Showing papers by "Zvonimir Sipus published in 2009"

Analysis of Uniaxial Multilayer Cylinders Used for Invisible Cloak Realization

Abstract: Uniaxial multilayer cylinder cloaks have recently been proposed to prevent scattering of electromagnetic waves. These cloaks are based on spatial variation of constitutive parameters within the structure by means of metamaterials. A mathematical description of invisibility in such cloaks is provided. The analysis approach is based on modal analysis of cylindrical multilayer structures. In order to analyze structures with arbitrary number of layers, an algorithm to calculate spectral-domain Green's functions of uniaxial multilayer structures has been developed. The results show that the recently introduced metamaterial cloaks with reduced variation of constitutive parameters can decrease the total scattering width up to a factor of three, but only in a narrow frequency band due to dispersion in the filling material. Furthermore, it is shown that in the analyzed structures already 6 layers of constant permittivity and permeability approximate well the continuous dependence of constitutive parameters.

Efficient Analysis of Aperture Antennas on Generally Shaped Convex Multilayered Surfaces Using a Hybrid SD-UTD Method

Abstract: A novel hybrid method is described for analyzing convex multilayered conformal array antennas. The hybrid method is based on the spectral domain approach in combination with the ray-based uniform theory of diffraction (UTD) method. The analysis is divided in two parts. First, the spectral domain approach is accelerated by using an asymptotic extraction technique where the extracted term of the Green's function is calculated using UTD. It is shown that this new approach results in significant acceleration of the existing spectral domain algorithm without losing accuracy. The modified spectral domain method is then used in the second part where generally shaped convex multilayered surfaces are analyzed by using sets of canonically shaped surfaces (spheres and/or circular cylinders). Their radii are obtained using the UTD formulation, which contains important information such as distance and curvature of the generally shaped surface along each geodesic. The results obtained using the new algorithm are compared to the available results (calculated and measured) for different conformal antennas, showing very good agreement.

Ant colony approach in optimization of base station position

Abstract: We are witnesses of the growing interest in providing and improving signal strength coverage for mobile phones and Wireless Local Area Networks (WLANs) in indoor environments. In such cases it is difficult to rigorously obtain the signal strength distribution. A neural network is used as alternative technique to predict signal strength at any point of given environment. It has advantages of deterministic (high accuracy) and empirical (short computation) approaches. The neural network architecture, based on the multilayer perceptron, is used to absorb the knowledge about the given environment through training based on measurements. Such network is capable to predict signal strength that includes absorption and reflection effects. In our approach we used neural network model as a cost function in the optimization of the base station and access point's positions. In this paper we used an algorithm based on global search method known as the ant colony optimization (ACO) method. This optimization method is based on the behaviour of ant colonies in obtaining food and carrying it back to the nest. This algorithm is well suited for discrete problems, so in our case it is needed some modifications to satisfy continuous problem. The pheromone matrix is generated with matrix elements that represent a location for ant movement, i.e. they represent locations of base stations. The population of ants is randomly distributed along the pheromone matrix. They move from one position to another on probabilistic manner that depends of pheromone concentration. The achieved minimum of the cost function represents optimal position of base station or access point. The ACO results are compared with PSO and GA results.

Efficient analysis of curved frequency selective surfaces

Abstract: This article presents the analysis of frequency selective surfaces (FSS) embedded in a curved supporting structure. This very complex task is simplified by combining rigorous spectral domain approach with simple free-space analysis method. To enhance efficiency and accuracy, the new hybrid method splits the analysis in two distinct parts depending on the curvature and distance. In the first part asymptotic extraction technique is used to achieve faster convergence of the spectral domain technique, and it is used for closely placed elements. For distant elements the analysis is based on the free-space approach modified with an additional correction term. Verification of the new method is shown using a spherical FSS composed of circular rings, but also a way to extend the method to arbitrary curved structures is proposed. Finally, the radiation properties of curved FSS are compared with an equivalent metal reflector.

Local wave Green's functions of parallel plate metamaterial-based gap waveguides with one hard wall

Abstract: The purpose of the presented paper is to give the derivation of the Green's functions of oversized waveguides with one hard wall. Two different realizations of the hard wall were considered: idealized PEC/PMC model and the corrugated hard surface. The applied analysis method avoids the complexity of the Floquet mode approach by using appropriate asymptotic boundary conditions for the observed periodic structure. This is possible because the period of the used periodic structures is very small compared to the wavelength. The developed Green's functions have poles corresponding to surface wave modes and waveguide modes, and their properties are determined by considering the pole distribution. We have also developed a program that calculates the electromagnetic field distribution inside the considered waveguides. The computed results are compared with the results obtained with the general electromagnetic solver, and the agreement is very good.

Height reduction of circular-cylindrical dielectric lens antennas

Abstract: A fast and accurate CAD tool for synthesizing layered circular-cylindrical dielectric lens antennas fed by pyramidal horn antennas has been recently developed. It is based on the G1DMULT (Green's functions for 1D MULTilayered structures) algorithm which presumes that circular-cylindrical lenses are of infinite length. In this paper we study the effects of length truncation upon the lens radiation characteristics and define the range of validity of G1DMULT. Two height reduction techniques are also proposed. It is shown that they allow reducing the antenna size by a factor two, while preserving fan-beam radiation patterns and beam scanning capabilities.

Multilayer hemi-spheroidal lenses for vehicle-mounted scanning antennas

Abstract: Methods for designing vehicle-mounted scanning lens antennas are presented. Spectral domain analysis approach is used for spherical and hemispherical structures with ability to model different kinds of feeds. In case of horn antennas, the feed can be optimized simultaneously with the lens using the equivalent surface current model. Spheroidal antennas are analyzed by high-frequency techniques (namely combination of geometrical and physical optics) and general electromagnetic solver, and some guidelines regarding the design of such, reduced height, antennas are given.

Input admittance of circular waveguide opening mounted on dielectric-covered grounded spherical surface

Abstract: The paper deals with a circular waveguide antenna mounted on a grounded spherical surface with a dielectric coating. The analysis method is based on the moment method in the spectral domain. The influence of thickness and permittivity of dielectric radome is discussed. The analysis method is verified by comparing the theoretical results with measurements.

Novel photonic guiding structures - analysis and measurements

Abstract: This paper presents an analysis and measurements of periodic holey fibers which belong to a continuously growing group of novel photonic crystal fibers. To analyze the modal properties of these fibers the transverse electric field is decomposed into a series of orthogonal modes which are then used to solve the vector wave equation for optical fibers. Orthogonal functions used in the considered case were Hermite - Gaussian functions because a very small number of modes is needed to achieve reasonable accuracy. The results of this analysis for fiber under test are compared with the measurements performed and are found to be in a very good agreement. Furthermore, additional measurements of bending properties were performed showing that holey fibers have significantly lower bending losses compared to standard G.652 fibers that are mostly in use today.

Raytracing analysis of lens antennas - practical approach

Abstract: In this paper an overview of the raytracing approach for analyzing dielectric lens antennas is given with special attention paid to practical realization of such program. The raytracing technique is very well known, but in most scientific texts the algorithm is described only in basic concepts. Here we shall describe the basic steps required to write a raytracing software including the definition of the structure, finding the ray-surface intersections, calculating the directions of the reflected and refracted rays in general 3D case and finally the steps needed for performing the physical optics integration and calculation of far-field radiation pattern.

Specialty Optical Fibers: Analysis and Characterization

Abstract: This paper gives a closer insight to novel optical fibers by ap plying specialized numerical analysis approaches. First we have described a newly developed algorithm based on the spectral domain approach, which is employed in the analysis of multilayer optical fibers with circular symm etry. It computes the Green’s functions of the considered fiber and locates the appropriate poles, which correspond to the actual propagating modes of the fiber. Through further analysis of the poles it is possible to obtain inform ation about modal fields, attenuation, bending losses and dispersion properties for all modes present in fibers with ve ry complicated refractive index profile. Furthermore, an approach for the analysis of novel types of fibers belongin g to the so-called Photonic Crystal Fibers family is presented. Since the considered fibers can contain variou s inclusions in the core or the cladding structure, the analysis is in this case based on the modal decomposition of t he electric field in spatial domain. In both analysis cases the obtained results were compared to the previously p b ished or where possible measured results, showing very good agreement.