592 research outputs found
Analytical and numerical investigations of ring resonators
No full textSevgi, Levent (Dogus Author)Rectangular vs. cylindrical finite-difference time-domain (FDTD) electromagnetic modeling is discussed, and characteristic tests and comparisons are presented depending on the analytical and numerical analysis of ring type circular resonators. A ring resonator is modeled with both rectangular- and cylindrical-FDTD packages which are also calibrated against analytical exact solution derived in terms of cylindrical Bessel functions. Applications of the periodic boundary condition in modeling circular (rectangular) structure via rectangular (circular) FDTD is also given
An Approximation for Direct and Inverse Problems Related to Longitudinally Inhomogeneous Waveguides
No full textAn approximation is derived for longitudinally inhomogeneous waveguides (LIW). Total reflections are adopted for a rough approximation through the tracking of the main reflected and transmitted fields in the multilayered dielectric structure. This approximation can be used to determine the thickness of layers of which the dielectric properties are known. Although the obtained thickness values are a rough approximation for the exact values, they provide a utility for iterative optimization algorithms for which the initial values are vital
Remedy for Inverse Scattering Problems via Space-Mapping Inverse-Difference Approach
No full textAn inverse-scattering algorithm (ISA) based on integral-equation formulation is used as an inverse coarse model in the space-mapping inverse-difference (SMID) method and named as ISA-based SMID (ISA-SMID). The enhancement of ISA-SMID optimization over inverse-scattering algorithms is tested. To this aim, extraction of dielectric profile problem is examined with ISA-SMID, which is obtained by the hybridization of SMID and Newton-based inverse-scattering algorithm. The method of moments solution of integral equation is used in the utilized inverse algorithm for the reconstruction of complex permittivity of the inhomogeneous object loaded in the waveguide
Analysis of Direct and Inverse Problems Related to Circular Waveguides Loaded With Inhomogeneous Lossy Dielectric Objects
No full textAn integral-equation-based analysis for direct and inverse problems related to circular waveguides loaded with inhomogeneous and arbitrarily shaped lossy dielectric material is introduced. The problem is formulated as a system of integral equations composed of the well-known data and object equations, which contain the dyadic Green's function (DGF) of the empty circular waveguide. Both the direct and inverse algorithms are based on this 3-D system of equations. In the direct problem, the scattering parameters are calculated using the scattered electric fields caused by the inhomogeneous lossy dielectric objects located in circular waveguide, while in the inverse algorithm, the scattered fields are assumed to be known and used for the determination of the complex permittivity variation of the object loaded in the waveguide through a Newton-type iterative approach. In both algorithms, the integral equations are solved via a method-of-moments-based discretization, where the accurate integration of the DGF at each discrete 3-D cell is achieved by a special integration technique. The validity region and the reliability of the direct and inverse algorithms are examined analytically and numerically through elaborative examples
Time And Frequency Domain Numerical Modeling For Ground Wave Propagation
No full textTez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2002Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2002Dünya üzerinde seçilen herhangi iki nokta arasındaki yer dalgası yayılım karakteristiğinin elde edilmesi yirminci yüzyılın başından bu yana ilgi odağı olmasına karşın, problemin karmaşıklığı nedeniyle henüz sayısal olarak hesaplanabilen analitik tam çözüm ya da genel olarak uygulanabilecek üç boyutlu sayısal çözüm bulunamamıştır. Bu yüzden, bugüne kadar analitik yaklaşık çözümler veya iki boyutlu sayısal yaklaşımlar kullanılmıştır. Bu çalışmada, öncelikle problemin çözümü için kullanılmakta olan ışın – mod çözümü gibi yaklaşık analitik ve Fourier dönüşümü yardımı ile parabolik denklem çözümüne dayanan yarı analitik –sayısal teknikler geçerlilik bölgeleri ile birlikte ayrıntılı olarak incelenmiş, daha sonra da yer dalgası yayılım probleminin çözümü için zamanda sonlu farklar tekniğine dayanan, saf sayısal yeni bir yöntem önerilmiştir. TDWP olarak adlandırılan bu yöntem, yayılım ortamının FDTD hesap uzayından her zaman daha geniş olması nedeni ile ilerleyen dalganın kayan bir pencere yardımı ile izlenmesine dayanmaktadır. TDWP ile elde edilen sonuçlar ışın-mod çözümleri ve/veya parabolik denklem sonuçları ile geçerlilik bölgeleri içinde karşılaştırılmış ve çok iyi bir uyum gözlenmiştir.Although the ground wave propagation problem has been a subject of interest from the beginning of the 20th century, because of the complexity of the proposed problem neither a full-wave numerically computable analytical solution, nor a three-dimensional, generally applicable numerical solution has yet appeared to determine the ground wave propagation characteristics between two points, which may be selected anywhere on or above the ground. Therefore analytical approximate solutions or two-dimensional numerical approaches have so far been used. In this study, first we consider ray –mode solutions and Split Step Parabolic Equation (SSPE) method, and then introduce a novel numerical method, based on the Finite-Difference Time Domain (FDTD) technique and applicable for a broad range of propagation problems. Since the propagation region is always larger than the available FDTD space, the propagating pulse is traced within a sliding window and this method is named as Time-Domain Wave-Propagator (TDWP). The results obtained via TDWP are compared with the data obtained via ray – mode solutions and/or SSPE technique within their domains of validity and good agreement is observed.DoktoraPh
Determination of Complex Permittivity of Arbitrarily Shaped Homogenous Materials via Waveguide Measurements
No full textIn this study, complex permittivity of arbitrarily shaped homogenous materials loaded in different shaped waveguides (rectangular waveguide and circular waveguide) is determined through an iterative inverse algorithm. S-parameters are calculated for chosen material numerically via Method of Moments (MoM), in place of real measurement data for sake of testing algorithm as a first step. Inverse algorithm depends on calculating transmission-reflection coefficients (two port S parameters) with respect to updated complex permittivity, started with an initial guess. At each step, calculated S parameters (either Si) and/or Su) can be used to update complex permittivity in sense of Newton-Raphson numerical approach. Problem is reduced to finding the roots of iterative equation, which is a function of complex permittivity
An approximation for direct and inverse problems related to longitudinally inhomogeneous waveguides
No full textDielectric Loaded Waveguide Filter Design
No full textA hollow waveguide is loaded with dielectric slabs to be operated as a band-pass filter in the X-band. Realizing the followed approach practically, the permittivity values of the slabs are chosen. The thickness values of the slabs are optimized to meet the filter requirements. Levenberg-Marquardt optimization algorithm is utilized to solve the given non-linear optimization problem. Drawbacks of the proposed approach are analyzed in terms of optimization parameters and shown with numerical examples
Groundwave propagation in a nonhomogeneous atmosphere: Prediction using 3D parabolic equation
No full textInternational audienc
Detection Of Three Dimensional Objects Buried In A Half Space By The Use Of Surface Impedance
No full textTez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2010Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2010Bu çalışmada, bir yarı-uzayda gömülü üç boyutlu cismlerin konumlarının tespit edilmesine yönelik bir yöntem sunulmuştur. Yöntem ara yüzeyden yukarıda yapılan elektrik ve manyetik alan ölçümlerinden yararlanılarak saçılan alanın gelen dalgayla aynı yöndeki bileşenlerini içeren bir yüzey empedans fonksiyonunun oluşturulmasına dayanır. Çalışma kapsamında bu alan değerleri moment metodu kullanılarak üretilmiştir. Üç boyutlu cisimlerin varlığı durumunda yüzey empedansı konuma bağlı iki boyutlu bir fonksiyon halini alır ve bu fonsiyonunun tepe noktaları gömülü cisimlerin konumlarını gösterir. Yüzey empedans fonksiyonunun hesaplanması için gerekli olan ara yüzeydeki elektrik alan değerinin ölçüm sonuçlarından elde edilmesinde Fourier dönüşümüne dayalı bir teknik kullanılmıştır. Sayısal uygulama sonuçları cisimler çok yakın veya çok derine gömülü olmadığı sürece empedans fonksiyonunun konumlarının belirlenmesinde oldukça etkili olduğunu ve ayrıca cisimlerin göreli derinlik, boyut, malzeme yapısı gibi karakteristik özellikleri hakkında bilgi sağladığını göstermektedir.A method to determine the locations of the three dimensional objects buried in a half space medium is presented. The method is based on the reconstruction of a surface impedance function defined by the co-polarized tangential components of electric and magnetic fields via remote field measurements of scattered field. In this work, the values obtained by those measurements are calculated by the use of method of moments. In the presence of 3-D objects, surface impedance becomes a two dimensional function of location and the locations of the peaks of this function indicates the position of buried objects. The scattered field distribution and its derivative on the half space boundary which are required for determination of surface impedance function are obtained through a continuation method based on Fourier transform of the measured data to the interface. Numerical simulations show that unless the objects are very close, the surface impedance is highly effective in locating the buried objects and also carries information about relative depth, size or material properties of the objects.Yüksek LisansM.Sc
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