Browsing by Author "Altintas, A."

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Open Access
Analysis of an arbitrary conic section profile cylindrical reflector antenna, H-polarization case
(IEEE (Institute of Electrical and Electronics Engineers), 2004) Oğuzer, T.; Nosich, A. I.; Altintas, A.
Two-dimensional scattering of waves by a perfectly electric conducting reflector having arbitrary smooth profile is studied in the H-polarization case. This is done by reducing the mixed-potential integral equation to the dual-series equations and carrying out analytical regularization. To simulate a realistic primary feed, directive incident field is taken as a complex source point beam. The proposed algorithm shows convergence and efficiency. The far field characteristics are presented for the reflectors shaped as quite large-size curved strips of elliptic, parabolic, and hyperbolic profiles.
Open Access
Analysis of an arbitrary-profile, cylindrical, impedance reflector surface illuminated by an E-polarized complex line source beam
(VSP BV, 2014) Kuyucuoglu, F.; Oǧuzer, T.; Avgin, I.; Altintas, A.
Electromagnetic scattering from a cylindrical reflector surface having an arbitrary conic section profile is studied. We assumed an electrically thin layer antenna illuminated by a complex line source in E-polarization mode. Our boundary value formulation, without loss of generality, involves an integral equation approach having impedance-type thin-layer boundary conditions. For simplicity, we also considered both faces of the reflector of the same uniform impedance value. Our computation employs the Method of Analytical Regularization (MAR) technique: the integral equations are converted into the discrete Fourier transform domain yielding two coupled dual series equations, which are then solved by the Fourier inversion and Riemann Hilbert Problem techniques. We demonstrate the accuracy and the convergence behaviors of our numerically solved MAR results that can serve as an accurate benchmark for comparison with widely used results obtained by approximate boundary conditions. © 2013 Taylor and Francis.
Open Access
Analysis of the nonconcentric radome-enclosed cylindrical reflector antenna system, e-polarization case
(Taylor & Francis, 2005) Oğuzer, T.; Altintas, A.
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.
Open Access
Analysis of the nonconcentric reflector antenna in radome system by the iterative reflector antenna and radome interaction
(Taylor & Francis, 2007) Oğuzer, T.; Altintas, A.
Nonconcentric reflector antenna-in-radome systems are used in applications especially requiring smaller radome coverages. The analysis of this two-dimensional geometry is performed for the E-polarization case. Here the geometry is decomposed into two parts and the analysis is performed by imposing the boundary conditions on each part as an iterative manner. This is known as the iterative boundary condition method in the literature. Convergence and accuracy are established numerically and it is seen that the expectations are really verified in reasonable CPU times. Also various numerical results are obtained for the description of the radiation characteristics.
Open Access
Archimedes' principle as an application of the divergence theorem
(Institute of Electrical and Electronics Engineers, 1990) Altintas, A.
A derivation is provided that allows one to obtain Archimedes' principle as an application of the divergence theorem.
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.
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.
Open Access
Characteristic basis function method for solving electromagnetic scattering problems over rough terrain profiles
(IEEE, 2010-3-1) Yagbasan, A.; Tunc, C. A.; Erturk V. B.; Altintas, A.; Mittra, R.
A computationally efficient algorithm, which combines the characteristic basis function method (CBFM), the physical optics (PO) approach (when applicable) with the forward backward method (FBM), is applied for the investigation of electromagnetic scattering fromand propagation overlarge-scale rough terrain problems. The algorithm utilizes high-level basis functions defined on macro-domains (blocks), called the characteristic basis functions (CBFs) that are constructed by aggregating low-level basis functions (i.e., conventional sub-domain basis functions). The FBM as well as the PO approach (when applicable) are used to construct the aforementioned CBFs. The conventional CBFM is slightly modified to handle large-terrain problems, and is further embellished by accelerating it, as well as reducing its storage requirements, via the use of an extrapolation procedure. Numerical results for the total fields, as well as for the path loss are presented and compared with either measured or previously published reference solutions to assess the efficiency and accuracy of the algorithm.
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.
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.
Open Access
Complex source radiation in a cylindrical radome of metal-dielectric grating
(IEEE, 1999) Altintas, A.; Ouardani, S.; Yurchenko, V. B.
The radiation fields of a line source enclosed in a circular dielectric radorne with grating consisting of an array of thin lossy metal strips are analyzed. The variations of the directivity of the source beam with respect to the beam direction are studied. The possibility of damping these variations by an appropriate design of the radome is demonstrated.
Open Access
Double-sided split-step MM-wave Fresnel lenses: design, fabrication and focal field measurements
(European Optical Society : Rapid Publications, 2014-09) Yurchenko, V. B.; Ciydem, M.; Gradziel, M.; Murphy, J. A.; Altintas, A.
Design, fabrication and focal field measurements of W-band Fresnel lenses to produce well-shaped Gaussian beams in the focal domain are presented. Two lenses, one of Teflon and another of Polyamide, have been compared in their performance with a double-sided Fresnel lens of different design, with full height of Fresnel steps. Experiments show that a good focusing ability of all the lenses is achieved. They produce the target beam according to the design. Yet, the lenses of split-step design are thinner, have lower insertion losses, and a greater focal depth as compared to more conventional Fresnel lenses.
Open Access
Effect of imperfect flat earth on the vertically polarized radiation of a cylindrical reflector antenna
(IEEE, 2000-02) Boriskina, S.; Nosich, A. I.; Altintas, A.
The radiation of a circular cylindrical reflector antenna in the presence of imperfect flat earth is treated in an accurate manner. The boundary value problem is formulated in terms of a full-wave integral equation converted to the dual-series equations and then regularized by using analytical inversion of the static part. The resulting Fredholm second-kind matrix equation is solved numerically with guaranteed accuracy. The feed directivity is included in the analysis by using the complex source-point method. Various antenna features, which include the overall directivity, efficiency, gain, and radiated and absorbed power fractions have been calculated and compared with the free-space antenna characteristics. They show some phenomena not predicted by approximate techniques.
Open Access
Effective cut-offs for modes on helical fibres
(Kluwer Academic Publishers, 1990) Altintas, A.; Love, J. D.
Every mode of a single-mode or multimode helical fibre is always leaky but, for practical purposes, can be treated as being bound with an effective cut-off wavelength. The leakage loss for each mode is quantified, showing that, for fixed core offset and source wavelength, the cut-off pitch increases with increasing mode order. The value of the cut-off pitch for each mode is in agreement with experimental measurements. © 1990 Chapman and Hall Ltd.
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.
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.
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.
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.
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.
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.
Open Access
An examination of the effect of polarization on the radiation losses of bent optical fibres
(Kluwer Academic Publishers, 1993) Altintas, A.; Tanyer, S. G.
It has long been recognized that the bending losses in weakly guiding optical fibres are independent of the polarization for large bend radius. Here, we show this fact using the volume equivalent current method. The procedure is then applied to a helically bent fibre and it is shown that the radiation from the helical fibre is also independent of the polarization as long as the fibre is weakly guiding.
Open Access
Experimental conditions for the excitation of thin disk whispering-gallery-mode resonators
(Electromagnetics Academy, 2013) Yurchenko, V. B.; Altintas, A.; Ciydem, M.; Koc, S.
Measurements of mm-wave excitation spectra of highorder whispering gallery modes in free-space cylindrical disk resonators as functions of resonator thickness have been made. Resonators in the form of tight stacks of thin dielectric disks excited via dielectric waveguides have been used in the experiment. Experimental conditions for the excitation of thin-disk resonators have been found. A simple approach for the modeling of resonator spectra and recovery of dielectric parameters has been proposed.
Open Access
Extraction of primary and secondary frequency control from active power generation data of power plants
(Elsevier Ltd, 2015) Ozer, B.; Arıkan, Orhan; Moral, G.; Altintas, A.
Frequency control is a vital component of a secure and robust power grid and it ought to be closely monitored. Frequency control consists of two main components; primary and secondary control and their contributions are usually aggregated in the active power generation data of a plant, which is acquired via Supervisory Control And Data Acquisition. In many cases, such as in Turkey, they are demanded to be evaluated separately due to different impacts on power system or different financial policies. However, this is not usually a straightforward process since primary and secondary response cannot be obtained distinctly. In this work, Extraction of Primary and Secondary Control (EPSCon) algorithm is introduced to extract primary and secondary response over active power generation data. Based on time and frequency domain characteristics of primary and secondary response, EPSCon is developed on a Expectation-Maximization type recursive scheme employing Generalized Cross Correlation and ℓ1 Trend Filtering techniques. Favorably, EPSCon uses a simple plant model built upon basic governor and plant load controller technical characteristics as an initial estimate of primary and secondary response.
Open Access
Focusing of THz waves with a microsize parabolic reflector made of graphene in the free space
(Springer International Publishing, 2017) Oguzer T.; Altintas, A.; Nosich A.I.
Background: The scattering of H- and E-polarized plane waves by a two-dimensional (2-D) parabolic reflector made of graphene and placed in the free space is studied numerically. Methods: To obtain accurate results we use the Method of Analytical Regularization. Results: The total scattering cross-section and the absorption cross-section are computed, together with the field magnitude in the geometrical focus of reflector. The surface plasmon resonances are observed in the H-case. The focusing ability of the reflector is studied in dependence of graphene’s chemical potential, frequency, and reflector’s depth. Conclusions: It is found that there exists an optimal range of frequencies where the focusing ability reaches maximum values. The reason is the quick degradation of graphene’s surface conductivity with frequency. © 2017, The Author(s).
Open Access
Integral equation anlaysis of an arbitrary-profile and varying-resistivity cylindrical reflector illuminated by an E-polarized complex-source-point beam
(Optical Society of America, 2009-06-09) Oguzer, T.; Altintas, A.; Nosich, A. I.
A two-dimensional reflector with resistive-type boundary conditions and varying resistivity is considered. The incident wave is a beam emitted by a complex-source-point feed simulating an aperture source. The problem is formulated as an electromagnetic time-harmonic boundary value problem and cast into the electric field integral equation form. This is a Fredholm second kind equation that can be solved numerically in several ways. We develop a Galerkin projection scheme with entire-domain expansion functions defined on an auxiliary circle and demonstrate its advantage over a conventional moment-method solution in terms of faster convergence. Hence, larger reflectors can be computed with a higher accuracy. The results presented relate to the elliptic, parabolic, and hyperbolic profile reflectors fed by in-focus feeds. They demonstrate that a partially or fully resistive parabolic reflector is able to form a sharp main beam of the far-field pattern in the forward half-space; however, partial transparency leads to a drop in the overall directivity of emission due to the leakage of the field to the shadow half-space. This can be avoided if only small parts of the reflector near the edges are made resistive, with resisitivity increasing to the edge.