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Item Open Access Accuracy: The Frequently Overlooked Parameter in the Solution of Extremely Large Problems(IEEE, 2011) Ergul, O.; Gürel, LeventShow more We investigate error sources and their effects on the accuracy of solutions of extremely large electromagnetics problems with parallel implementations of the multilevel fast multipole algorithm (MLFMA). Accuracy parameters and their effects on the accuracy of MLFMA solutions are studied for large-scale problems discretized with hundreds of millions of unknowns. We show that some error sources are more dominant and should be suppressed for more accurate solutions; identifying less-effective error sources may allow us to derive more efficient implementations. Based on our analysis, we determine a set of benchmark problems that can be used to compare the accuracy of solvers for large-scale computations. A benchmarking tool is provided at www.cem.bilkent.edu.tr/ benchmark.Show more Item Open Access Advanced partitioning and communication strategies for the efficient parallelization of the multilevel fast multipole algorithm(IEEE, 2010) Ergül O.; Gürel, LeventShow more Large-scale electromagnetics problems can be solved efficiently with the multilevel fast multipole algorithm (MLFMA) [1], which reduces the complexity of matrix-vector multiplications required by iterative solvers from O(N 2) to O(N logN). Parallelization of MLFMA on distributed-memory architectures enables fast and accurate solutions of radiation and scattering problems discretized with millions of unknowns using limited computational resources. Recently, we developed a hierarchical partitioning strategy [2], which provides an efficient parallelization of MLFMA, allowing for the solution of very large problems involving hundreds of millions of unknowns. In this strategy, both clusters (sub-domains) of the multilevel tree structure and their samples are partitioned among processors, which leads to improved load-balancing. We also show that communications between processors are reduced and the communication time is shortened, compared to previous parallelization strategies in the literature. On the other hand, improved partitioning of the tree structure complicates the arrangement of communications between processors. In this paper, we discuss communications in detail when MLFMA is parallelized using the hierarchical partitioning strategy. We present well-organized arrangements of communications in order to maximize the efficiency offered by the improved partitioning. We demonstrate the effectiveness of the resulting parallel implementation on a very large scattering problem involving a conducting sphere discretized with 375 million unknowns. ©2010 IEEE.Show more Item Open Access Analysis of an arbitrary profile reflector antenna having resistive-type surface-H-polarization case(IEEE, 2008-06-07) Oǧuzer, T.; Altıntaş, Ayhan; Nosich, A. I.Show more A regularization-based numerical solution is obtained for arbitrary-shape conic section profile reflector antenna in 2-D, for the H-polarization case. New point is that the reflector surface is assumed a resistive-type material. The problem is treated by reducing the singular integral equation obtained from the boundary condition to the dual series equations and application of the Riemann Hilbert Problem (RHP) technique. The resulting matrix equation has regularized form. Sample numerical results are obtained for various values of the eccentricity of the conic section contour of reflector and the resistivity of its surface. © 2008 IEEE.Show more Item Open Access Analysis of circular reflectors by complex source-dual series approach(IEEE, 1993-06-07) Oğuzer, Taner; Altıntaş, Ayhan; Nosich, A. L.Show more In the present paper, two dimensional circular reflector antennas are analyzed by a rigorous analytical-numerical technique for both E and H polarization cases. The method is used in combination with the complex source approach. The convergence of the solution is guaranteed and any desired accuracy can be obtained. Some principal results of reflector antennas are examined by the exact circular reflector solution.Show more Item Open Access Analysis of Lossy Dielectric Objects with the Multilevel Fast Multipole Algorithm(IEEE, 2011) Ergul, O.; Gurel, LeventShow more Rigorous solutions of electromagnetics problems involving lossy dielectric objects are considered. Problems are formulated with two recently developed formulations, namely, the combined-tangential formulation (CTF) and the electric and magnetic current combined-field integral equation (JMCFIE), and solved iteratively using the multilevel fast multipole algorithm (MLFMA). Accuracy and efficiency of solutions are compared for different objects and conductivity values. We show that iterative solutions of CTF are significantly accelerated as the conductivity increases and CTF becomes a good alternative to JMCFIE in terms of efficiency. Considering also its high accuracy, CTF seems to be a suitable formulation for the analysis of lossy dielectric objects.Show more Item Open Access Analysis of the 2D nonconcentric large reflector antenna-in-radome system: H-polarization case(IEEE, 2004) Oğuzer, TanerShow more 2D nonconcentric reflector antenna-in-radome system is analyzed for H-polarization case. Rigorous formulation is performed depending on the MoR, Green's function and CSP methods. Larger geometries are solved in reasonable cpu times by using the FFT based algorithm in the computation of the Green's function.Show more Item Open Access Analysis of the nonconcentric radome-enclosed cylindrical reflector antenna system, e-polarization case(Taylor & Francis, 2005) Oğuzer, T.; Altintas, A.Show more Two-dimensional (2-D) radiation of a directive complex line source is analyzed in the presence of a perfectly conducting (PEC) reflector antenna system and nonconcentrically located dielectric radome. Similar problem was studied in the literature by using method of regularization and Green's function formulation for the H-polarization case. Here the same techniques are used for E-polarization case but in this case the scattered part of the Green's function is computed by using an FFT based algorithm. This provides us to solve the larger geometries accurately in reasonable computer times. So this approach can be considered as another alternative for the analysis of the E-polarized radome-enclosed reflector antenna system. Various numerical results are presented to support the convergence and accuracy of the technique and at the same time these results can be considered as reference data.Show more Item Open Access Benchmark Solutions of Large Problems for Evaluating Accuracy and Efficiency of Electromagnetics Solvers(IEEE, 2011) Ergul, O.; Gürel, LeventShow more We present a set of benchmark problems involving conducting spheres and their solutions using a parallel implementation of the multilevel fast multipole algorithm (MLFMA). Accuracy of the implementation is tested by comparing the computational results with analytical Mie-series solutions. Reference solutions are made available on an interactive website to evaluate and compare the accuracy and efficiency of fast solvers. We also demonstrate the capabilities of our solver on real-life problems involving complicated targets, such as the Flamme.Show more Item Open Access Chiral metamaterials: From negative index to asymmetric transmission(IEEE, 2013) Mutlu, Mehmet; Li, Zhaofeng; Özbay, EkmelShow more Chiral metamaterials are attractive for their intriguing properties such as negative refractive index, optical activity and circular dichroism, and asymmetric transmission. In this paper, we review the research we have conducted for the purpose of investigating these exciting properties. © 2013 EurAAP.Show more Item Open Access Choices of expansion and testing functions for the method of moments applied to a class of electromagnetic problems(IEEE, 1993-03) Aksun, M. I.; Mittra, R.Show more It is well known that the choice of expansion and testing functions plays an important role in determining the rate of convergence of the integrals associated with the moment method matrix, and that an improper choice can lead to erroneous results. The main objective of this paper is to critically examine this convergence issue and to provide criteria for the choice of these expansion and testing functions. The question of whether these functions need to satisfy the Holder condition is also investigated and the convergence behavior of the integrals involved in the spatial and spectral domain moment method is discussed for some representative expansion and testing functions.Show more Item Open Access Closed-form green's function representations for mutual coupling calculations between apertures on a perfect electric conductor circular cylinder covered with dielectric layers(IEEE, 2011-06-07) Akyüz, M. S.; Ertürk, V. B.; Kalfa, M.Show more Closed-form Green's function (CFGF) representations are developed for tangential magnetic current sources to calculate the mutual coupling between apertures on perfectly conducting circular cylinders covered with dielectric layers. The new representations are obtained by first rewriting the corresponding spectral domain Green's function representations in a different form (so that accurate results for electrically large cylinders, and along the axial line of a cylinder can be obtained). Then, the summation over the cylindrical eigenmodes is calculated efficiently. Finally, the resulting expressions are transformed to the spatial domain using a modified two-level generalized pencil of function method. Numerical results are presented showing good agreement when compared to CST Microwave Studio results.Show more Item Open Access Çok kullanıcılı çok antenli sistemlerde iş birlikli iletim(IEEE, 2008-04) Yazarel, Y. K.; Aktaş, DefneShow more Bu çalışmada işbirlikli, çok kullanıcılı, ve çok antenli bir haberleşme sisteminde telsiz erişim terminallerinin en iyi veri iletimi tekniğine ortaklaşa karar vermeleri problemini inceliyoruz. Burada pek çok çalışmadan farklı olarak kullanıcıların bireysel başarım hedefleri ve anten başına iletim gücü kısıtlamaları olduğu durumu ele alıyoruz. Önceki bir çalışmamızda bu kısıtlamalar altında en iyi sonucu bulan döngülü bir algoritma sunmuştuk. Ancak bu algoritma merkezi bir yapıda olduğu için tam anlamıyla dağıtılmış şekilde gerçeklenememektedir. Bununla birlikte basit yaklaşıklıklar kullanarak en iyiye yakın bir başarım sağlayan ve kısıtlı ve yerel veri iletimi ile gerçeklenebilecek etkin bir algoritma öneriyoruz.Show more Item Open Access Computational analysis of complicated metamaterial structures using MLFMA and nested preconditioners(IEEE, 2007-11) Ergül, Özgür; Malas, Tahir; Yavuz, Ç; Ünal, Alper; Gürel, LeventShow more We consider accurate solution of scattering problems involving complicated metamaterial (MM) structures consisting of thin wires and split-ring resonators. The scattering problems are formulated by the electric-field integral equation (EFIE) discretized with the Rao-Wilton- Glisson basis functions defined on planar triangles. The resulting dense matrix equations are solved iteratively, where the matrix-vector multiplications that are required by the iterative solvers are accelerated with the multilevel fast multipole algorithm (MLFMA). Since EFIE usually produces matrix equations that are ill-conditioned and difficult to solve iteratively, we employ nested preconditioners to achieve rapid convergence of the iterative solutions. To further accelerate the simulations, we parallelize our algorithm and perform the solutions on a cluster of personal computers. This way, we are able to solve problems of MMs involving thousands of unit cells.Show more Item Open Access Design and analysis of slotted sectoral waveguide array antennas embedded in cylindrically stratified media(IEEE, 2014-07) Kalfa, Mert; Ertürk Vakur B.Show more Slotted waveguide antennas are being widely used in military and commercial applications for many decades. Low cross-polarization, high power capacity, ease of fabrication in microwave bands, and the ability to form arrays make them excellent candidates for phased array antennas in radar applications. However, due to slots being highly resonant (narrow-band, high Q) radiators, their design parameters are very sensitive; hence, accurate design and analysis methods are required for a successful antenna design. Moreover, slotted waveguide array antennas are low-profile structures, which makes them suitable candidates for conformal and structure-integrated applications. Conformal and structure-integrated system solutions are especially required for air platforms, where aerodynamics, radar cross-section (RCS) and efficient use of real estate are of utmost importance. Although the accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays are of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, one of the widely used solvers are integral equation (IE) based ones that utilize the method of moments (MoM). However, IE solvers suffer from long matrix fill times, especially for matrix entries related to the cylindrically stratified media. © 2014 IEEE.Show more Item Open Access Development of micro-structured metamaterials for innovative antenna layouts(IEEE, 2007) Bilotti F.; Capolino F.; Gonzalo, R.; Özbay, Ekmel; Romeu J.; Schuchinsky, A.; Tretyakov, S.; Vardaxoglou, Y.Show more In this paper, we present the joint activities developed in the frame of the FP6 European Network of Excellence METAMORPHOSE in the field of new micro-structured materials for antenna applications. One of the key scientific goals of the research efforts developed within this network is to design innovative micro-structured materials to improve the performances of conventional radiators. Miniaturization, multi-functionality, reduced interference with electronic circuitry, are some of the main challenges in the design of antennas for the next generation of electronic transceivers in the microwave frequency range. The employment of different classes of metamaterials in innovative antenna layouts has been demonstrated to be effective to reach most of the desired goals. The theoretical, numerical and experimental efforts carried out by the leading European institutions working in this field, in fact, show how metamaterial antennas with unusual features are ready to push the innovation in the antenna research. The collaborative dimension of this research is described in this paper, as well as the related scientific achievements. © 2007 EuMA.Show more Item Open Access Digital microfluidics for reconfigurable antennas(IEEE, 2014) Damgaci, Y.; Çağatay, Engin; Bıyıklı, Necmi; Cetiner, B. A.Show more Usage of fluids (microfluidic or otherwise) in antennas provides a conceptually easy reconfiguration mechanism in the aspect of physical alteration. However, a requirement of pumps, valves, etc. for liquid transportation makes the antenna implementations rather impractical for the real-life scenarios. This work reports on the theoretical calculations and experiments conducted to evaluate the electrowetting on dielectric (EWOD) driven digital microfluidics as a reconfiguration mechanism for antennas and RF circuits. © 2014 European Association on Antennas and Propagation.Show more Item Open Access Downlink beamforming under individual SINR and per antenna power constraints(IEEE, 2007-08) Yazarel, Y. K.; Aktaş, DefneShow more In this paper we consider the problem of finding the optimum beamforming vectors for the downlink of a multiuser system, where there are individual signal to interference plus noise ratio (SINR) targets for each user. Majority of the previous work on this problem assumed a total power constraint on the base stations. However, since each transmit antenna is limited by the amount of power it can transmit due to the limited linear region of the power amplifliers, a more realistic constraint is to place a limit on the per antenna power. In a recent work, Yu and Lan proposed an iterative algorithm for computing the optimum beamforming vectors minimizing the power margin over all antennas under individual SINR and per antenna power constraints. However, from a system designer point of view, it may be more desirable to minimize the total transmit power rather than minimizing the power margin, especially when the system is not symmetric. Reformulating the transmitter optimization problem to minimize the total transmit power subject to individual SINR constraints on the users and per antenna power constraints on the base stations, the algorithm proposed by Yu and Lan is modified. Performance of the modified algorithm is compared with existing methods for various cellular array scenarios. ©2007 IEEE.Show more Item Open Access E-polarized beam scattering by an open cylindrical PEC strip having an arbitrary "conical-section" profile(John Wiley & Sons, Inc., 2001) Oğuzer, T.; Nosich, A. I.; Altintaş, A.Show more Two-dimensional (2-D) scattering of waves by a conducting strip with a canonical profile is simulated in the E-polarization case. This analysis is performed by reducing a singular integral equation (IE) to the dual-series equations, and making their analytical regularization. Furthermore, the incident field is taken as a complex source point (CSP) beam. This is an extension of our previous studies about circular and parabolic reflector antennas. The algorithm features are demonstrated. Far-field characteristics are presented for quite large-size curves strips of elliptic, parabolic, and hyperbolic profiles.Show more Item Open Access Effective preconditioners for large integral-equation problems(IET, 2007-11) Malas, Tahir; Ergül, Özgür; Gürel, LeventShow more We consider effective preconditioning schemes for the iterative solution of integral-equation methods. For parallel implementations, the sparse approximate inverse or the iterative solution of the near-field system enables fast convergence up to certain problem sizes. However, for very large problems, the near-field matrix itself becomes too crude approximation to the dense system matrix and preconditioners generated from the near-field interactions cannot be effective. Therefore, we propose an approximation strategy to the multilevel fast multipole algorithm (MLFMA) to be used as a preconditioner. Our numerical experiments reveal that this scheme significantly outperforms other preconditioners. With the combined effort of effective preconditioners and an efficiently parallelized MLFMA, we are able to solve targets with tens of millions of unknowns in a few hours.Show more Item Open Access Efficient surface integral equation methods for the analysis of complex metamaterial structures(IEEE, 2009) Yla-Oijala, P.; Ergül, Özgür; Gürel, Levent; Taskinen, M.Show more Two approaches, the multilevel fast multipole algorithm with sparse approximate inverse preconditioner and the surface equivalence principle algorithm, are applied to analyze complex three-dimensional metamaterial structures. The efficiency and performance of these methods are studied and discussed.Show more

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