Dual-band Sierpinski fractal monopole antenna

A new top-loaded reduced-sized dual-band (1.9 GHz and 3.5 GHz) monopole antenna for wireless communications is presented. The antenna provides better than -15 dB input return loss and keeps the same radiation pattern over both bands. In addition, a low-profile performance is achieved.Peer ReviewedPostprint (published version

Why Are Asset Markets Modeled Successfully, But Not Their Dealers?

Market-level microstructure models of asset pricing succeed where dealer-level models do not. This study addresses this empirical difficulty in the context of foreign exchange dealers. New evidence is presented rejecting the latter models' specifications of how information asymmetry and inventory accumulation affect dealer pricing. This rejection is consistent with those of other dealer-level empirical studies. A new modeling avenue may be to reconsider optimal price setting while relaxing assumptions that specify incoming orders as the only component through which dealer inventories evolve. This approach is consistent with inventory evolution data and with market-level models' assumptions about currency markets. Copyright 2005, International Monetary Fund

Modeling grain boundaries in solids using a combined nonlinear and geometrical method

The complex arrangements of atoms near grain boundaries are difficult to understand theoretically. We propose a phenomenological (Ginzburg-Landau-like) description of crystalline phases based on symmetries and fairly general stability arguments. This method allows a very detailed description of defects at the lattice scale with virtually no tunning parameters, unlike usual phase-field methods. The model equations are directly inspired from those used in a very different physical context, namely, the formation of periodic patterns in systems out-of-equilibrium ({\it e.g.} Rayleigh-B\'enard convection, Turing patterns). We apply the formalism to the study of symmetric tilt boundaries. Our results are in quantitative agreement with those predicted by a recent crystallographic theory of grain boundaries based on a geometrical quasicrystal-like construction. These results suggest that frustration and competition effects near defects in crystalline arrangements have some universal features, of interest in solids or other periodic phases.Comment: 10 pages, 3 figure

Compression of morbidity and the labor supply of older people

"This paper tests whether there is evidence of compression of morbidity using data from the American Health and Retirement Study and analyzes the effects of this on the labor supply of older people. We find younger cohorts to suffer less from functional problems than older cohorts at given ages. Furthermore, we observe that instrumentalized disability has a negative effect on labor force participation. According to the cohort analysis and the multivariate analysis, it can be concluded that individuals will be able to work longer because of the delay in the onset of disability problems." (Author's abstract, IAB-Doku) ((en))Lebenserwartung, ältere Menschen, alte Menschen, Gesundheitszustand, Krankheit, chronische Krankheit, Behinderung, Erwerbsbeteiligung, Lebensarbeitszeit, USA

Multiband Sierpinskl fractal patch antenna

The multiband behavior of the Sierpinski patch antenna is described, and a new technique to improve the multiband behavior from the point of view of the radiation patterns is introduced. The technique suppresses the effects of the high order modes and a patch antenna with similar radiation patterns can be designed. Once the high order mode has been suppressed for the second band, the next step is to try eliminate it for the third and fourth band breaking the appropriate junctions.Peer ReviewedPostprint (published version

Dual band fss with fractal elements

Experimental and computed results of a frequency selective surface (FSS) based on a certain type of fractal element are presented. The fractal element is a two iteration Sierpinski gasket dipole. Owing to the dual band behaviour of the two iteration Sierpinski gasket dipole, two stopbands are exhibited within the operating frequency band. This behaviour is obtained by arraying one simple element in a single layer frequency selective surface (FSS)Peer ReviewedPostprint (published version

Truncation errors in cylindrical near to far field transform. A plane wave synthesis approach

Cylindrical Near to far field antenna transformation is expressed as a plane wave synthesis problem where a plane wave is produced by a cylindrical current distribution that encloses the Antenna Under Test (AUT). The current distribution that produces the plane wave is directly derived from the near to far field transformation algorithm.Peer ReviewedPostprint (published version

near field to far field tranfsormation of bipolar measurements by equivalent magnetic current approach

An equivalent magnetic current (EMC) approach has been proposed as an alternative method to the classical modal formulation for computing the far-field pattern of a radiating antenna from planar near-field measurements. The attractiveness of this EMC approach includes the possibility of producing the correct far-field pattern in all regions in front of a planar antenna under test (AUT), a drawback of the classical modal formulation due to its dependence on the Fourier transform and assumptions which must be made about the field outside of the measurement zone, and its adaptability to both irregularly sampled and non-canonical near-field measurement surfaces. A drawback of the EMC approach, however, is its significantly larger computational requirements. A comparison of the EMC and classical modal approaches for near-field to far-field transformation is examined in terms of the resultant far-field patterns. Measurement results for a waveguide-fed slot array using the UCLA bi-polar planar near-field measurement scanner are presented. An implementation of the EMC approach for the bi-polar geometry is described and results obtained using different subsets of the measured bi-polar near-field data are presented and compared to that obtained using the classical modal approach.Peer ReviewedPostprint (published version