4,215 research outputs found
Polarizablity of 2D and 3D conducting objects using method of moments
Fundamental antenna limits of the gain-bandwidth product are derived from
polarizability calculations. This electrostatic technique has significant value
in many antenna evaluations. Polarizability is not available in closed form for
most antenna shapes and no commercial electromagnetic packages have this
facility. Numerical computation of the polarizability for arbitrary conducting
bodies was undertaken using an unstructured triangular mesh over the surface of
2D and 3D objects. Numerical results compare favourably with analytical
solutions and can be implemented efficiently for large structures of arbitrary
shape.Comment: Peer-reviewed articl
An investigation into the Gustafsson limit for small planar antennas using optimisation
The fundamental limit for small antennas provides a guide to the
effectiveness of designs. Gustafsson et al, Yaghjian et al, and
Mohammadpour-Aghdam et al independently deduced a variation of the
Chu-Harrington limit for planar antennas in different forms. Using a
multi-parameter optimisation technique based on the ant colony algorithm,
planar, meander dipole antenna designs were selected on the basis of lowest
resonant frequency and maximum radiation efficiency. The optimal antenna
designs across the spectrum from 570 to 1750 MHz occupying an area of were compared with these limits calculated using the
polarizability tensor. The results were compared with Sievenpiper's comparison
of published planar antenna properties. The optimised antennas have greater
than 90% polarizability compared to the containing conductive box in the range
, so verifying the optimisation algorithm. The generalized
absorption efficiency of the small meander line antennas is less than 50%, and
results are the same for both PEC and copper designs.Comment: 6 pages, 10 figures, in press article. IEEE Transactions on Antennas
and Propagation (2014
Point Contacts in Modeling Conducting 2D Planar Structures
Use of an optimization algorithm to improve performance of antennas and
electromagnetic structures usually ends up in planar unusual shapes. Using
rectangular conducting elements the proposed structures sometimes have
connections with only one single point in common between two neighboring areas.
The single point connections (point crossing) can affect the electromagnetic
performance of the structure. In this letter, we illustrate the influence of
point crossing on dipole and loop antennas using MoM, FDTD, and FEM solvers.
Current distribution, radiation pattern, and impedance properties for different
junctions are different. These solvers do not agree in the modeling of the
point crossing junctions which is a warning about uncertainty in using such
junctions. However, solvers agree that a negligible change in the junction
would significantly change the antenna performance. We propose that one should
consider both bridging and chamfering of the conflicting cells to find
optimized structures. This reduces the simulation time by 40% using FDTD
modeling, however no significant reduction is obtained using the MoM and FEM
methods.Comment: 4 pages, 10 figures, 1 table, accepted for publication in IEEE
Antennas Wireless Propag. Let
Photoelectron spectra of an Al70Pd21Mn9 quasicrystal and the cubic alloy Al60Pd25Mn15
Photoelectron spectra of a fivefold quasicrystalline alloy Al70Pd21Mn9 and a related cubic alloy Al60Pd25Mn15 reveal two noteworthy features. The first is that the Pd 3dlines fall at binding energies which are 2.2 eV higher than in pure Pd. A similar shift is observed for Pd in other alloys. The second noteworthy feature is that the Mn 2p3/2 line is very sharp in the quasicrystal. Fitting the experimental peaks with a Doniach-Sunjic line shpae suggests that the position and density of Mn states near EFis very sensitive to the structural and/or chemical environment of Mn in the alloys, and that this accounts for the shape of the 2p3/2 Mn line. The sharpness of the Mn line may be a fingerprint of the quasicrystalline phase within the AlPdMn family
Radiographic features of liver allograft rejection
The radiographic features of 19 transplanted patients with failure of the liver allograft were evaluated. These features were: poor filling, stretching, attenuation of intrahepatic biliary ducts documented by T-tube cholangiogram, attenuation of branches of the hepatic artery seen on angiogram as well as a decrease of blood flow through the liver seen on angiogram and nuclear medicine dynamic scintigram. These findings were secondary to swelling of the transplanted liver and were not specific for rejection; they may also be present in hepatic infarction or infection
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