1,720,959 research outputs found
Time Domain Reflectometry of Glass Beads/Magnetite Mixtures: a Time and Frequency Domain Study
No full textThe measurements of the time domain reflectometry sTDRd signal propagating along a coaxial
probe, filled with dry mixtures of glass beads and magnetite, are analyzed for deriving the
attenuation factor a and the bandwidth. Samples with different percentages s5%–25%d of magnetite
are considered. The a values are obtained by two different methods: sid wave amplitude at the
second reflection, siid electromagnetic parameters and widths of the passing bands. The two methods
provide consistent a values within the experimental uncertainties. The agreement supports the
possibility of measuring the attenuation factor from the second TDR reflection, for anhydrous
samples similar to the investigated mixtures
Effective Frequency and Attenuation Measurements of Glass Beads/Magnetite Mixtures by Time-Domain Reflectometry
No full textSamples with different percentages (5–30%) of magnetite and different ranges of grain size are
analysed by time-domain reflectometry (TDR). The passband, the attenuation factor α and the
effective frequency are derived from the TDR measurements. The passband was obtained from the
second edge of the sample by eliminating the DC component and applying a Fourier transform to
the residual signal. The bandwidth of the resulting spectral shape decreases as the magnetite percentage
increases. The second edge was also used to evaluate the effective attenuation factor αe,
using multiple-reflection theory, terminated at the second reflection. The attenuation values α can
be estimated independently by means of the electromagnetic parameters obtained using an LCR
meter. To compare α and αe, an effective TDR frequency νe is introduced. The agreement between
the values obtained from TDR and LCR-meter techniques supports the reliability of the attenuation
derived from the second TDR reflection, and that computed from the effective frequency. The
shapes of the passbands and the αe and νe values for magnetite/glass-beads mixtures, which simulate
dry soils with increasing iron oxides content, are reported and discussed
Role of dispersive effects in determining probe and electromagnetic parameters by Time Domain Reflectometry
No full textTime domain reflectometry (TDR) measurements, performed in a cylindrical tube
filled with air, glass beads, and water, are analyzed in the time and frequency domains. In
the time domain, air and water provide contrasting results of the probe length if the
standard tangent line fitting procedure is used. An alternative method, based on the time
derivative of the TDR response, is introduced. Its application suggests that the anomalous
result provided by the tangent method in water is explained by dispersion effects. To
account for the frequency dependence of the permittivity, an analysis in the frequency
domain is performed. The problems of the input function choice and of the noise control
are investigated. This allows us to correct the experimental scatter function S(n) and to
perform the best fit of its parameterized expression, which is theoretically evaluated
from the Debye model of the permittivity. Consistent values of the probe length are
derived. It is concluded that the tangent method cannot be applied for determining the
geometrical and electromagnetic parameters of a probe filled by dispersive media. As an
application, the calibrated probe is used for measuring the TDR trace of glass beads, which
simulate sandy soils, and for deriving the frequency-dependent permittivity within the
Debye model
Electromagnetic parameters of dielectric and magnetic mixtures evaluated by Time Domain Reflectometry
No full textThe frequency-domain analysis of time-domain reflectometry
(TDR) data can be used to evaluate the electromagnetic
(EM) parameters of a sample under test. The goal of this
letter was to use TDR to determine EM parameters, assumed
to be frequency independent, for various magnetite/glass-beads
mixtures. The EM parameters are, in turn, used to determine
the velocity and attenuation of propagating waves. The latter
quantity is also obtained through the TDR voltage method. Velocities
decrease, and attenuations increase with increasing magnetite
content. The measurements of the present work are compared
with the velocities and attenuations reported in the literature
(measured via the network analyzer (NA) and LCR meter techniques).
The velocities calculated using the various methods are
in good agreement. In contrast, the attenuations determined by
fitting the TDR data only agree with the NA measurements at
high frequencies (450 MHz), while those obtained by the TDR
voltage method match the low-frequency attenuations determined
through the LCR meter. The reasons for these behaviors are
discussed, and the need for precise handling of the TDR data is
emphasized. The TDR fit procedure is recommended to obtain
reliable EM parameters of materials
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Full text linkWe 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
- …
