Tayfun Ozdemir

Hybrid finite-element methodologies for antennas and scattering

This paper is an overview of the finite-element method (FEM) as applied to electromagnetic scattering and radiation problems. A brief review of the methodology is given with particular emphasis on new developments over the past five years relating to feed modeling, parallelization, and mesh truncation. New applications which illustrate the methods capabilities, versatility, and utility for general purpose application are discussed.

Triangular prisms for edge-based vector finite element analysis of conformal antennas

This paper deals with the derivation of the edge-based shape functions for the distorted triangular prism and their applications for the analysis of doubly curved conformal antennas in the context of the finite element method (FEM). Although the tetrahedron is often the element of choice for volume tessellation, mesh generation using tetrahedra is cumbersome and the central processing unit (CPU) intensive. On the other hand, the distorted triangular prism allows for meshes which are unstructured in two dimensions and structured in the third dimension. This leads to substantial simplifications in the meshing algorithm, and many conformal printed antenna and microwave circuit geometries can be easily tessellated using such a mesh. The new edge-based shape functions are first validated by computing the eigenvalues of three different cavities (rectangular, cylindrical, and pie-shell). We then proceed with their application to computing the input impedance of conformal patch antennas on planar, spherical, conical, and other doubly curved (ogival) platforms, where the FEM mesh is terminated using an artificial absorber applied conformal to the platform. Use of artificial absorbers for mesh termination avoids introduction of Greens functions and, in contrast to absorbing boundary conditions, a knowledge of the principal radii of curvature of the closures boundary is not required.

A hybridization of finite-element and high-frequency methods for pattern prediction for antennas on aircraft structures

This paper considers the hybridization of the finite-element and high-frequency methods for predicting the radiation pattern of printed antennas mounted on aircraft platforms. The finite-element method is used to model the cavity-backed antennas, whereas the interactions between the radiators and the substructures are treated via a high-frequency technique, such as the GTD, PO/PTD, or SBR. We present comparisons between measurements and calculations, along with a qualitative description of the finite-element and high-frequency codes employed.

Analysis of the log-periodic folded slot array

In recent years then has been significant interest in conformal, low-profile, very wideband antennas. To this end various configurations of log-periodic antenna structures have been developed, mostly consisting of planar logarithmic spirals, toothed planar and wire structures, and the ubiquitous dipole array. Unfortunately, none of these designs easily lend themselves to low-profile, conformal mounting. A log-periodic antenna structure more suited to this type of mounting has been developed by Greiser and is called the log-periodic folded slot array (LPFSA). Accurate knowledge of the radiation patterns and impedance characteristics would facilitate the development and optimization of a design procedure for the LPFSA. The purpose of this investigation is to gain a more complete understanding of the LPFSA, with the goal of developing an optimized design procedure.<<ETX>>

Antenna array processing for radar applications using support vector machines

Support vector machines are a good candidate for the solution of antenna array processing problems such as beamforming and the angle of arrival estimation, because these algorithms provide superior performance in generalization ability and computational complexity. In this work we introduce three new approaches for antenna array beamforming based on support vector machines. The first one is based on the use of the linear support vector regressor. The second algorithm uses a nonlinear multiregressor to find the parameters of a linear beamformer and the third is an implementation of a nonlinear beamformer using a nonlinear support vector machine. Comparisons with conventional beamforming strategies and simulation results are provided to demonstrate the advantages of the support vector machine approach

Fast parameter optimization using Kriging metamodeling [antenna EM modeling/simulation]

In this paper, we propose a hybrid optimizer, which combines the Kriging macro-modeling and the divided rectangles method (DIRECT) to perform global optimization. The latter yields a deterministic answer with fast convergence rate and possesses local as well as global optimization properties. Two examples are presented, where DIRECT optimizer is combined with Kriging metamodeling. The first example involves optimization of the shape of a slot array Frequency Selective Surface (FSS). In the second example, the electromagnetic coupling between the harness and the FM antenna on a Deville model automobile is minimized. Both examples demonstrate the exceptional rate of convergence and the flexibility of the proposed optimizer.

A comparative study of an ABC and an artificial absorber for truncating 3-D finite element meshes

The type of mesh termination used in the context of finite element formulations plays a major role on the efficiency and accuracy of the field solution. In this work, we evaluate the performance of an absorbing boundary condition (ABC) and an artificial absorber (a new concept) for terminating the finite element mesh. The artificial absorber consists of a metal-backed lossy dielectric layer which was designed to minimize the TE and TM reflection coefficients at all (real) incidence angles. Based on this design, the thickness of the metal-backed artificial absorber was selected to be 0.15/spl lambda/ and its relative permittivity and permeability were selected to be /spl epsiv//sub r/=/spl mu//sub r/=1.-j2.7. In this paper, the two approximate mesh truncation schemes (one using ABCs and the other artificial absorber) are compared with the exact finite element-boundary integral (FEM-BI) method in terms of accuracy and efficiency. This analysis is done in connection with the problem of scattering by a three dimensional finite slot array in a thick ground plane. The array slots are of thickness d and, for this study, they are assumed to have a square aperture of width W. In all cases, it is assumed that the excitation is a plane impinging from above at some angle with respect to the array normal.<<ETX>>

A thin cavity-backed Archimedean slot spiral for 800-3000 MHz band coverage

We show that cavity-backed slot spirals (rather than printed) are particularly attractive for conformal applications. Since slot radiation is enhanced by the presence of a shallow metal backed cavity, very thin (less than a 1/4 inch thick) antennas can be considered without compromising the spirals broadband performance. A specific design proposed by Nurnberger and Volakis (see 19th. Meeting and Symposium of Antenna measurement Techniques Association, Boston, Massachusetts, 1997) is considered for analysis and improvement.

Meanderline antenna for RFID-chemical sensor applications

An RFID-based chemical sensor is being developed for detecting biological warfare agents (BWA). When interrogated with a Reader, the sensor responds with a positive or a negative reading. Future versions of the sensor will also identify the type of agent present. Figure 1 shows the sensor board housing a chemical sensor, the RFID chip and the antenna. The entire board occupies a space of 65mm × 30mm × 2.36mm. The chemical sensor comes with its own packaging, is glued to the substrate and interfaces with the RFID chip via a circuit. The antenna occupies most of the space and is connected to the ATMEL ATA5590 RFID chips antenna ports. The focus of this paper is the design, fabrication and testing of the antenna.

Training of adaptive antennas using simulated data

GPS is vulnerable to multipath interference and intentional jamming, and many variants of multipath mitigation systems have been proposed and deployed. Such GPS receivers make use of adaptive arrays that employ spatial processing to place nulls in the direction of interfering signals. Although this approach is adequate for narrowband signals, it may be inadequate for broadband operation, especially when multipath is present. A new approach, based on machine learning and support vector machines (SVM) has been developed in order to enhance the spatial and temporal capabilities of the existing adaptive array antennas used by GPS receivers. The new approach makes the antenna array intelligent, so that when one of the antenna elements in the array fails, the performance of the GPS array degrades gracefully for both narrowband and broadband signals. The beamforming algorithms need to be improved using inter-element coupling models and trained by data contaminated with multipath interference. Experimental data is expensive to obtain. Therefore, VirAntenn/spl trade/ antenna array simulation software from Virtual EM Inc. has been interfaced with a high frequency propagation modeler (HFPM) (also by Virtual EM) to produce the training data. Inter-element coupling is provided by VirAntenn/spl trade/ alone, while multipath interference has been simulated by integrating the two software.