher has been then fabricated at f = 30 GHz. The measured transverse electric field component shows a satisfactory agreement with full-wave numerical simulations. IEEE

In this paper, an alternative approach to design controlled switches at millimeter and sub-millimeter waves, based on coupled tunable Bessel-beam launchers in the radiative near field region, is proposed. In particular, we investigate the possibility of tuning the electromagnetic properties of the radiating apertures through the application of a driving voltage in order to dynamically switch the cover distance of the launchers. Such a controlled switch is intended to work in a bi-stable mode: when the control signal is high (low), the launchers exhibit an efficient (a poor) electromagnetic coupling through the Bessel-beam main lobe. Therefore, the device acts as a switching transistor, i.e., as in digital applications, and also as an opto-isolator at lower frequencies.

Scattering of Bessel beams can be matter of great interest in many practical areas such as near-field radar cross section (RCS) measurement, ground penetrating radar (GPR) and optical microscopy. In this contribution, we explore the advantages of performing inverse scattering through Bessel beams to obtain better results when compared to standard inversion approaches dealt with standard incident fields such as plane waves, current filaments or fields radiated by dipoles/loops in tridimensional imaging. Preliminary numerical assessments are given towards metallic scatterers by exploiting the well-known standard back-propagation technique.

Nowadays, the use of CubeSats for telecommunications and interplanetary missions is ever-increasing, thanks to their appealing low-cost character, as well as the space environment, which poses challenging multiphysics constraints on the antenna design. In this framework, the use of Ka-band for communication is explored. We present the design of a stacked patch antenna working across the down- and uplink Ka-bands. Materials and geometry of the radiator have been selected by accounting for the trade-off between electromagnetic, thermal and mechanical requirements. The design of the antenna is performed with a particle swarm optimization algorithm developed to control the bandwidth and matching. A bandwidth of 5.53@33.185 GHz has been obtained, with a gain around 8 dB. Furthermore, a multiphysics thermal analysis is performed to verify the operational stability of the optimized array, mounted on a 1 U satellite, in a case-study mission. The temperature patterns in the array are evaluated during the orbital period, and the influence of the operative temperature on the antenna responses and gain was considered. We found that the thermal loads can affect the antenna matching. However, thanks to the optimized design, the proposed stacked antenna can operate from −100 $^{\circ }$ C to 100 $^{\circ }$ C, with an almost constant gain. Finally, following a damage-tolerant approach, the level of mechanical deformation, which could be induced on the communication system, was studied. The stress analysis reveals that the stacked geometry can be used in a space mission. From the investigation of the strain and displacement field, we found a negligible impact on the antenna performance.

The scattering of a Bessel beam (BB) incident field by a dielectric circular cylinder is here discussed and compared to more standard plane wave (PW) scattering, in such a way to clearly show that the scattering by BBs is more localized than that by PWs. Such an inherent localized scattering process, especially if applied to dense dielectric scatterers, offers new possibilities for imaging techniques at millimeter waves, especially of biological media.

In this paper, we present the design and fabrication of two dual circularly polarized (CP) patch antennas that can be profitably used as feeders for reflector systems normally adopted for satellite applications. In the first part of the manuscript, we propose the optimization of a dual-CP patch antenna, loaded by a fence of passive monopoles around it to increase antenna gain for high elevation angles. To achieve dual-CP operation mode, the circular patch has been sequentially-fed by three pins, whose mutual phase-shift is equal to 120 ∘ . The antenna feeding network was placed on the antenna back and designed using microstrip technology. Two different input ports provide both right-hand (RH) and left-hand (LH) circular polarizations. A prototype of such an antenna was fabricated and measured at f = 8 . 25 GHz. Furthermore, to test the versatility of the proposed single radiating patch, in the second part of the manuscript, we present the results of geometrically scaling at f = 2 . 2 GHz (higher UHF band) and this was used as a building block for the design of a dual-CP sequentially-fed 2-by-2 array antenna. The results for both the proposed antennas are satisfactory in terms of impedance bandwidth, broad radiation pattern, gain and cross-polarization rejection, thus they can be profitably used as feeders for reflectors at relatively low frequencies.

In this paper, we present a simple technique to achieve electronically-controlled beam scanning, by properly acting on the exhibited average surface susceptances placed in correspondence of the sidewalls of a slotted rectangular waveguide. Such an approach allows to avoid the surface susceptance spatial modulation, thus it inherently simplifies the active elements biasing-network design. The preliminary results in the case of a 1D traveling-wave array demonstrates the concept and paves the way for the full development of 2D scanning arrays and for surface reactance synthesis by means of active elements, such as PIN diodes.

In this paper, two different but complementary techniques developed to generate limited-diffractive electromagnetic (EM) beams and pulses by using radial line slot arrays (RLSA) and leaky-wave (LW) planar devices at microwaves/millimeter waves are presented. In the first part of the paper, two kinds of RLSA Bessel beam launchers are proposed, obtained by implementing an inward cylindrical traveling-wave aperture distribution in the longitudinal and transverse electric field components. Moreover, in the second part, two different LW launchers are presented, namely based on forward and backward leaky-waves.

Geometric and energy metrics are here proposed to gauge the confinement properties of localized electromagnetic beams (e.g., Bessel beams) and pulses (e.g., X-waves) in both the nondispersive and dispersive cases. As is well known, the frequency superposition of Bessel beams sharing the same axicon angle leads to the generation of tightly bounded wavepackets, namely X-waves, with remarkable spatio–temporal confinement properties. In the nondispersive case, the axicon angle does not change with frequency, and these features can be predicted by closed-form expressions. Remarkably, it is found that the volumetric confinement of such pulses can be optimized for a specific axicon angle. Since the geometric metrics do not account for the unavoidable presence of energy tails outside the main spot, energy metrics are here introduced and compared with the purely geometric definition. Finally, the dispersive case is investigated, accounting for the typical wavenumber dispersion of radial waveguides. As a result, the axicon angle changes with the frequency, and fundamental limits exist for the maximum theoretical fractional bandwidth. Interestingly, design criteria based on approximate analytical expressions are derived to optimize the confinement, even in the dispersive case.

In this paper, a comparative analysis of Machine learning (ML) and physics-based (PB) optimizations of the monopole fence surrounding a dual circular polarized (CP) antenna, operating in the X-band, is presented and discussed. The aim is to enhance the antenna performances in terms of achieved co-polar gain and cross-polarization rejection at grazing angles, by preserving good impedance matching and input CP port isolation. The results are found to be satisfactory for the specific V2X application, since an acceptable co-polar (LHCP) gain is achieved on a broad angular range with a good cross-polarization (RHCP) rejection, without increasing dramatically the final antenna complexity.

In this paper we provide explicit formulas to calculate the phase center location of an arbitrary antenna. Such a formulation allows the description of the phase center dependence from the observation aspect and from the frequency.

In this paper, we deal with the proper definition of the near-field wireless power transfer efficiency between two antennas, by adopting both spatial and spectral approaches to evaluate it. Such a figure of merit allows the evaluation of wireless link performances in the radiating near-field by using RLSA launchers. Numerical results to establish the performances of uniform, zero- th order Bessel, and zero- th order first kind Hankel wireless links are presented. In the final part of the paper, an example of RHCP RLSA Bessel beam launcher at f = 30 GHz implementing a zero-th order, first kind Hankel aperture distribution is proposed as building block for a near-field wireless link, showing that its performances can be considered satisfactory in terms of efficiency, provided that a suitable distance between transmitting/receiving antennas is chosen.

In the last decades [1], [2], non-diffractive waves have gained increasing interest among researchers in electromagnetics engineering and physics, due to the considerable applications in near-field focusing, high data-rate transfer, medical imaging, ground-penetrating radars (GPR), only to mention the most important ones.

In this paper, we present an overview of radial line slot array (RLSA) antenna design and optimization techniques developed in the past years at the University of Siena, both for near and far field applications at microwaves/millimeter waves. Moreover, also the possibility of generating localized pulses in time domain by means of RLSA is briefly sketched.

In this paper, we present a technique for the automatic optimization of a right-hand circular-polarized (RHCP) linear slotted array in substrate integrated waveguide (SIW) technology for the so-called “last-mile”communications, in which a narrow (broad) beam is required in the elevation (azimuthal) plane. The procedure can be divided into two steps; i) the optimization of isolated single slot pairs, by means of two physics-based functionals able to minimize both slot pair back-scattering and axial-ratio, ii) the optimization of the entire linear array of slot pairs, by means of a global functional able to match the co-polar component of the synthesized aperture distribution to the target one, i.e., a uniform magnetic current distribution to radiate a narrow beam in the elevation plane. A prototype of array was optimized at 24.3GHz and measured in the bandwidth [24.1, 24.5]GHz, showing a fair agreement against full-wave simulations, thus validating the exposed techniques.

Orbital angular momentum (OAM) is gaining great attention in the physics and electromagnetic community owing to an intriguing debate concerning its suitability for widening channel capacity in next-generation wireless communications. While such a debate is still a matter of controversy, we exploit OAM generation for microwave imaging within the classical first order linearized models, i.e., Born and Rytov approximation. Physical insights into different fields carrying ℓ-order OAM are conveniently exploited to propose possible alternative imaging approaches and paradigms in microwave imaging.

In this paper, an automatic procedure based on physical optics (PO) for the design and optimization of a sectoral pattern shaped reflector at millimeter waves is proposed. As a starting guess for the PO-based iterative procedure, a fast optimization based on geometrical optics (GO) has been developed. In order to preserve the smoothness of the reflector shape, a reduced set of control points on its surface is defined, and an interpolation is performed between them. The results show the effectiveness of the proposed technique, that can profitably find applications in the design of RADAR antennas for surveillance purpose.

Bessel-beam launchers are promising and established technologies for focusing applications at microwaves. Their use in time-domain leads to the definition of a new class of devices, namely, the X-wave launchers. In this work, we discuss the focusing features of such devices with a specific interest at millimeter waves. The spatial resolutions of such systems are described under a rigorous mathematical framework to derive novel operating conditions for designing X-wave launchers. These criteria might be particularly appealing for specific millimeter-wave applications. In particular, it is shown that an electrically large aperture is not strictly required, as it seemed from previous works. However, the use of an electrically small aperture would demand a considerably wideband capability. The various discussions presented here provide useful information for the design of X-wave launchers. This aspect is finally shown with reference to the practical design of two different X-wave launchers.

Metasurface (MTS)antennas have been shown to be a versatile class of planar antenna. In particular, they allow for beam shaping and pattern control through the modulation of the boundary conditions, while keeping unchanged the overall antenna structure. This feature makes them suitable for dynamic reconfiguration. In this contribution, we show how an electronic beam scanning can be achieved by controlling either the average equivalent impedance or the modulation period of the modulated MTS.

International audience; This chapter illustrates the capabilities of non-diffractive Bessel beams for near-field focusing. After a brief introduction of the non-diffractive phenomenon and its origin, the generation of non-diffractive Bessel beams by inward cylindrical traveling waves is analyzed in detail. A ray interpretation is proposed for such beams for infinite and finite radiating apertures. Their main radiation capabilities and limitations in terms of focusing, bandwidth, and operating range are discussed. In particular, it is shown that inward cylindrical traveling wave aperture field distributions can generate non-diffractive Bessel beams over a large bandwidth. As a practical implementation of the discussed theory, a class of launchers based on radial waveguides loaded by metallic gratings or slots is proposed, for which an efficient design and optimization technique is described. In addition, a different and unconventional leaky-wave approach is also adopted for the design of radial waveguides loaded by metasurfaces. The chapter ends by outlining the future research activities and possible applications of non-diffractive beams. © Springer International Publishing AG 2018.

International audience; In this work, useful criteria for dimensioning X-wave launchers are provided based on the definition of an original metric of confinement. The mathematical expressions for evaluating the nondiffractive features of a spectrally-flat X-wave are first reviewed. Then, a novel criterion for dimensioning the aperture size of an X-wave launcher is proposed to obtain a pulse which is confined over both the transverse and the propagating axes. Such an analysis may be particularly useful for the development of X-wave launchers especially in the microwave/millimeter-wave range, where the available fractional bandwidths are subjected to severe technological constraints.

In this paper, the dispersion properties of RLSA Bessel beam launchers, realized by enforcing both cylindrical standing wave and inward traveling wave aperture distributions, are compared. In particular, it is shown that such launchers can be profitably used for the generation of electromagnetic localized pulses at millimeter waves.

International audience; In this paper, the generation of twisted pulses carrying orbital angular momentum is discussed. Such pulses are generated by polychromatic superposition of higher-order Bessel beams. The non-negligible effect of the wavenumber frequency-dispersion on the pulse propagation at millimeter waves is taken into account. Large operating bandwidths are required to synthesize spatially confined electromagnetic pulses. For this reason, a wideband Bessel-beam launcher is proposed, implementing an inward cylindrical traveling-wave aperture distribution with an azimuthal phase variation. Preliminary results clearly show the possibility of successful generation of twisted electromagnetic pulses at millimeter waves. © 2017 Euraap.

In this paper, a comparison is presented between Bessel beam launchers at millimeter waves based on either a cylindrical standing wave (CSW) or a cylindrical inward traveling wave (CITW) aperture distribution. It is theoretically shown that CITW launchers are better suited for the generation of electromagnetic short pulses because they maintain their performances over a larger bandwidth than those realizing a CSW aperture distribution. Moreover, the wavenumber dispersion of both the launchers is evaluated both theoretically and numerically. To this end, two planar Bessel beam launchers, one enforcing a CSW and the other enforcing a CITW aperture distribution, are designed at millimeter waves with a center operating frequency of f¯=60GHz and analyzed in the bandwidth 50 - 70 GHz by using an in-house developed numerical code to solve Maxwell's equations based on the method of moments. It is shown that a monochromatic Bessel beam can be efficiently generated by both the launchers over a wide fractional bandwidth. Finally, we investigate the generation of limited-diffractive electromagnetic pulses at millimeter waves, up to a certain non-diffractive range. Namely, it is shown that by feeding the launcher with a Gaussian short pulse, a spatially confined electromagnetic pulse can be efficiently generated in front of the launcher.

The surface wave dispersion of a grounded layer of liquid crystals (LCs) is investigated by taking into account the inherent electrical reconfigurability of such a material. The dyadic impedance Green's function of the tunable LC grounded slab is calculated and the dispersion curve of the fundamental mode supported by the structure is presented, showing that the orientation of the optical axis of the LCs modifies the surface wave dispersion diagram and can be applied for surface wave propagation control. Furthermore, it is demonstrated that the presence of an inductive metasurface (MTS) on top of the LC layer reduces the resonance frequency and increases the sensitivity to the voltage biasing.

International audience; We discuss the generation and propagation of nondiffracting twisted pulses at microwaves, obtained through polychromatic spectral superposition of higher-order Bessel beams. The inherent vectorial structure of Maxwell's equations has been considered to generalize the nondiffracting solution of the scalar wave equation with azimuthal phase variation. Since a wide frequency bandwidth is necessary to synthesize time-limited pulses, the non-negligible wavenumber frequency dispersion, which commonly affects the propagation in the microwave range, has been taken into account. For this purpose, a higher-order Bessel beam is generated by enforcing an inward cylindrical traveling-wave distribution over a finite aperture. We present and discuss the main aspects of the generation of twisted pulses in the microwave range, showing the promising possibility to carry orbital angular momentum through highly focused X-shaped pulses up to the nondiffractive range. © 2017 Author(s).

This contribution addresses the use of reconfigurable modulated metasurfaces (MTSs) for the realization of electronically scanning antennas without the use of phase shifters. Different approaches can be used to electronically control the average equivalent impedance or the modulation period of the modulated MTS.

The surface wave dispersion of a grounded layer of liquid crystals (LCs) is investigated by taking into account the inherent electrical reconfigurability of such a material. The spectral dyadic impedance Green’s function of the tunable LC grounded slab is calculated and the dispersion curve of the fundamental mode supported by the structure is presented, showing that the orientation of the optical axis of the LCs modifies the surface wave dispersion curve significantly enough to be applied for surface wave propagation control. Furthermore, it is demonstrated that the presence of an inductive metasurface on top of the LC layer impressively reduces the resonance frequency and increases the sensitivity to the continuous voltage biasing.

International audience; In this work we discuss the limitations of the localized transmission of electromagnetic energy at microwaves. The nondiffractive features of localized pulses are evaluated in terms of the main spot size over both the propagating and the transverse axis. By defining an original theoretical framework, we show that, even in the nondispersive case, it is not always possible to efficiently confine electromagnetic pulses. Results show that the considered fractional bandwidth plays a key role in the confinement properties of such pulses, posing well-defined constraints to the practical realization of localized pulses in the microwave range. © 2016 IEEE.

International audience; In this work we analyze the possibility to generate limited-diffraction electromagnetic pulses in the millimeter-wave range. The propagating and focusing features of such pulses are discussed in terms of spot-lengths over both the propagating and the transverse axes. Starting from a new theoretical framework, we show that, even in the ideal case of non dispersive and infinite radiating apertures, some limitations exist for the spatial confinement of a nondiffracting electromagnetic pulse. However, our results show that practical realizations of such pulses are possible when considering a certain defined spatial region and time interval. © 2016 European Association of Antennas and Propagation.

International audience; In this paper the focusing capability of a radiating aperture implementing an inward traveling wave current distribution directed along a fixed unit vector is investigated. In particular, it is shown that such a current distribution generates a field profile with its transverse component (with respect to the normal of the radiating aperture) taking the form of a zero-th order Bessel profile. As a practical implementation of the theoretical analysis, a circular-polarized (CP) Bessel beam launcher, made by a radial parallel-plate waveguide loaded with several slot pairs and arranged on a spiral pattern, is designed and optimized. The designed launcher performance agrees with the theoretical model and exhibits an excellent polarization purity.

International audience; In this work, we analyze the spatial and temporal features of electromagnetic X-waves propagating in free space and generated by planar radiating apertures. The performance of ideal X-waves is discussed and compared to practical cases where the important effects related to the finiteness of the radiating aperture and the wavenumber dispersion are taken into account. In particular, a practical device consisting of a radial waveguide loaded with radiating slots aligned along a spiral path is considered for the practical case in the millimeter-wave range. A common mathematical framework is defined for a precise comparison of the spatiotemporal properties and focusing capabilities of the generated X-wave. It is clearly shown that the fractional bandwidth of the radiating aperture has a key role in the longitudinal confinement of an X-wave in both ideal and practical cases. In addition, the finiteness of the radiating aperture as well as the wavenumber dispersion clearly affect both the transverse and the longitudinal profiles of the generated radiation as it travels beyond the depth-of-field of the generated X-wave. Nevertheless, the spatiotemporal properties of the X-wave are preserved even in this "dispersive-finite" case within a defined region and duration related to the nondiffractive range and fractional bandwidth of the spectral components of the generated X-wave. The proposed analysis may open new perspectives for the efficient generation of X-waves over finite radiating apertures at millimeter waves where the dispersive behavior of realistic devices is no longer negligible. Published by AIP Publishing.