1,721,011 research outputs found
Radiation of a source on a convex NURBS surface
No full textIn this paper we propose a technique for computing the ray caustic distance parameter on an arbitrary convex surface described by Non-Uniform Rational B-splines (NURBS) surface representation. The ray caustic distance parameter is essential for evaluating the UTD surface diffracted fields that are diffracted from a convex surface. Examples validating the accuracy of the numerical approach proposed are presented
Design of Reconfigurable Intelligent Surfaces by Using S-Parameter Multiport Network Theory - Optimization and Full-Wave Validation
No full textMultiport network theory has been proved to be a suitable abstraction model for analyzing and optimizing reconfigurable intelligent surfaces (RISs) in an electromagnetically consistent manner, especially for studying the impact of the electromagnetic mutual coupling among radiating elements that are spaced less than half of the wavelength apart and for considering the interrelation between the amplitude and phase of the reflection coefficients. Both representations in terms of Z-parameter (impedance) and S-parameter (scattering) matrices are widely utilized. In this paper, we embrace multiport network theory for analyzing and optimizing the reradiation properties of RIS-aided channels, and provide four new contributions. (i) First, we offer a thorough comparison between the Z-parameter and S-parameter representations. This comparison allows us to unveil that typical scattering models utilized for RIS-aided channels ignore the structural scattering from an RIS, which is well documented in antenna theory. We show that the structural scattering results in an unwanted specular reflection. (ii) Then, we develop an iterative algorithm for optimizing, in the presence of electromagnetic mutual coupling, the tunable loads of an RIS based on the S-parameters representation. We prove that small perturbations of the step size of the algorithm result in larger variations of the S-parameter matrix compared with the Z-parameter matrix, resulting in a faster convergence rate. (iii) Subsequently, we generalize the proposed algorithm to suppress the specular reflection due to the structural scattering, while maximizing the received power towards the direction of interest, and analyze the effectiveness and tradeoffs of the proposed approach. (iv) Finally, we validate the theoretical findings and algorithms with numerical simulations and a commercial full-wave electromagnetic simulator based on the method of moments
Accurate calculation of creeping ray parameters for radiation of sources on convex NURBS surfaces
No full textIn this paper we present a numerical approach for computing the creeping ray geometrical parameters associated with high frequency source radiation and diffraction on an arbitrary NURBS convex surface. These parameters are essential for accurately estimate the UTD surface diffracted fields that are diffracted from the convex surface. Examples validating the accuracy of the proposed numerical approach are presented
Fringe integral equation method for a truncated grounded dielectric slab
Full text linkThe problem of scattering by a semi-infinite grounded dielectric slab illuminated by an arbitrary incident TMz polarized electric field is studied by solving a new set of “fringe” integral equations (F-IEs), whose functional unknowns are physically associated to the wave diffraction processes occurring at the truncation. The F-IEs are obtained by subtracting from the surface/surface integral equations pertinent to the truncated slab, an auxiliary set of equations obtained for the canonical problem of an infinite grounded slab illuminated by the same source. The F-IEs are solved by the method of moments by using a set of subdomain basis functions close to the truncation and semi-infinite domain basis functions far from it. These latter functions are properly shaped to reproduce the asymptotic behavior of the diffracted waves, which is obtained by physical inspection. The present solution is applied to the case of an electric line source located at the air-dielectric interface of the slab. Numerical results are compared with those calculated by a physical optics approach and by an alternative solution, in which the integral equation is constructed from the field continuity through an aperture orthogonal to the slab. The applications of the solution to an array of line currents are also presented and discusse
UHF and L-Band Microwave Measurements of the Antarctic Firn-Layer Complex Permittivity Depth Profile
No full textIn order to characterize the complex permittivity depth profile of the firn constituting the Antarctic ice sheet, a microwave probe, based on the open-coaxial re-entrant cavity method, is designed, realized and then used during an Antarctic campaign. Preliminary results of the measurements of firn permittivity depth profile, are presented and discussed highlighting very good agreement with expected values
Recent developments in diffraction theory for impedance structures
No full textA brief overview of the most recent results relevant to the impedance wedge canonical problem has been presented in this paper. It is worth noting that there are still open problems to be faced with, as well as a continuous interest in the above canonical problems due to the need of efficient high frequency techniques in effective engineering numerical codes. The authors are presently working on revisiting some known exact solutions for both isotropic and anisotropic impedance wedges on the basis of the results recently obtained by Prof. Tiberio. Work is also in progress to derive heuristic approximate diffraction coefficients for edges in penetrable screens, starting from the exact solution for the scattering at the edge of a semi-infinite planar strip gratin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Including high-frequency surface diffraction in a hybrid FEBI-MLFMM-UTD method
No full textIn this paper we propose a technique for including UTD surface diffracted fields which are diffracted from curved convex objects into a multilevel fast multipole implementation (MLFMM) of the Finite Element Boundary Integral method. We use Non-Uniform Rational B-splines (NURBS) representation for efficiently describing arbitrarily shaped objects. The dyadic Green's function associated with UTD field scattered by these convex curved object is determined using a high frequency ray-based method and then inserted at the various levels of the MLFMM. Results show that the proposed hybrid MLFMM-FEBI-UTD technique for convex object allows a significant reduction in memory storage and computational effort, preserving however the accuracy in the field estimates
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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