1,720,972 research outputs found
Radial GRIN Lenses based on the Solution of a Regularized Ray Congruence Equation
No full textWe introduce a novel formulation for flat radial GRadient INdex (GRIN) lenses allowing for the optimal lens design through closed form expressions. The validity of the proposed formulation covers a very large range of GRIN lens design parameters (focal distance, lens thickness and maximum refractive index). The formulation is based on the derivation of a new form of non-linear integral equation representing the equalization of all the optical ray-path lengths, denoted as Regularized Ray Congruence (RRC) equation, and on its closed form solution. An analytical form of aperture efficiency is given for standard feed patterns. The application of the formulas presented here allows for an instantaneous design for medium/high gain antennas with controllable total aperture efficiency till 80%. The accuracy of the formulation is tested by a full-wave analysis and compared with other formulations available in the literature. We found that the new formulation proposed here significantly reduces the phase error in a wide range of the lens parameters, thus allowing for a more efficient, accurate and flexible design for GRIN lenses. As a proof of concept, a thin GRIN lens antenna for operation in E-band (60-90 GHz) is designed, demonstrating a high aperture efficiency (63%) across a wide frequency range
Notes on Profile Inversion and Closed Form Formulation of Compact GRIN Lenses
Get PDFThis note is an addendum to the paper Profile Inversion and Closed Form Formulation of Compact GRIN Lenses [1] and summarizes a few comments obtained after publication of [1]
Ray-tracing in Dielectric Inhomogeneous Metalenses
No full textA flexible and efficient algorithm, based on Geometrical Optics (GO) ray-tracing, is presented. The algorithm allows the analysis of graded index (GRIN) lenses, namely dielectric inhomogeneous lenses with radially varying refractive index. The source incident field in absence of the lens is supposed to be known by simulation or by measurements. Phase and amplitude distributions at the output interface of the lens are obtained by solving the ray-equation and the energy transport equations, respectively. Once the field distribution at the lens-antenna aperture has been achieved, the radiation pattern is derived by aperture type radiation integrals. The ray-tracing algorithm is validated by full-wave analysis, after predetermining the feed source, the lens dimensions and the refractive index profile. Moreover, beam scanning capability of GRIN lenes can be investigated with the aid of the presented algorithm. Results achieved by the raytracing algorithm and the full-wave analysis have shown to be in good agreement, demonstrating that the proposed algorithm is a valuable alternative to time-consuming full-wave simulations in the GRIN lens analysis
Numerical and Workbench Design of 2.35 T Double-Tuned (1H/23Na) Nested RF Birdcage Coils Suitable for Animal Size MRI
Get PDFThe birdcage Radio Frequency (RF) coil is one of the most used configurations in Magnetic Resonance Imaging (MRI) scanners for the detection of the proton (1H) signal over a large homogeneous volume. More recently, birdcage RF coils have been successfully used also in the field of X-nuclei MRI, where the signal of a second nucleus (e.g. 13C, 23Na, 31P, and many others) needs to be detected with high sensitivity and spatial homogeneity. To this purpose several technical solutions have been adopted to design Double Tuned (DT) volume RF coils, including the recent configuration of the nested birdcage RF coils. One of the main problems in the design of DT RF coils is the decoupling between the 1H and X channels, and a number of solutions have been adopted over the years. In this work, based on numerical and workbench methods, we report the decoupling optimization of DT (1H/23Na) nested RF birdcage coils suitable for 2.35 T MRI scanners encompassing an inner Low-Pass (LP) birdcage used for X-nuclei, an outer High-Pass (HP) birdcage for 1H and an external cylindrical RF shield. We show that a suitable geometrical selection of the two coaxial RF birdcage coils (relative angular orientation, diameters and lengths) and RF shield (diameter, length) allows a significant decoupling optimization. We also provide valuable information about the RF B1+ field homogeneity and efficiency. Our approach was validated both with numerical simulations and workbench testing using DT nested RF coil prototypes
Profile Inversion and Closed Form Formulation of Compact GRIN Lenses
Get PDFNew formulas for the design of cylindrical Graded-Index (GRIN) lens-Antennas with integrated feeder are presented. The possibility of integrating the feeder within the lens makes the system more compact, avoids complex mechanical design and alignment errors. The lens is characterized in analytical form by Geometrical Optics (GO). The expression of the radially varying refractive index is derived by applying the conservation of the momentum inside the lens and imposing parallel rays at the lens output interface. The mathematical procedure to retrieve the refractive index is based on the inversion of a truncated Abel transform. The Poynting vector at the lens aperture is derived by applying the conservation of energy in each elementary ray-Tube. By approximating the Poynting vector to a Gaussian function the total efficiency of the lens-Antenna is derived in a closed form, allowing for a quick lens design. The proposed formulation has been successfully validated by using ray-Tracing and a full-wave simulations. Finally, we present examples of practical design of GRIN lenses by using holes of different shapes in a dielectric ABS/Teflon host media
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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