459 research outputs found
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Assessment of a low-profile planar antenna for a wireless sensor network monitoring the local water distribution network
This paper presents an assessment on the suitability of a low-profi le planar antenna for a
Wireless Sensor Network (WSN) application monitoring the water supply at Fire Hydrants
(FHs). The antenna must have a low pro le so that it can be mounted on the FH lid; it must
have an omnidirectional radiation pattern so that it can communicate with base stations at
low elevations; and it must operate in the 2.4 GHz Industrial, Scienti c and Medical (ISM)
band. Measurements show that for the majority of the 2.4 GHz ISM band, the antenna has
a return loss of at least -10 dB and e ciency greater than 60 %.
For the FH WSN assessment, the antenna was deployed as a transmitter mounted on
the FH lid above the underground FH chamber and a vertically polarised monopole antenna
mounted on a mast at various speci ed heights above ground level was used to measure the
received power as a function of distance. The path loss results were compared with those from
a previous deployment, where the FH antenna was located in the FH chamber, and it is found
that using the low-pro le antenna reduced the path loss by at least 10 dB over the measured
transmitter and receiver separation.This paper is a postprint of a paper submitted to and accepted for publication in IET Wireless Sensor Systems and is subject to Institution of Engineering and Technology Copyright. The final version is available the IET Digital Library
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Characterizing the spectral properties and time variation of the in-vehicle wireless communication channel
To deploy effective communication systems in vehicle cavities, it is critical to understand the time variation of the in-vehicle channel. Initially rapid channel variation is addressed, which is characterised in the frequency domain as a Doppler spread. It is then shown that for typical Doppler spreads, the in-vehicle channel is underspread, and therefore the
information capacity approaches the capacity achieved with perfect receiver channel state information in the infinite bandwidth limit. Measurements are performed for a number of channel variation scenarios (absorptive motion, reflective motion, one antenna moving, both antennas moving), at a number of carrier frequencies and for a number of cavity loading scenarios. It is found that the Doppler spread increases with carrier frequency,
however the type of channel variation and loading appear to have little effect.
Channel variation over a longer time period is also measured, to characterise the slower channel variation. Channel variation is a function of the cavity occupant motion, which is difficult to model theoretically, therefore an empirical model for the slow channel
variation is proposed, which leads to an improved estimate of the channel state.This work is supported by the U.K. Engineering and Physical Sciences
Research Council (EPSRC) and National Physical Laboratory (NPL) under an
EPSRC-NPL Industrial CASE studentship programme on the subject of intra-Vehicular Wireless Sensor Networks. The work of T. H. Loh was supported by
the 2009 - 2012 Physical Program and 2012 - 2015 Electromagnetic Metrology
Program of the National Measurement Office, an Executive Agency of the
U.K. Department for Business, Innovation and Skills, under Projects 113860
and EMT13020, respectively.This is the author accepted manuscript. The final version can be found on the publisher's website at: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=682581
On the analogy between vehicle and vehicle-like cavities with reverberation chambers
Deploying wireless systems in vehicles is an area of current interest. Often, it is implicitly assumed that the electromagnetic environment in vehicle cavities is analogous to that in reverberation chambers, it is therefore important to assess to what extent this analogy is valid. Specifically, the cavity time constant, electromagnetic isolation and electric field uniformity are investigated for typical vehicle and vehicle-like cavities.
It is found that the time constant is a global property of the cavity (i.e., it is the same for all links). This is important, as it means that the root mean square delay spread for any link is also a property of the cavity, and thus so is the coherence bandwidth. These properties could be exploited by wireless sytems deployed in vehicles. It is also found that the field distribution is not homogeneous (and is therefore not uniform), but can be isotropic. For situations where the field distribution is isotropic, the spatial coherence is well defined, and therefore Multiple-Input-Multiple-Output antenna arrays can be used to improve performance of wireless systems. For situations where the field distribution is not isotropic, the angular spread is not uniform, and therefore beam-forming can be used to improve performance of wireless systems.This is the author's accepted manuscript and will be under embargo until publication. The final version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=692843
Approximate Analytical Equations for the Stirrer Angular Correlation in a Reverberation Chamber
IEEE In a reverberation chamber (RC), the angular correlation coefficient of a stirrer is an important parameter. It has been used to evaluate the performance of a stirrer or to estimate the number of independent samples in a measurement. In the previous work, the angular correlation coefficient (ACC) was evaluated numerically and no analytical equation was proposed. In this study, we propose an approximate analytical equation to fit the measured angular correlation that shows good agreements with measurement results. General properties of ACC are explored with physical insights; the equivalency of the mean value of the angular correlation and the -factor is revealed. This study provides further understandings on the control of the stirrer angular correlation and the -factor in an RC
The Cop Number of the One-Cop-Moves Game on Planar Graphs
Cops and robbers is a vertex-pursuit game played on graphs. In the classical
cops-and-robbers game, a set of cops and a robber occupy the vertices of the
graph and move alternately along the graph's edges with perfect information
about each other's positions. If a cop eventually occupies the same vertex as
the robber, then the cops win; the robber wins if she can indefinitely evade
capture. Aigner and Frommer established that in every connected planar graph,
three cops are sufficient to capture a single robber. In this paper, we
consider a recently studied variant of the cops-and-robbers game, alternately
called the one-active-cop game, one-cop-moves game or the lazy-cops-and-robbers
game, where at most one cop can move during any round. We show that Aigner and
Frommer's result does not generalise to this game variant by constructing a
connected planar graph on which a robber can indefinitely evade three cops in
the one-cop-moves game. This answers a question recently raised by Sullivan,
Townsend and Werzanski.Comment: 32 page
A Quantum Monte Carlo algorithm for non-local corrections to the Dynamical Mean-Field Approximation
We present the algorithmic details of the dynamical cluster approximation
(DCA), with a quantum Monte Carlo (QMC) method used to solve the effective
cluster problem. The DCA is a fully-causal approach which systematically
restores non-local correlations to the dynamical mean field approximation
(DMFA) while preserving the lattice symmetries. The DCA becomes exact for an
infinite cluster size, while reducing to the DMFA for a cluster size of unity.
We present a generalization of the Hirsch-Fye QMC algorithm for the solution of
the embedded cluster problem. We use the two-dimensional Hubbard model to
illustrate the performance of the DCA technique. At half-filling, we show that
the DCA drives the spurious finite-temperature antiferromagnetic transition
found in the DMFA slowly towards zero temperature as the cluster size
increases, in conformity with the Mermin-Wagner theorem. Moreover, we find that
there is a finite temperature metal to insulator transition which persists into
the weak-coupling regime. This suggests that the magnetism of the model is
Heisenberg like for all non-zero interactions. Away from half-filling, we find
that the sign problem that arises in QMC simulations is significantly less
severe in the context of DCA. Hence, we were able to obtain good statistics for
small clusters. For these clusters, the DCA results show evidence of non-Fermi
liquid behavior and superconductivity near half-filling.Comment: 25 pages, 15 figure
Congenital bipartite lunate presenting as a misdiagnosed lunate fracture: a case report
<p>Abstract</p> <p>Introduction</p> <p>A rare case of congenital bipartite lunate in a child is reported. Carpal variants are very uncommon as independent entities, with only three previous reports of this condition in the English literature.</p> <p>Case presentation</p> <p>An 11-year-old Caucasian boy presented with pain in the left wrist after a fall. Radiographs in the emergency department demonstrated a lunate that was divided into palmar and dorsal parts, causing a misdiagnosis of fractured lunate. Magnetic resonance imaging was then used to differentiate between the two diagnoses.</p> <p>Conclusion</p> <p>Very few cases of bipartite lunate have been reported in the literature, and unless awareness is raised about congenital anomalies such as this variant, confusion may arise.</p
Strong, omnidirectional radar backscatter from subwavelength, 3D printed metacubes
This is the author accepted manuscript. The final version is available from IET via the DOI in this recordMetallic metacubes formed of six metal plate faces connected via a metal jack are shown to backscatter microwave radiation extremely powerfully. Experimental radar scattering cross-section (RCS) data from three-dimensional (3D) printed samples agrees very well with numerical model predictions, showing a monostatic RCS of 15 times the geometric cross-section. The principal resonance of the metacubes demonstrates near-complete independence of the incident angle or polarisation of the radiation, making the metacube an omnidirectional scatterer. The metacubes are fabricated via additive manufacturing from metal-coated polymer, and are extremely lightweight, making them excellent candidates for improving the radar return signals from small objects such as drones and cubesats.Engineering and Physical Sciences Research Council (EPSRC
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Phase-matched multi-layer based polarisation-independent spot-size converter for silicon nanowire
The efficient coupling of optical power from a silicon nanowire (NW) to an optical fibre is challenging for both the quasi-TE and quasi-TM polarisations. Here, we propose a polarisation-independent spot-size converter (PI-SSC) based on phase-matched multi-layer waveguides for efficient coupling between a silicon NW and an optical fibre for both the polarisations. The fabrication process of the proposed PI-SSC is compatible with the complementary metal-oxide-semiconductor (CMOS) process. The optimisation for the proposed PI-SSC is studied by using a numerically efficient algorithm, combining a rigorous H-field based full-vectorial finite element method (VFEM) and the least squares boundary residual (LSBR) method. The simulation results show that using an eleven-layer based PI-SSC, the coupling losses between a silicon NW and a lensed fibre of radius 2 μm can be reduced to only 0.34 dB and 0.25 dB for the quasi-TE and quasi-TM polarisations, respectively. Furthermore, the output multi-layer is horizontally tapered, which further reduces the coupling loss for both the polarisations and the end face is easy to be polished
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