6,920 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
Influence of strain in Ag on Al(111) and Al on Ag(100) thin film growth
We demonstrate the influence of interfacial strain on the growth modes of Ag films on Al(111), despite the small magnitude of the lattice misfit in this system. The strain is relieved by the formation of stacking fault domains bounded by Shockley partial dislocations. The growth mode and the step roughness appear to be strongly connected. Growth is three-dimensional (3D) as long as the steps are straight, but switches to 2D at higher coverage when the steps become rough. Anisotropic strain relaxation and straight steps seem to be related. We also report related observations for Al deposited on Ag(100)
Further steps of hepatic stimulatory substance purification
The hepatic stimulatory substance (HSS) extracted from weanling rat livers was purified 381,000-fold using chromatographic techniques including nondissociating polyacrylamide gel electrophoresis (nondenaturing PAGE). The activity of this highly purified HSS, named Acr-F4, was assessed in two in vivo models. In 40% hepatectomized rats, it produced a fivefold increase in the proliferative rate normally seen following this partial hepatectomy. In Eck fistula dogs, the level of base increase in hepatocyte renewal was amplified threefold by an infusion of Acr-F4 (50 ng/kg/day). Acr-F4 had no influence on the regenerative response of the kidney following a unilateral nephrectomy or of the bowel following a 40% resection of the small bowel. On the basis of these findings, it can be concluded that HSS (Acr-F4) has a high biological activity and is organ specific. Ā© 1991 Plenum Publishing Corporation
How to avoid potential pitfalls in recurrence plot based data analysis
Recurrence plots and recurrence quantification analysis have become popular
in the last two decades. Recurrence based methods have on the one hand a deep
foundation in the theory of dynamical systems and are on the other hand
powerful tools for the investigation of a variety of problems. The increasing
interest encompasses the growing risk of misuse and uncritical application of
these methods. Therefore, we point out potential problems and pitfalls related
to different aspects of the application of recurrence plots and recurrence
quantification analysis
Cyclosporine augments hepatic regenerative response in rats
A number of mechanisms participate in the hepatic injury that occurs during and following liver transplantation. A normal allograft regenerative response is probably essential for a successful transplant outcome. In this study, the effect of cyclosporine, a potent immunosuppressant used routinely after liver transplantation, on the regenerative response of the liver after partial hepatectomy was investigated. Male Wistar rats were pretreated for one week with either cyclosporine or the olive oil vehicle and were subjected to either a two-thirds partial hepatectomy or a sham operation. Animals were sacrificed at various times postoperatively and the remnant livers were weighed to determine the liver weight to body weight ratio, two biochemical measures of a regenerative response (cytosolic ornithine decarboxylase activity and thymidine kinase activity), and the hepatic content of estrogen and androgen receptors, as the content of these receptors has been shown to modulate, at least in part, the subsequent hepatic regenerative response. The preoperative hepatic cytosol content of ornithine decarboxylase, thymidine kinase, and estrogen receptor was significantly greater (P<0.05) in rats pretreated with cyclosporine than in those treated with the vehicle alone. A significant increase in ornithine decarboxylase and thymidine kinase activities occurred after partial hepatectomy in both the cyclosporine-pretreated and vehicle-pretreated animals. The absolute levels for each parameter were also greater in the cyclosporine-treated animals than in the vehicle-treated controls at 24 hr after partial hepatectomy (P<0.05). The pattern of change in the hepatic cytosolic content of estrogen and androgen receptors in both groups of animals was comparable with those described previously for regenerating liver. These data suggest that cyclosporine may predispose the liver to respond to either a regenerative signal or perceived need and thereby fortuitously enhance liver graft performance after successful surgical implantation. Ā© 1989 Plenum Publishing Corporation
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