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Item Open Access Achieving transparency and maximizing scattering with metamaterial-coated conducting cylinders(American Physical Society, 2007) Irci, E.; Ertürk, V. B.Show more In this work, the electromagnetic interaction of plane waves with infinitely long metamaterial-coated conducting cylinders is considered. Different from "conjugate" pairing of double-positive (DPS) and double-negative (DNG) or epsilon-negative (ENG) and mu-negative (MNG) concentric cylinders, achieving transparency and maximizing scattering are separately achieved by covering perfect electric conductor (PEC) cylinders with simple (i.e., homogeneous, isotropic, and linear) metamaterial coatings. The appropriate constitutive parameters of such metamaterials are investigated for Transverse Magnetic (TM) and in particular for Transverse Electric (TE) polarizations. For TE polarization it is found out that the metamaterial-coating permittivity has to be in the 0< εc < ε0 interval to achieve transparency, and in the - ε0 < εc <0 interval to achieve scattering maximization. However, unlike the "conjugate" pairing of DPS-DNG or ENG-MNG cases, when the transparency for metamaterial-coated PEC cylinders are considered, the analytically found relation between εc and the ratio of core-coating radii, γ, should be modified in a sense that scattering from the PEC core is canceled by the coating. Furthermore, replacing ε by μ (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. On the other hand, scattering maximization can also be achieved in the TM polarization case when coating permeability μc <0, whereas transparency requires large | μc | for this polarization. Numerical results in the form of normalized monostatic and bistatic echo widths, which demonstrate the transparency and scattering maximization phenomena, are given and possible application areas are discussed.Show more Item Open Access Analysis of finite arrays of axially directed printed dipoles on electrically large circular cylinders(IEEE, 2004) Ertürk, V. B.; Rojas, R. G.; Lee, K. W.Show more Various arrays consisting of finite number of printed dipoles on electrically large dielectric coated circular cylinders are investigated using a hybrid method of moments/Green's function technique in the spatial domain. This is basically an "element by element" approach in which the mutual coupling between dipoles through space as well as surface waves is incorporated. The efficiency of the method comes from the computation of the Green's function, where three types of spatial domain Green's function representations are used interchangeably, based on their computational efficiency and regions where they remain accurate. Numerical results are presented in the form of array current distributions, active reflection coefficient and far-field pattern to indicate the efficiency and accuracy of the method. Furthermore, these results are compared with similar results obtained from finite arrays of printed dipoles on grounded planar dielectric slabs. It is shown that planar approximations, except for small separations, can not be used due to the mutual coupling between the array elements. Consequently, basic performance metrics of printed dipole arrays on coated cylinders show significant discrepancies when compared to their planar counterparts. © 2004 IEEE.Show more Item Open Access Analysis of finite arrays of circumferentially oriented printed dipoles on electrically large cylinders(Wiley, 2004) Ertürk, V. B.; Güner, B.Show more An efficient and accurate hybrid method of moments (MoM)/Green's function technique in the spatial domain is developed for the rigorous analysis of large, finite phased arrays of circumferentially oriented printed dipoles on electrically large, dielectric-coated, circular cylinders. Basic performance metrics (in the form of array current distribution, active reflection coefficient, far-field patterns, and so forth) of several arrays have been obtained and compared with similar printed arrays on grounded planar substrates. Certain discrepancies have been observed and discussed. © 2004 Wiley Periodicals, Inc.Show more Item Open Access Analysis of slotted sectoral waveguide arrays with multilayered radomes(Institute of Electrical and Electronics Engineers Inc., 2016) Kalfa, M.; Ertürk, V. B.Show more A method of moments/Green's function (MoM/GF) technique in the space domain is used for the rigorous and fast analysis of cylindrically conformal slotted sectoral waveguide arrays (SSWGAs) in the presence of multilayered cylindrical dielectric radomes. Representing the slots by fictitious magnetic currents via the equivalence principle, the geometry is divided into two regions and separate GF representations for each region are developed to be used in conjunction with the MoM procedure. Particularly, in the region that constitutes the cylindrically stratified medium, the newly developed closed-form GF representations for magnetic currents are valid for all source and observation points, including the source region (where source and observation points are on the top of each other). Basic performance metrics of an SSWGA such as equivalent slot currents, S-parameters, radiation patterns in the presence of a multilayered cylindrical radome are presented to assess the accuracy and efficiency of the proposed technique. © 1963-2012 IEEE.Show more Item Open Access Application of iterative techniques for electromagnetic scattering from dielectric random and reentrant rough surfaces(Institute of Electrical and Electronics Engineers, 2006) Inan, K.; Ertürk, V. B.Show more Stationary [e.g., forward-backward method (FBM)] and nonstationary [e.g., conjugate gradient squared, quasi-minimal residual, and biconjugate gradient stabilized (Bi-CGSTAB)] iterative techniques are applied to the solution of electromagnetic wave scattering from dielectric random rough surfaces with arbitrary complex dielectric constants. The convergence issues as well as the efficiency and accuracy of all the approaches considered in this paper are investigated by comparing obtained scattering (in the form of normalized radar cross section) and surface field values with the numerically exact solution, computed by employing the conventional method of moments. It has been observed that similar to perfectly and imperfectly conducting rough surface cases, the stationary iterative FBM converges faster when applied to geometries yielding best conditioned systems but exhibits convergence difficulties for general geometries due to its inherit limitations. However, nonstationary techniques are, in general, more robust when applied to arbitrarily general dielectric random rough surfaces, which yield more ill-conditioned systems. Therefore, they might prove to be more suitable for general scattering problems. Besides, as opposed to the perfectly and imperfectly conducting rough surface cases, the Bi-CGSTAB method and FBM show two interesting behaviors for dielectric rough surface profiles: 1) FBM generally converges for reentrant surfaces when the vertical polarization is considered and 2) the Bi-CGSTAB method has a peculiar convergence problem for horizontal polarization. Unlike the other nonstationary iterative techniques used in this paper, where a Jacobi preconditioner is used, convergent results are obtained by using a block-diagonal preconditioner.Show more Item Open Access Capacity of printed dipole arrays in the MIMO channel(Institute of Electrical and Electronics Engineers, 2008-10) Tunc, C. A.; Aktas, D.; Ertürk, V. B.; Altintas, A.Show more Moments performance of printed dipole arrays in the MIMO channel is investigated using a channel model based on the Method of solution of the electric-field integral equation. Comparisons with freestanding dipoles are given in terms of channel capacity. Effects of the electrical properties (such as the dielectric thickness and permittivity) on the MIMO capacity are explored. Various dielectric-substrate configurations yielding high-capacity MIMO arrays are presented.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 Closed-form Green's function representations in cylindrically stratified media for method of moments applications(IEEE, 2009) Karan, S.; Ertürk, V. B.; Altintas, A.Show more Closed-form Green's function (CFGF) representations for cylindrically stratified media, which can be used as the kernel of an electric field integral equation, are developed. The developed CFGF representations can safely be used in a method of moments solution procedure, as they are valid for almost all possible source and field points that lie on the same radial distance from the axis of the cylinder (such as the air-dielectric and dielectric-dielectric interfaces) including the axial line (ρ = ρ′ and φ = φ′), which has not been available before. In the course of obtaining these expressions, the conventional spectral domain Green's function representations are rewritten in a different form so that i) we can attack the axial line problem and ii) the method can handle electrically large cylinders. Available acceleration techniques that exist in the literature are implemented to perform the summation over the cylindrical eigenmodes efficiently. Lastly, the resulting expressions are transformed to the spatial domain using the discrete complex image method with the help of the generalized pencil of function method, where a modified two-level approach is used. Numerical results are presented in the form of mutual coupling between two current modes to assess the accuracy of the final spatial domain CFGF representations.Show more Item Unknown A closed‐form solution to the asymptotic part of the MOM impedance matrix and the MOM excitation vector for printed structures on planar grounded dielectric slabs(John Wiley & Sons, Inc., 2007) Bakýr O.; Ertürk, V. B.Show more In the spectral domain method of moments (MoM) solution of printed structures on planar grounded dielectric slabs, the infinite double integrals which appear in the asymptotic parts of the MoM impedance matrix and the MoM excitation vector elements, have been previously transformed to one-dimensional finite integrals, which have been numerically computed using the highly specialized "International Mathematics and Statistics Library" subroutines. In this paper, these one-dimensional integrals are evaluated in closed-form, resulting in an improved efficiency and accuracy for the rigorous investigation of printed antennas and complex millimeter and microwave integrated circuits. Numerical results in the form of mutual impedance between two expansion functions and input impedance of various microstrip antennas are presented to assess the accuracy of these closed-form expressions.Show more Item Open Access A comparative investigation of SRR‐ and CSRR‐based band‐reject filters: Simulations, experiments, and discussions(Wiley-Blackwell, 2008) Öznazı, V.; Ertürk, V. B.Show more A comparative investigation of split-ring resonator (SRR)and complementary split-ring resonator (CSRR)-based band-reject filters is performed. These compact band-reject filters are obtained by loading simple 50-Ω microstrip lines with SRRs and CSRRs that have exactly the same shape and dimensions. Unlike the previous studies, stopband characteristics of these filters, such as resonance frequency, band-width, sharpness, and amount of attenuation in the rejection region based on the number of SRR or CSRR stages, are investigated in a detailed and comparative manner. Based on simulations that are accompanied by experimental results, it has been observed that some of the aforementioned stopband characteristics of SRR-based band-reject filters are significantly different than those of CSRR-based band-reject filters. This makes SRR- or CSRR-based filters preferable depending on applications.Show more Item Open Access Determining the effective constitutive parameters of finite periodic structures: Photonic crystals and metamaterials(Institute of Electrical and Electronics Engineers, 2008) Aksun, M. I.; Alparslan, A.; Karabulut, E.P .; Irci, E.; Ertürk, V. B.Show more A novel approach to find the effective electric and magnetic parameters of finite periodic structures is proposed. The method uses the reflection coefficients at the interface between a homogenous half-space and the periodic structure of different thicknesses. The reflection data are then approximated by complex exponentials, from which one can deduce the wavenumber, and the effective electric and magnetic properties of the equivalent structure by a simple comparison to the geometrical series representation of the generalized reflection from a homogenous slab. Since the effective parameters are for the homogenous equivalent of the periodic structure, the results obtained are expected to be independent of the number of unit cells used in the longitudinal direction. Although the proposed method is quite versatile and applicable to any finite periodic structure, photonic crystals and metamaterials with metallic inclusions have been used to demonstrate the application of the method in this paper.Show more Item Open Access Efficient analysis of input impedance and mutual coupling of microstrip antennas mounted on large coated cylinders(IEEE, 2003-04) Ertürk, V. B.; Rojas, R. G.Show more An efficient and accurate hybrid method, based on the combination of the method of moments (MoM) with a special Green’s function in the space domain is presented to analyze antennas and array elements conformal to electrically large material coated circular cylinders. The efficiency and accuracy of the method depend strongly on the computation of the Green’s function, which is the kernel of the integral equation that is solved via MoM for the unknown equivalent currents representing only the antenna elements. Three types of space-domain Green’s function representations are used, each accurate and computationally efficient in a given region of space. Consequently, a computationally optimized analysis tool for conformal microstrip antennas is obtained. Input impedance of various microstrip antennas and mutual coupling between two identical antennas are calculated and compared with published results to assess the accuracy of this hybrid method.Show more Item Open Access Efficient analysis of large phased arrays using iterative MoM with DFT-based acceleration algorithm(John Wiley & Sons, Inc., 2003) Ertürk, V. B.; Chou, H-T.Show more A discrete Fourier transform (DFT)-based iterative method of moments (IMoM) algorithm is developed to provide an O(Ntot) computational complexity and memory storages for the efficient analysis of electromagnetic radiation/scattering from large phased arrays. Here, Ntot is the total number of unknowns. Numerical results for both printed and free-standing dipole arrays are presented to validate the algorithm's efficiency and accuracy.Show more Item Open Access Efficient analysis of phased arrays of microstrip patches using a hybrid generalized forward backward method/green's function technique with a DFT based acceleration algorithm(Institute of Electrical and Electronics Engineers, 2008) Bakir, O.; Çivi, Ö. A.; Ertürk, V. B.; Chou, H.-T.Show more A hybrid method based on the combination of generalized forward backward method (GFBM) and Green's function for the grounded dielectric slab together with the acceleration of the combination via a discrete Fourier transform (DFT) based algorithm is developed for the efficient and accurate analysis of electromagnetic radiation/scattering from electrically large, irregularly contoured two-dimensional arrays consisting of finite number of probe-fed microstrip patches. In this method, unknown current coefficients corresponding to a single patch are first solved by a conventional Galerkin type hybrid method of moments (MoM)/Green's function technique that uses the grounded dielectric slab's Green's function. Because the current distribution on the microstrip patch can be expanded using an arbitrary number of subsectional basis functions, the patch can have any shape. The solution for the array currents is then found through GFBM, where it sweeps the current computation element by element. The computational complexity of this method, which is originally O(Ntot 2 being the total number of unknowns) for each iteration, is reduced to O(Ntot) using a DFT based acceleration algorithm making use of the fact that array elements are identical and the array is periodic. Numerical results in the form of array current distribution are given for various sized arrays of probe-fed microstrip patches with elliptical and/or circular boundaries, and are compared with the conventional MoM results to illustrate the efficiency and accuracy of the method.Show more Item Open Access Efficient computation of nonparaxial surface fields excited on an electrically large circular cylinder with an impedance boundary condition(Institute of Electrical and Electronics Engineers, 2006) Alisan, B.; Ertürk, V. B.; Altintas, A.Show more An alternative numerical approach is presented for the evaluation of the Fock-type integrals that exist in the uniform geometrical theory of diffraction (UTD)-based asymptotic solution for the nonparaxial surface fields excited by a magnetic or an electric source located on the surface of an electrically large circular cylinder with an impedance boundary condition (IBC). This alternative approach is based on performing numerical integration of the Fock-type integrals on a deformed path on which the integrands are nonoscillatory and rapidly decaying. Comparison of this approach with the previously developed one presented in [1], which is based on invoking the Cauchy's residue theorem by finding the pole singularities numerically, reveals that the alternative approach is considerably more efficient.Show more Item Open Access An equivalent circuit model for nested split-ring resonators(Institute of Electrical and Electronics Engineers Inc., 2017) Ozbey, B.; Altintas, A.; Demir, Hilmi Volkan; Ertürk, V. B.Show more In this paper, an equivalent circuit model for nested split-ring resonators (NSRRs) is proposed. NSRRs are an emerging class of split ring resonators, preferred in a range of areas from sensing in biomedical or civil engineering applications to antenna design, due to their more compact size and enhanced sensitivity/resolution characteristics over the conventional SRRs. In the proposed model, the NSRR structure is treated as a combination of basic elements, i.e., strips and gaps, and the electromagnetic characteristics of the whole geometry are expressed in terms of capacitances and inductances of each of these elements. The outputs of the model are compared with those obtained via full-wave simulations using the package programs as well as measurements. The variation of NSRR resonance frequency (fres) with all important design parameters is also compared with full-wave simulations. In all comparisons, the results demonstrate agreement, showing that the proposed model can correctly explain the electromagnetics of the NSRR structure and that it provides an intuitive way for a better and easier analysis and a preliminary design of normally complex structures.Show more Item Open Access Examination of existent propagation models over large inhomogeneous terrain profiles using fast integral equation solution(Institute of Electrical and Electronics Engineers, 2005-09) Tunc, C. A.; Altintas, A.; Ertürk, V. B.Show more The accuracyof most widelyused empirical models are investigated using the spectrallyaccelerated forward-backward (FBSA) method as a benchmark solution. First, FBSA results are obtained for propagation over large scale terrain profiles and compared with measurements to assess the accuracyof FBSA. Then, accuracyof some International Telecommunication Union (ITU) and Federal Communications Commission (FCC) propagation models are investigated. It has been observed that, for rural areas, the prediction of the most recent ITU recommended propagation model (Rec. 1546) deviates much more than older models do.Show more Item Open Access Extension of forward-backward method with DFT-based acceleration algorithm for the efficient analysis of large periodic arrays with arbitrary boundaries(John Wiley & Sons, 2005) Civi, Ö. A.; Ertürk, V. B.; Chou, H.-T.Show more An extension of the discrete Fourier transform (DFT)-based forward-backward algorithm is developed using the virtual-element approach to provide a fast and accurate analysis of electromagnetic radiation/scattering from electrically large, planar, periodic, finite (phased) arrays with arbitrary boundaries. Both the computational complexity and storage requirements of this approach are O(Ntot) (Ntot is the total number of unknowns). The numerical results for both printed and freestanding dipole arrays with circular and/or elliptical boundaries are presented to validate the efficiency and accuracy of this approach.Show more Item Open Access Frequency‐selective loading for a transmitting active integrated antenna(Wiley, 2001-10-05) Ertürk, V. B.; Rojas, R. G.; Roblin, P.Show more A simple frequency‐selective load in the form of a compensating network is attached to an oscillator‐type transmitting active integrated antenna (AIA) to suppress undesired low‐frequency oscillations and to improve the robustness of the oscillator. Although the input impedance of the active device (transistor) remains unchanged at the upper frequencies, it changes drastically at the lower frequencies.Show more Item Open Access Incomplete-leaf multilevel fast multipole algorithm for multiscale penetrable objects formulated with volume integral equations(Institute of Electrical and Electronics Engineers Inc., 2017) Takrimi, M.; Ergül, Ö.; Ertürk, V. B.Show more Recently introduced incomplete-leaf (IL) tree structures for multilevel fast multipole algorithm (referred to as IL-MLFMA) is proposed for the analysis of multiscale inhomogeneous penetrable objects, in which there are multiple orders of magnitude differences among the mesh sizes. Considering a maximum Schaubert-Wilton-Glisson function population threshold per box, only overcrowded boxes are recursively divided into proper smaller boxes, leading to IL tree structures consisting of variable box sizes. Such an approach: 1) significantly reduces the CPU time for near-field calculations regarding overcrowded boxes, resulting a superior efficiency in comparison with the conventional MLFMA where fixed-size boxes are used and 2) effectively reduces the computational error of the conventional MLFMA for multiscale problems, where the protrusion of the basis/testing functions from their respective boxes dramatically impairs the validity of the addition theorem. Moreover, because IL-MLFMA is able to use deep levels safely and without compromising the accuracy, the memory consumption is significantly reduced compared with that of the conventional MLFMA. Several examples are provided to assess the accuracy and the efficiency of IL-MLFMA for multiscale penetrable objects.Show more