Browsing by Author "Ertürk, Vakur B."

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Open Access
Analysis of cylindrically conformal antennas using closed-form Green's function representations
(IEEE, 2015-04) Kalfa, Mert; Karan, S.; Ertürk, Vakur B.
Probe-fed microstrip patch antennas and slotted sectoral waveguide array antennas embedded in cylindrically stratified media are analyzed with a hybrid Method of Moments/Green's function technique, where closed-form Green's function representations for electric and magnetic current sources are used as the kernel of the associated integral equations. Various patch and slot antennas are analyzed using the proposed method. Numerical results in the form of input impedance, S-parameters, and radiation patterns are presented and compared to the results obtained from CST Microwave Studio™ and HFSS™.
Open Access
Analysis of slotted sectoral waveguide array antennas with multilayer radomes and nonzero metal thickness
(IEEE, 2015-07) Kalfa, Mert; Ertürk, Vakur B.
Slotted waveguide array antennas offer low cross-polarization and high power handling capacity, and they are low-profile which enables them to be used in conformal and structurally-integrated antenna solutions. Hence, they are excellent candidates for phased array antennas in radar applications, especially in air platforms. Aerodynamics and radar cross section (RCS) for an air platform are critical design considerations for air platforms; therefore, conformal and structurally-integrated solutions with integrated multilayer (sandwich) radomes are desired. Although the accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays is 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 when cylindrically stratified media are considered. © 2015 IEEE.
Open Access
Application of fast methods to large scale rough surface scattering and propagation problems
(2006) Altıntaş, Ayhan; Ertürk, Vakur B.; Tunç, Celal; Topçu, Satılmış
Open Access
Broadband analysis of multiscale electromagnetic problems: Novel incomplete-leaf MLFMA for potential integral equations
(IEEE, 2021-06-24) Khalichi, Bahram; Ergül, Ö.; Takrimi, Manouchehr; Ertürk, Vakur B.
Recently introduced incomplete tree structures for the magnetic-field integral equation are modified and used in conjunction with the mixed-form multilevel fast multipole algorithm (MLFMA) to employ a novel broadband incomplete-leaf MLFMA (IL-MLFMA) to the solution of potential integral equations (PIEs) for scattering/radiation from multiscale open and closed surfaces. This population-based algorithm deploys a nonuniform clustering that enables to use deep levels safely and, when necessary, without compromising the accuracy resulting in an improved efficiency and a significant reduction for the memory requirements (order of magnitudes), while the error is controllable. The superiority of the algorithm is demonstrated in several canonical and real-life multiscale geometries.
Open Access
Broadband multilevel fast multipole algorithm for large-scale problems with nonuniform discretizations
(IEEE, 2016) Ergül, Ö.; Karaosmanoğlu, B.; Takrimi, Manouchehr; Ertürk, Vakur B.
We present a broadband implementation of the multilevel fast multipole algorithm (MLFMA) for fast and accurate solutions of multiscale problems involving highly nonuniform discretizations. Incomplete tree structures, which are based on population-based clustering with flexible leaf-level boxes at different levels, are used to handle extremely varying triangulation sizes on the same structures. Superior efficiency and accuracy of the developed implementation, in comparison to the standard and broadband MLFMA solvers employing conventional tree structures, are demonstrated on practical problems.
Open Access
A broadband multilevel fast multipole algorithm with incomplete-leaf tree structures for multiscale electromagnetic problems
(IEEE, 2016) Takrimi, Manouchehr; Ergül, Ö.; Ertürk, Vakur B.
An efficient, broadband, and accurate multilevel fast multipole algorithm (MLFMA) is proposed to solve a wide range of multiscale electromagnetic problems with orders of magnitude differences in the mesh sizes. Given a maximum RWG population threshold, only overcrowded boxes are recursively bisected into smaller ones, which leads to novel incomplete-leaf tree structures. Simulations reveal that, for surface discretizations possessing highly overmeshed local regions, the proposed method presents a more efficient and/or accurate results than the conventional MLFMA. The key feature of such a population-based clustering scenario is that the error is controllable, and hence, regardless of the number of levels, the efficiency can be optimized based on the population threshold. Numerical examples are provided to demonstrate the superior efficiency and accuracy of the proposed algorithm in comparison to the conventional MLFMA.
Open Access
Broadband solutions of potential integral equations with NSPWMLFMA
(IEEE, 2019-06) Khalichi, Bahram; Ergül, Ö.; Ertürk, Vakur B.
In this communication, a mixed-form multilevel fast multipole algorithm (MLFMA) is combined with the recently introduced potential integral equations (PIEs), also called as the A-φ system, to obtain an efficient and accurate broadband solver that can be used for the solution of electromagnetic scattering from perfectly conducting surfaces over a wide frequency range including low frequencies. The mixed-form MLFMA uses the nondirective stable planewave MLFMA (NSPWMLFMA) at low frequencies and the conventional MLFMA at middle/high frequencies. Various numerical examples are presented to assess the validity, efficiency, and accuracy of the developed solver.
Open Access
Computation of surface fields excited on arbitrary smooth convex surfaces with an impedance boundary condition
(2012) Alişan, Burak; Ertürk, Vakur B.
Open Access
Derivation of Green's function representations for the analysis of sectoral waveguides embedded in cylindrically stratified media
(IEEE, 2013) Kalfa, Mert; Ertürk, Vakur B.
The design and analysis of dielectric-covered slotted waveguide arrays are of great interest in many military and civil applications, because of their low-profile, high efficiency and high power handling capabilities. Regarding the efficient and accurate analysis of such antennas, integral equation (IE) based methods that utilize the method of moments (MoM) or a hybrid combination of MoM with an appropriate Green's function (referred to as MoM/Green's function technique) are widely used. For the hybrid MoM/Green's function technique, appropriate dyadic Green's function representations for the waveguide interior and the exterior stratified media are required. However, in the case of a cylindrically conformal dielectric-covered slotted waveguide array, which may be desired due to aerodynamic and/or radar cross section (RCS) concerns, the abovementioned IE-based analysis that utilize a hybrid MoM/Green's function technique becomes a greater challenge due to the difficulties in the evaluation of the entries of MoM impedance/admittance matrices, especially for the terms related to cylindrically stratified media. © 2013 IEEE.
Open Access
Efficient analysis of large finite arrays via mom formulation with dft based acceleration algorithms
(2003) Çivi, Ö. A.; Ertürk, Vakur B.; Chou, H.-T.
A DFT based acceleration algorithm is combined with iterative methods to accelerate the computation of Method of Moments (MoM) analysis of electromagnetic radiation/scattering from large, finite free-standing and printed elements of phased arrays. Computational complexity of this approach is O(Ntot), where Ntot is the number of unknowns. Numerical results are presented to validate the efficiency and accuracy of the method.
Open Access
An electromagnetic sensing system incorporating multiple probes and single antenna for wireless structural health monitoring
(IEEE, 2017) Özbey, Burak; Altıntaş, Ayhan; Demir, Hilmi Volkan; Ertürk, Vakur B.; Kurç, Ö.
In this study, a wireless and passive displacement/strain sensing system is proposed for structural health monitoring (SHM). The wireless and passive interrogation of the sensing unit [a variant of a nested split-ring resonator (NSRR)] is achieved through the near-field interaction and electromagnetic coupling between the single antenna in the system and the multiple sensors called the NSRR probes. It is demonstrated that the system can acquire data from more than one NSRR probe simultaneously in a real-life scenario, where the probes are confined within concrete inside a beam, while the antenna monitors them from outside.
Open Access
Error analysis of MLFMA with closed-form expressions
(IEEE, 2021-04-06) Kalfa, Mert; Ertürk, Vakur B.; Ergül, Özgür
The current state-of-the-art error control of the multilevel fast multipole algorithm (MLFMA) is valid for any given error threshold at any frequency, but it requires a multiple-precision arithmetic framework to be implemented. In this work, we use asymptotic approximations and curve-fitting techniques to derive accurate closed-form expressions for the error control of MLFMA that can be implemented in common fixed-precision computers. Moreover, using the proposed closed-form expressions in conjunction with the state-of-the-art scheme, we report novel design curves for MLFMA that can be used to determine achievable error limits, as well as the minimum box sizes that can be solved with a given desired error threshold for a wide range of machine precision levels.
Open Access
Error control in MLFMA with multiple-precision arithmetic
(Institution of Engineering and Technology, 2018-04) Kalfa, Mert; Ergül, Ö.; Ertürk, Vakur B.
We present a new error control method that provides the truncation numbers as well as the required digits of machine precision for the translation operator of the multilevel fast multipole algorithm (MLFMA). The proposed method is valid for all frequencies, whereas the previous studies on error control are valid only for high-frequency problems (i.e., electrically large translation distances). When combined with a multiple-precision implementation of MLFMA, the proposed method can be used to solve low-frequency problems that are problematic with a fixed-precision implementation. Numerical results in the form of optimal truncation numbers and machine precisions for a variety of box sizes and desired relative error thresholds are presented and compared with the methods or numerical surveys available in the literature.
Open Access
Extension of forward backward method with DFT based acceleration algorithm for the efficient analysis of large periodic arrays with arbitrary boundaries
(IEEE, 2003) Çivi, Ö. A.; Chou, H. T.; Ertürk, Vakur B.
An extension of Forward-Backward (FB) with Discrete Fourier Transform (DFT) based acceleration approach is presented. This is given to provide a relatively efficient analysis of EM radiation/scattering from an electrically large, planar, periodic, finite arrays with arbitrary boundaries, such as arrays with circular or elliptical boundaries. It is shown that only very few significant DFT terms are sufficient to provide accurate results.
Open Access
Fast acceleration algorithm based on DFT expansion for the iterative MoM analysis of electromagnetic radiation/scattering from two-dimensional large phased arrays
(IEEE, 2002) Ertürk, Vakur B.; Chou, H. T.
An acceleration algorithm based on Discrete Fourier Transform (DFT) is developed to reduce the computational complexity and memory storages of iterative methods of moment (IMoM) solution to O(Ntot), where Ntot is the total number of elements in the array. As such, numerical results for free-standing dipoles obtained using IMoM-DFT approach are presented and compared with the conventional MoM solution.
Open Access
Fast integral equation techniques for propagation problems
(IEEE, 2004-09) Babaoğlu, Barış; Tunç, Celal A.; Altıntaş, Ayhan; Ertürk, Vakur B.
In this paper, the Method of Moments (MoM) solution is achieved for scattering problems by using the stationary Spectrally Accelerated Forward-Backward method (FBSA) and the non-stationary Spectrally Accelerated BiConjugate Gradient Stabilized (SA-BiCGSTAB) method, with a storage requirement and a computational cost of O(N) per iteration where N is the number of surface unknowns in the discretized integral equation. The SA-BiCGSTAB method is applied over rough terrain profiles as well as re-entrant surfaces which can not be handled by any conventional stationary iterative technique.
Open Access
Geniş ölçekli pürüzlü yüzeylerden saçınım ve yayınım problemleri için hızlı yöntemler ve uygulamaları
(2006) Ertürk, Vakur B.; Tunç, Celal; Topçu, Satılmış; Altıntaş, Ayhan
Open Access
Integral equation based method for the fast analysis of irregularly contoured large finite phased arrays
(Institution of Engineering and Technology, 2007) Ertürk, Vakur B.; Çivi, Ö. A.
A fast and accurate integral equation based hybrid method that can investigate electrically large, arbitrarily contoured finite planar arrays of printed elements is developed. The method is a hybridization of the Galerkin type method of moments (MoM) and generalized forward backward method (GFBM) with the grounded dielectric slab's Green's function; and the acceleration of the resultant hybrid method by a discrete Fourier transform (DFT) based acceleration algorithm. Numerical results in the form of array current distribution are given for arbitrarily contoured as well as thinned arrays of probe fed microstrip patches where current on each element expanded by more than one subsectional basis function.
Open Access
Investigation of metamaterial coated conducting cylinders for achieving transparency and maximizing radar cross section
(IEEE, 2007) Ircı, Erdinç; Ertürk, Vakur B.
Recently, reducing the radar cross sections (RCS) of various structures to achieve transparency and obtaining resonant structures aimed at increasing the electromagnetic intensities, stored or radiated power levels have been investigated. The transparency and resonance (RCS maximization) conditions investigated in are mainly attributed to pairing of "conjugate" materials: materials which have opposite signs of constitutive parameters [e.g., double-positive (DPS) and double- negative (DNG) or epsilon-negative (ENG) and mu-negative (MNG)]. In the present work, we extend the transparency and resonance conditions for cylindrical structures when the core cylinder is particularly perfect electric conductor (PEC). The appropriate constitutive parameters of such metamaterials are investigated for both TE and TM polarizations. For TE polarization it is found out that, the metamaterial coating permittivity has to be in the 0 < epsivc < epsiv0 interval to achieve transparency, and in the -epsiv0 < epsivc < 0 interval to achieve RCS maximization. As in the case of "conjugate" pairing, transparency and resonance are found to be heavily dependent on the ratio of core-coating radii, instead of the total size of the cylindrical structure. However, unlike the "conjugate" pairing cases, replacing epsiv by mu (and vice versa) does not lead to the same conclusions for TM polarization unless the PEC cylinder is replaced by a perfect magnetic conductor (PMC) cylinder. Yet, RCS maximization can also be achieved in the TM polarization case when coating permeability muc < 0, whereas transparency requires large \muc\ for this polarization. Numerical results, which demonstrate the transparency and RCS maximization phenomena, are given in the form of normalized monostatic and bistatic echo widths.
Open Access
Investigation of planar and conformal printed arrays for MIMO performance analysis
(IEEE, 2006) Tunç, Celal Alp; Ircı, Erdinç; Bakır, Onur; Aktaş, Defne; Ertürk, Vakur B.; Altıntaş, Ayhan
MIMO channel capacity of printed arrays with dipole elements is analyzed. A MIMO channel model based on electric fields is used. The effects of mutual interactions among the array elements through space and surface waves are included into the channel matrix using a full-wave hybrid Method of Moments (MoM)/Green's function technique in the spatial domain. MIMO capacity of printed arrays is then compared with that of free standing thin wire dipole arrays. Results show better performance of printed arrays.