Browsing by Subject "Radar cross section"
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Item Open Access A comparative study of RCS predictions of canonical rectangular and circular cavities with double-layer material coating(Denshi Jouhou Tsuushin Gakkai,Institute of Electronics Information and Communication Engineers, 1997-11) Koshikawa, S.; Çolak, D.; Altıntaş, A.; Kobayashi, K.; Nosich, A. I.A rigorous radar cross section (RCS) analysis is carried out for two-dimensional rectangular and circular cavities with double-layer material loading by means of the Wiener-Hopf (WH) technique and the Riemann-Hilbert problem (RHP) technique, respectively. Both E and H polarizations are treated. The WH solution for the rectangular cavity and the RHP solution for the circular cavity involve numerical inversion of matrix equations. Since both methods take into account the edge condition explicitly, the convergence of the WH and RHP solutions is rapid and the final results are valid over a broad frequency range. Illustrative numerical examples on the monostatic and bistatic RCS are presented for various physical parameters and the far field scattering characteristics are discussed in detail. It is shown that the double-layer lossy material loading inside the cavities leads to the significant RCS reduction.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.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.Item Open Access The examination of new equivalent edge currents in the prediction of high frequency backscattering from flat plates(Bilkent University, 1991) Oğuzer, TanerEquivalent edge currents based on the geometrical theory of dilfraction (GTD) have been utilized for the prediction of electromagnetic scattering from edged bodies. These equivalent currents are use Keller’s diffraction coefficient and therefore not valid for arbitrary aspect of observation. More general expressions for equivalent edge currents are later obtained by Michaeli. Those expressions become infinite at certain observation directions. These infinities are later eliminated by the same author for the fringe component of the equivalent currents l)y choosing a skew coordinate system on the half plane to be used for the asymptotic integration. A similar approach is employed here to eliminate the infinities in the physical optics(PO) component of the equivalent edge currents. It is also shown that the radiation from the fringe and PO equivalent currents is unique and yields the GTD field. The fringe and PO equivalent currents are then applied to the backscattering problems from the perfectly conducting square and triangular plates. The higher order interactions between the edges are also included into the analysis. Some improvements are obtained over the previous solutions.Item Open Access Fast solution of electromagnetic scattering problems with multiple excitations using the recompressed adaptive cross approximation(IEEE, 2014) Kazempour, Mahdi; Gürel, LeventWe present an algebraic compression technique to accelerate the computation of multiple monostatic radar cross sections of arbitrary 3-D geometries. The method uses adaptive cross approximation, followed by a recompression technique to reduce the CPU time and the memory consumption. Each scattering problem due to a single excitation is solved with the multilevel fast multipole algorithm. The numerical results demonstrate the efficiency and accuracy of the proposed method. © 2014 IEEE.Item 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.Item Open Access Large flat plate models in the physical optics method for RCS calculations(IEEE, 2004-09) Altıntaş, Ayhan; Çelik, AslıhanThe calculation of Radar Cross Section (RCS) of arbitrarily large perfectly conducting body is presented. The body is modelled as triangular meshes of any size by the help of graphical tools. For the calculation of scattered field, Physical Optics(PO) surface integral is analytically evaluated over each of the triangular meshes. Due to the analytical integration, there is no limitation on the size of the triangles.Item Open Access Manipulation of backscattering from a dielectric cylinder of triangular cross-section using the interplay of go-like ray effects and resonances(Institute of Electrical and Electronics Engineers, 2015) Sukharevsky, Ilya O.; Nosich, A. I.; Altıtaş, AyhanA triangular dielectric cylinder (dielectric prism) of the size, in cross-section, comparable to or moderately larger than the wavelength is a scatterer, which blends together two different types of electromagnetic behavior: geometrical optics (GO) and resonance. As shown in this paper, the first is responsible, for instance, for enhanced reflection from an isosceles 90° prism, if illuminated from the base. The second is responsible for the peaks in the total scattering and absorption cross-sections (ACSs) at the natural-mode frequencies. The numerical analysis is performed by solving the well-conditioned Muller-type boundary integral equation (IE) discretized using an algorithm with controlled accuracy.Item Open Access Radar cross-section study of cylindrical cavity-backed apertures with outer or inner material coating: the case of E-polarization(IEEE, 1993) Çolak, D.; Nosich, A. I.; Altintaş, A.A dual-series-based solution is obtained for the scattering of an E-polarized plane wave from a cavity-backed aperture which is formed by a slitted infinite circular cylinder coated with absorbing material. The material coating can be done on the inner or outer surface of the cylinder. For both cases, numerical results are presented for the radar cross section and comparisons are given for two different realistic absorbing materials. The radar cross-section results are also given for the aspect angle of the screen. Finally, the dependence of radar cross section on the thickness of the absorbing layer is presented. © 1993 IEEEItem Open Access Radar cross-section study of cylindrical cavity-backed apertures with outer or inner material coating: the case of H-polarization(Institute of Electrical and Electronics Engineers, 1995-05) Çolak, D.; Nosich, A. I.; Altıntaş, A.A dual-series-based solution is obtained for the scattering of an H-polarized plane wave from a slitted infinite circular cylinder coated with absorbing material from inside or outside. For both cases, numerical results are presented for the radar cross section and comparisons are given for two different realistic absorbing materials. The radar cross-section dependencies are also given for the aspect angle of the scatterer and the thickness of the absorbing layer.Item Open Access RCS study of cylindrical cavity-backed apertures with outer or inner material coating: the case of e-polarization(IEEE, 1993) Çolak, Dilek; Altıntaş, Ayhan; Nosich, A. I.Cavity-backed apertures (CBA) are encountered as parts of any airborne or spaceborne radar targets. Most familiar of them are, probably, air inlets and engine tubes, known to contribute a great deal to radar cross section (RCS) of jet aircraft. In the present study, the dual-series-based solution is obtained for the scattering of an E-polarized plane wave from the cavity-backed aperture which is formed by a slitted infinite circular cylinder coated with absorptive material. The material coating can be done on the inner or outer surface of the cylinder. For both cases, numerical results are presented for the RCS and comparisons of the suppression of RCS are given for two different realistic absorptive materials.Item Open Access Validation through comparison: measurement and calculation of the bistatic radar cross section of a stealth target(Wiley-Blackwell Publishing, Inc., 2003) Gürel, Levent; Bağcı, H.; Castelli, J. C.; Cheraly, A.; Tardivel, F.Bistatic radar cross section (BRCS) values of a stealth airborne target are predicted by performing both scaled-model measurements and numerical simulations. In order to achieve the solution of large-scale electromagnetic problems in the numerical simulation environment, the fast multipole method (FMM) is implemented and used. The FMM has produced remarkably accurate results, in addition to its efficiency. The efficiency of the FMM is due to its reduced computational complexity and memory requirement, which are both O(N1.5). Comparison of the measured and computed BRCS values has resulted in a striking agreement, which serves to validate both of the prediction techniques.Item Open Access Wave propagation in metamaterial structures and retrieval of homogenization parameters(Bilkent University, 2007) Ircı, ErdinçElectromagnetic wave propagation in metamaterial structures (metamaterial slabs, metamaterial cylinders, metamaterial coated conducting cylinders etc.) are investigated. Scattered and transmitted electromagnetic fields by these structures due to electric line sources or plane wave illuminations are found. A generic formulation of these wave propagation problems is done, enabling any kind of metamaterial or conventional material to be used, having any sign combination of constitutive parameters and having any electric and/or magnetic losses. For one of these propagation problems i.e., metamaterial coated conducting cylinders illuminated normally with plane waves, achieving transparency and maximizing scattering are investigated thoroughly. It is found out that, rigorous derivation of transparency and resonance (scattering maximization) conditions for PEC core cylinder case under the sub-wavelength limitations yields the same conditions of two electrically small concentric layers of conjugately paired cylinders, given in the literature (when the inner core layer is also taken to the PEC limit). These transparency and resonance conditions are found to be heavily dependent on the permittivity of the metamaterial coating (for TE polarization) and the ratio of core-shell radii. The relations between the permittivity of the coating and the ratio of core-shell radii are investigated for achieving transparency and scattering maximization. Numerical results show that these analytical relations are quite successful and work better when the cylindrical scatter is electrically very small. A novel homogenization method for the retrieval of effective constitutive parameters of metamaterials is proposed and implemented. The method is based on the simple idea that the total reflection coefficient from a finite metamaterial structure has to resemble the reflection from an homogeneous equivalent. While implementing the method, 1, 2, . . ., 20 unit cells of the same metamaterial structure are stacked and their reflection coefficients are collected. The homogenization quality of the metamaterial is evaluated in terms of various factors, which showed that the method is very successful to retrieve the effective constitutive parameters of the metamaterial. Finally, another method has been proposed for the retrieval of surface wave propagation constants on any periodic or non-periodic grounded slab medium. As a preliminary, the method is applied to grounded dielectric slabs. The numerical results generally show good agreement with their theoretical counterparts.