1,721,049 research outputs found
Higher-order cylindrical leaky waves–Part I: Canonical sources and radiation formulas
No full textRadiation from cylindrical leaky waves (CLWs) supported by layered dielectric media is of interest in a variety of current applications. A comprehensive two-part study has been undertaken to investigate the generation and the radiation properties of CLWs with an arbitrary integer azimuthal order n. The first of this two-part sequence deals with the identification of continuous rings of canonical sources of electric and magnetic types, capable of exciting higher-order CLWs (HOCLWs) in planar open radial waveguides. The relevant far-field radiation formulas are then derived, generalizing those already available in the literature for cylindrical waves with azimuthal orders n=0 and n=1 , i.e., those excited by elementary vertical or horizontal dipoles. Design guidelines for the practical excitation of HOCLWs through discrete sources, namely, circular phased arrays, are provided in the companion paper along with numerical validations and illustrative results in both the near-field and the far-field regions
Tunable THz Leaky-Wave Radiation from Periodic Chains of Graphene-Coated Circular Rods
No full textThe beam-scanning properties at a fixed frequency in the low terahertz range of one-dimensional periodic leaky-wave antennas composed by a periodic chain of circular dielectric cylinders coated with graphene are studied. A rigorous full-wave modal solver, based on the lattice-sums technique combined with the transition-matrix approach, is applied to the analysis of transverse electric (TE) and magnetic (TM) leaky modes, where the transition matrix suitably takes into account the conductivity of the external graphene sheet. The reconfigurable features of the leaky-wave structure have been investigated in terms of the dispersion behaviors of the phase and attenuation constants of the radiating proper space harmonic of the Bloch leaky modes. The beam-scanning performance, variation of the beamwidth, and the appearance of grating lobes as functions of the graphene chemical potential are investigated. Three-dimensional structures composed by a finite number of graphene-coated dielectric rods sandwiched by metal or perfect magnetic conductor plates are finally validated through full-wave simulations, showing a good beam-scanning capability for both TE and TM polarizations, respectively, and a remarkable agreement with the theoretical leaky-wave model
3D Printing Materials for the Superstrate of a Resonant-Cavity Antenna
No full textThe 3D-printing of dielectric superstrates of Resonant Cavity Antennas has been demonstrating its advantages in fast prototyping and flexibility in the realization of customized layouts. A suitable combination of thickness, side extension and permittivity of the superstrate can enhance the antenna gain of the primary source over a broad frequency interval. With non-periodic and perforated layouts, instead, the radiative properties can be improved. In this work, the dielectric permittivity of different printing material is measured with a Vector Network Analyzer. The tested materials are all polymeric filaments with low dielectric permittivity, for fabrication with Fusion Deposition Modelling technique, and their application in the realization of an RCA superstrate is presented
A Resonant-Cavity Antenna with High-Gain and Wide Bandwidth with an All-Dielectric 3D-Printed Superstrate
No full textThe 3D-printing of dielectric superstrates of Resonant Cavity Antennas has advantages of fast prototyping and flexibility in the realization of customized layouts. In this work, the low dielectric permittivity of test filaments is experimentally measured for their use in superstrates of Resonant Cavity Antennas. A suitable combination of thickness, side extension and permittivity of the superstrate can enhance the antenna gain of the primary source over a broad frequency interval. With non-periodic and perforated layouts, instead, the radiative properties can be improved in terms of reductions of the Side-Lobe Level. The use of an all-dielectric superstrate in the design of an RCA is presented, demonstrating the possibility of obtaining a broadband gain enhancement with a single dielectric layer of low permittivity
On the input impedance of probe-fed electromagnetic bandgap antennas based on lattice modes
No full textThe impedance of a coaxial probe, feeding an Electromagnetic Bandgap (EBG) structure conceived for radiation shaping, is studied from the theoretical and numerical viewpoints. The EBG medium is a square arrangement of dielectric cylinders placed in a parallel-plate waveguide, where a suitable lattice mode is excited. A semi-analytical model is developed and used to derive an approximate, closed-form expression of the probe resistance. The model is based on a modal expansion in Floquet harmonics, on which a current distribution is projected according to the Lorentz reciprocity theorem to derive the amplitude of lattice modes propagating right above the bandgap along lattice axes. The dependence of probe impedance on lattice parameters is then investigated with the numerical simulations of a finite-element method, which is also used to validate the developed model. A broad set of parametric analyses is presented, showing that the reactive part weakly depends on probe position, cylinder radius and permittivity, while the heights of probe and parallel-plate waveguide play a major role in determining the resonance condition. As to the probe resistance, it decreases with cylinder radius and permittivity and decreases with probe and waveguide heights. The derived analytical formula correctly reproduces such functional dependences and its calculation is immediate, revealing its usefulness in antenna design. Matching issues are heuristically and experimentally approached by examples, demonstrating that the proposed work can be effectively employed to improve the electrical performance of EBG antennas with an embedded source
Tapered All-Dielectric EBGs with 3-D Additive Manufacturing for High-Gain Resonant-Cavity Antennas
No full textA resonant-cavity antenna is implemented assembling on an open-ended rectangular waveguide a wideband electromagnetic band gap (EBG), designed as a multilayer. Two different implementations of the multilayer are proposed, labeled as Cases A and B. As to Case A, a high-permittivity layer is embedded between two slabs of low permittivity, whereas a single layer of low permittivity, surrounded by two high-permittivity slabs, is employed in Case B. Successful performances of the broadband behavior are also dependent on the side length of the EBG, that is truncated to small footprints, with an effect of high sidelobe level (SLL). Therefore, grid layouts with nonuniform air holes are proposed for in-plane realization of the dielectric slabs. Their effect in the EBG superstrate is a nonuniform spatial reflectivity, that reduces the SLL keeping a good wideband response. In the EBG manufacturing, both numerically controlled machining techniques and 3-D printing have been employed. 3-D printing has been used for the low permittivity layers, realized in the polylactic acid (PLA) filament, allowing more flexibility in the fabrication of the grid slabs to custom layouts. Results of two antenna prototypes assembling the EBGs, with the parameters of Cases-A and -B and grid layout, to the waveguide source are reported
Semi-Analytical Method for the Analysis of Bound and Leaky Modes in Woodpile Dielectric Structures
No full textAn efficient semi-analytical method to study modes with real and complex wavenumbers in three-dimensional woodpile bandgap structures, that consist of layered periodic arrays of circular dielectric cylinders, is proposed. The reflection and transmission matrices of the woodpile structure are rigorously calculated using the transition-matrix approach combined with the lattice sums technique and the recurrence formulae for the layered structure. A dispersion equation is rigorously derived taking into account proper and improper determinations of each space-harmonic. Two different configurations of the woodpile cavity structures are investigated. The dispersion characteristics of bound and complex modes, in the closed stopband region, and of hybrid TE and TM leaky modes, propagating at arbitrary angles within the open woodpile cavity waveguide, are numerically calculated. The results are compared with those obtained by fully independent commercial software simulations. Suitable post-processing and radiation patterns based on leaky-wave theory are used for validation of the proposed method, and very good agreement between theoretical and full-wave results are obtained
Agile Bessel Beam Leaky-Wave Launcher With Suppressed Open Stopband and Multiport Feeding
No full textThis work investigates the generation of Bessel beams from a double strip bull-eye planar leaky-wave launcher. The structure offers wideband operation and an extended non-diffracting range thanks to the suppression of the open stopband, which typically affects periodic leaky-wave antennas. A comparative analysis is reported with a single-strip implementation, which shows the stopband and therefore offers a reduced non-diffracting range. To enhance functionality with respect to conventional bull-eye structures operating in the near field, the double strip launcher is excited through a flexible substrate integrated waveguide multiport feeder, which enables dual-polarization and controllable aperture fields and allows for the generation of both zeroth- and first-order Bessel beams. Both simulations and measurements are reported, demonstrating beam focusing over larger distances in the near field. For the first time, an adaptable and polarization agile launcher is studied and experimentally validated to increase the non-diffractive range of a planar and compact structure
2-D Beam Scanning with Cylindrical-Leaky-Wave-Enhanced Phased Arrays
No full textDirective pencil beams scannable in both elevation and azimuth are obtained through a planar phased array placed inside a Fabry-Perot cavity. The key element of the proposed approach is the exploitation of a conical element pattern (EP) with high directivity in elevation, obtained through the excitation of a dominant, weakly attenuated cylindrical TM leaky wave of azimuthal order n = 0 by means of a simple coaxial probe. Then, a highly reduced number N of such sources are arranged to form a phased array radiating directive pencil beams. Beam-angle reconfigurability with continuous scanning both in azimuth and elevation inside a wide solid angular range is achieved by varying the array phasing and the operating frequency. An investigation on the features of truncated cylindrical leaky waves is first developed to properly characterize the EP. Then, conventional array theory is exploited to calculate the pattern of the entire array. The radiation efficiency is also evaluated accounting for the spurious surface wave related to the undesired excitation of the quasi-TEM mode. The proposed array design provides a simple and inexpensive innovative solution for obtaining a high-gain pencil beam continuously scanning in the 3-D space without suffering gain losses. In the presented implementation, the elevation angular scan, which is generally constrained by the wideband capability of the feeding system, by the requirements on the sidelobe level, and by the cutoff of the relevant leaky mode, ranges from about 21° to about 68°. Possible applications are envisaged for the next generation of wireless power transfer devices, for advanced radar and surveillance systems, earth observation, as well as for ceiling-mounted indoor localization and tracking
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