Browsing by Author "Mittra, R."
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Item 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.Item Open Access Choices of expansion and testing functions for the method of moments applied to a class of electromagnetic problems(IEEE, 1993-03) Aksun, M. I.; Mittra, R.It is well known that the choice of expansion and testing functions plays an important role in determining the rate of convergence of the integrals associated with the moment method matrix, and that an improper choice can lead to erroneous results. The main objective of this paper is to critically examine this convergence issue and to provide criteria for the choice of these expansion and testing functions. The question of whether these functions need to satisfy the Holder condition is also investigated and the convergence behavior of the integrals involved in the spatial and spectral domain moment method is discussed for some representative expansion and testing functions.Item Open Access Closed-Form green's functions and their use in the method of moments(World Scientific Publishing Company, 1996) Aksun, M. Irsadi; Mittra, R.; Guran, A.; Mittra, R.; Moser, P. J.Derivation of the spatial-domain, closed-form Green's functions of the vector and scalar potentials are demonstrated for planar media, and their use in conjunction with the method of moments (MoM) is presented. As the first step of the derivation, the Green's functions are obtained analytically in the spectral domain for various sources viz., horizontal and vertical electric and magnetic dipoles embedded in a planar stratified media. The spatial-domain Green's function can be obtained from the Sommerfeld integral which is the Hankel transform of the corresponding Green's function in the spectral domain. The analytical evaluation of this transformation yields the closed-form, spatial-domain Green's functions which can be used in the solution of a mixed-potential integral equation (MPIE) via the MoM. This combination, i.e., the use of the closed-form Green's functions in conjunction with the MoM, results in a significant improvement in the fill-time of MoM matrices. In the conventional application of the spatial-domain MoM, the matrix elements are double integrals and they require the evaluation of the time-consuming Sommerfeld integral for the spatial-domain Green's function. In the approach presented herein, the spatial-domain Green's functions are in closed forms, and the remaining double-integrals in the matrix elements are evaluated analytically. Thus, there are two factors in this approach that contribute to the improvement in the computation time: (i) elimination of the numerical integration to obtain the spatial-domain Green's functions; (ii) circumventing the need to carry out the numerical integration in the calculation of the MoM matrix elements.Item Open Access Derivation of Closed-Form Green’s Functions for a General Microstrip Geometry(1992) Aksun, M.I.; Mittra, R.The derivation of the closed-form spatial domain Green’s functions for the vector and scalar potentials is presented for a microstrip geometry with a substrate and a super-state, whose thicknesses can be arbitrary. The spatial domain Green’s functions for printed circuits are typically expressed as Sommerfeld integrals, that are inverse Hankel transform of the corresponding spectral domain Green’s functions, and are quite time-consuming to evaluate. Closed-form representations of these Green’s functions in the spatial domains can only be obtained if the integrands are approximated by a linear combination of functions that are analytically integrable. In this paper, we show we can accomplish this by approximating the spectral domain Green’s functions in terms of complex exponentials by using the least square Prony’s method. © 1992 IEEEItem Open Access Efficient use of closed-form Green's functions for three-dimensional problems involving multilayered media(IEEE, 1994-06) Aksun, M. Irsadi; Mittra, R.With the use of casting the spatial domain Green's functions into closed forms approach, it was demonstrated that the computational efficiency of the method of moments (MoM) for the solution of the mixed potential integral equations can be improved significantly for planar microstrip geometries. However, this approach is not effective in the improvement in the computational efficiency achieved for three-dimensional geometries in planar layered media. In this paper, discussed are the difficulties involved in using the spatial domain, closed-form Green's functions in the Method of Moments formulation for three-dimensional geometries and proposed a technique to improve the computational efficiency of the MoM.Item Open Access Estimation of Spurious Radiation from Microstrip Etches Using Closed-Form Green’s Functions(IEEE, 1992) Aksun, M.I.; Mittra, R.The problem of spurious radiation from electronic packages is considered in this paper by investigating the power radiated from microstrip etches that are excited by arbitrarily-located current sources, and terminated by complex loads at both ends. The first step in the procedure is to compute the current distribution on the microstrip line by using the method of moments (MoM). Two novel contributions of this paper are: (i) employing the recently-derived closed-form Green’s functions in the spatial domain that permit an efficient computation of the elements of the MoM matrix; (ii) incorporating complex load terminations in a convenient manner with virtually no increase in the computation time. The computed current distribution is subsequently used to calculate the spurious radiated power and the result is compared with that derived by using an approximate, transmission line analysis. © 1992 IEEEItem Open Access A generalized eigenvalue method for fdtd analyses(John Wiley & Sons, 1993-07) Ko, W. L.; Aksun, M. I.; Mittra, R.In this article we apply the generalized eigenvalue method (GEM) to extract the complex exponentials from a truncated time record computed by the finite‐difference time‐domain (FDTD) code for analyzing microwave integrated circuits. To obtain accurate scattering parameters without further FDTD computations, the truncated FDTD time record is efficiently extended into the future by summing the complex exponentials with complex coefficients determined by the least‐squares method. Numerical GEM results with fewer poles are shown to be in good agreement with those obtained by the Prony method with a large number of poles.Item Open Access Numerically efficient analysis of planar microstrip configurations using closed-form Green's functions(1995-02) Park, I.; Mittra, R.; Aksun, M. I.An efficient technique for the analysis of a general class of microstrip structures with a substrate and a superstrate is investigated in this paper using newly-derived closed-form spatial domain Green's functions employed in conjunction with the Method of Moments (MoM). The computed current distributions on the microstrip structure are used to determine the scattering parameters of microstrip discontinuities and the input impedances of microstrip patch antennas. It is shown that the use of the closed-form Green's functions in the context of the MoM provides a computational advantage in terms of the CPU time by almost two orders of magnitude over the conventional spectral domain approach employing the transformed version of the Green's functions.Item Open Access On the evaluation of spatial domain MoM matrix entries containing closed form Green's functions(IEEE, 1997-07) Kinayman, Noyan,; Mittra, R.; Aksun, M. I.The method of moments (MoM) is widely-used for the solution of mixed potential integral equations (MPIE) arising in the analysis of planar stratified geometries. However, the application of this technique in the spatial domain poses some difficulties since the associated spatial-domain Green's functions for these geometries are improper oscillatory integrals, known as Sommerfeld integrals, that are very computationally-intensive to evaluate. It is possible to eliminate the time-consuming task of computing these integrals by using closed form versions of the spatial domain Green's functions and the time required to evaluate the reaction integrals in the MoM matrix can be reduced considerably. Furthermore, the reaction integrals resulting from the application of the MoM can also be evaluated analytically by using piecewise linear basis and testing functions (Alatan et al., 1996). Hence, an efficient EM simulation algorithm can be developed by using the closed form Green's functions in the MoM formulation that involves no numerical integration. However, despite the time-saving realized from the analytical evaluation of the reaction integrals with the closed-form Green's functions, the need for further reducing the matrix fill-time is not obviated for many problems. Thus the objective of this paper is to present a hybrid technique for the evaluation of the MoM reaction integrals in a numerically-efficient manner that further reduces the time needed for their computation. A microstrip patch antenna is used as an example.Item Open Access Spurious radiation from microstrip interconnects(IEEE, 1993-05) Aksun, M. I.; Mittra, R.The level of spurious radiation from microstrip interconnects, which are modeled here as either single or asymmetric parallel microstrip lines terminated by arbitrary complex load impedances, is investigated in this paper. The calculation of the spurious radiation requires a knowledge of the current distributions on the microstrip lines, and the first step is to compute these distributions efficiently. This is carried out here by using the method of moments in conjunction with closedform spatial domain Green’s functions that circumvent the need for time-consuming evaluation of Sommerfeld integrals. Once the current distributions on the etches have been obtained, the level of spurious radiation, which is defined as the radiated power crossing the plane parallel to the plane of interconnects, is calculated. The dependence of the spurious radiation on the lengths of the lines and on the termination impedances of the etches is also studiedItem Open Access Use of characteristic basis function method for scattering from terrain profiles(TÜBİTAK, 2008) Yağbasan, Atacan; Tunç, Celal Alp; Ertürk, Vakur B.; Altıntaş, Ayhan; Mittra, R.An integral equation (IE) based solution procedure is presented for the rigorous analysis of scattering from terrain profiles. The procedure uses characteristic basis function method (CBFM), which is hybridized with the forward-backward method (FBM), to reduce the storage requirements of the resultant Method of Moments (MoM) impedance matrix, as well as to accelerate the solution procedure. Numerical results in the form of induced current and scattered field are presented to assess the accuracy and efficiency of the solution procedure.