Showing papers by "George A. Kyriacou published in 2006"

Development of an Adaptive and a Switched Beam Smart Antenna System for Wireless Communications

Abstract: This study concentrates on the development of two separate Smart Antenna Systems for the 2.45 GHz ISM band. Both systems incorporate the RF-beamforming method. Each system has the ability to point the beam in a three dimensional space, both in azimuth and in elevation direction. The Switched Beam System adopts a passive network-based beamforming approach, using 2-D Butler matrix topologies. The Adaptive System utilizes the vector modulator method, but only in the azimuth direction, increasing beam-steering accuracy, whereas introducing complexity and cost. The system design is presented for both cases, along with some module design and testing examples. A comparison of the two systems will be also discussed.

On the Design of Patch Antennas Tuned by Transversely Magnetized Lossy Ferrite Including a Novel Resonating Mode

Abstract: Circularly symmetric patch antennas tuned by trans- versely magnetized lossy ferrite are studied. The circular and ring patch geometries printed on ferrite substrate or tuned by ferrite post and ferrite toroid are studied. Both saturated and partially magne- tized ferrite are considered. Strong effects on the dispersive properties of modes propagating under the patch and in turn on the antenna resonant frequency and input impedance are observed when the ferrite losses are taken into account. The patch antennas resonance at a novel mode propagating in the traditionally assumed switch-off frequency range of negative effective permeability constitutes an essential origi- nal contribution of this work. In all cases the dynamic control of the patch resonant frequency through the DC-biasing field is investigated. The "perfect magnetic walls approximation" was employed in the anal- ysis since it offers a valuable physical insight as well as simplified closed form expressions for the resonant conditions. These are used as engi- neering design formulas for an initial antenna design, which is in turn fine tuned with the aid of a numerical simulation-optimization scheme. The validity of the present method was verified through comparisons with published experimental results and numerical simulations.

Eigenvalue analysis of curved waveguides employing FDFD method in orthogonal curvilinear co-ordinates

Abstract: A two-dimensional finite difference frequency domain eigenvalue method employing orthogonal curvilinear co-ordinates is established. Its main strength is the accurate modelling of curved interfaces enabling the accurate evaluation of the complex propagation constants of curved waveguides. Numerical results for multilayer-multiconductor microstrip lines printed on curved substrates prove the validity of the method.

Localization of an Equivalent Central Cardiac Electric Dipole for Electrocardiography Applications

Abstract: In the dipole approach the activity of the heart is represented by one or two moving-rotating current dipoles. The basic underlying principle is to select the amplitudes and coordinates of these dipoles within an appropriate model of the torso such that calculated torsosurface potential distribution closely matches the measured body-surface-potential distribution. This source model was used by Gulrajani et al, [2, 3], in some earlier investigations and Guard et al, [4]. Also Armoundas et al. [5], and Abstract— An efficient and robust method for the solution of the non-linear and ill-posed inverse problem of electrocardiography is presented. The hearts activity is modeled by a central cardiac electric dipole, which within the present work is allowed only to rotate about a fixed origin. For this purpose a three-dimensional volume conductor model of the human body is constructed based on a classical anatomic atlas. This is excited by an assumed (initial guess) dipole located at the center of the heart. In turn a Least squares optimization scheme is employed, aiming at the matching of the potential distribution calculated on the torso surface to the corresponding distribution measured with the aid of multiple electrodes. The efficiency of the method stems from the employment of arbitrary shaped hexahedral elements within the finite element method for the minimization of the required computational resources while the model realistically reflects the body internal structure. Finally, the algorithm is successfully tested using measured data available online H Bruder et al, [6] used the single moving dipole to simulate the electrical activity of the heart. There are many other research groups, [7,8] in the inverse electrocardiography field which aim at the definition of epicardial potential distribution either by using realistic geometry anisotropic heart models or trying to exploit a priory information. The research status up to 1998 is given in the review paper [1]. A common characteristic of these inverse problems is their ill-posed nature, where this difficulty becomes worst when the number of unknown parameters is increased. Namely, the problem is best conditioned when a single cardiac equivalent electric dipole is considered, which in turn involves a compromise in modeling accuracy. The solution of this inverse problem is generally based on a “volume conductor model” representing the whole body, which enables the solution of the forward problem. Preferably this model should retain high spatial resolution around the heart. The assumed equivalent electric source for heart activity is modeled and the generalized Laplace or Poisson equation (forward problem) is solved to obtain a “calculated data set” for the body surface potentials. At this point a multiple lead (e.g. 24 electrodes or more) electrocardiographer is required to facilitate the “measured data set” on the actual human subject to be diagnosed. In turn an inverse problem solution algorithm like Newton’s, Newton-Raphson e.t.c can be employed for the minimization of a cost function, usually in the least squares means. Moreover, like most inverse problems this is a non-linear one. So, the initially assumed equivalent electric dipole parameters (e.g. 3 dipole moment components and 3 dipole coordinates) are iteratively updated until the differences between the measured and calculated data sets become comparable to an accepted error tolerance. The latter may be defined by the measurements errors and their noise as well as the computer modeling inaccuracies.