Browsing by Subject "Microwave antennas."
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Item Open Access Analysis of slotted sectoral waveguide antenna arrays embedded in cylindrically stratified media(Bilkent University, 2013) Kalfa, MertSlotted waveguide antenna arrays with dielectric covers are widely used in both military and civil applications due to their low-profile, high power handling capacity, and the ability to conform to the host platform. Conformity is especially required for air platforms where aerodynamics and radar cross section (RCS) of the vehicle are of utmost importance. For an air platform, one or more dielectric layers (monolithic or sandwich radomes) can be used to protect the antenna from the extreme flight conditions. Although accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays is of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, integral equation (IE) based ones that utilize the Method of Moments (MoM) are widely used. However, the IE solvers suffer from long matrix fill times, especially when cylindrically stratified media are considered. In this study, a slotted sectoral waveguide antenna, coaxially covered by multiple dielectric layers is analyzed with a hybrid MoM/Green’s function technique in the space domain. Only the fundamental mode of propagation (TE11) is assumed to be excited inside the sectoral waveguide. The longitudinal slots are on the broadside wall of the sectoral waveguide and are very thin in the transverse direction; therefore, only the TE modes are assumed to propagate. The solution domain is divided into two by using the equivalence theorem and fictitious magnetic current sources on the waveguide slots. Note that for the purposes of this study, the waveguide wall thickness is assumed to be zero. However, it can be incorporated into the problem by adding a third region which would be a sectoral cavity. The magnetic sources on the slots are expanded by piecewise sinusoid basis functions, and the continuity of the tangential magnetic fields across the iii iv slots is enforced to construct the integral equation. The integral equation is then converted into a matrix equation using Galerkin’s procedure. To compute the elements of the mutual admittance matrix, two Green’s function representations for the two solution regions are used. For the sectoral waveguide interior, the dyadic Green’s function components for a sectoral waveguide corresponding to longitudinal magnetic currents are rigorously derived. For the cylindrically stratified dielectric region, closed-form Green’s function representations for magnetic currents are developed, which are valid for all source and observation points, including the source region, where two magnetic current modes fully or partially overlap with each other. The proposed analysis method can be easily extended to include: slotted substrate integrated waveguides, slotted cavity antennas, and similar aperture type antennas embedded in cylindrically stratified media. Numerical results in the form of equivalent slot currents and far-zone radiation patterns for a generic slotted sectoral waveguide are presented, and compared to the results obtained from the commercially available full-wave electromagnetic solversItem Open Access Investigation of finite phased arrays of printed antennas on planar and cylindrical grounded dielectric slabs(Bilkent University, 2006) Bakır, OnurPrinted structures, in the form of a single printed antenna (printed dipole, patch, etc.) or an array of printed antennas on planar and cylindrical grounded dielectric slabs, are investigated. Full-wave solutions based on the hybrid method of moments (MoM)/Green’s function technique in two different domains, the spectral and the spatial domains are used to analyze these types of geometries. Several numerical problems, encountered in the evaluation of both the spectral and spatial domain integrals are addressed and solutions for these problems are presented. Among them the two important ones are: (1) The infinite double integrals which appear in the asymptotic parts of the spectral domain MoM impedance matrix and the MoM excitation vector elements for planar grounded dielectric slabs are evaluated in closed-form in this thesis, resulting an improved efficiency and accuracy for the rigorous investigation of printed antennas. (2) In the space domain MoM solution of printed structures on planar grounded dielectric slabs, an accurate way of treating the singularity problem of the self-term and overlapping terms as well as the MoM excitation vector is presented along with a way to halve the order of space domain integrals by employing a proper change of variables and analytical evaluation of one of the integrals for each double integral. Finally two different studies which use these improved methods are presented in order to asses their accuracy and efficiency: (1) Investigation of scan blindness phenomenon for finite phased arrays of printed dipoles on material coated electrically large circular cylinders, and its comparison with the same type of arrays on planar platforms. In this study effects on the scan blindness mechanism of several array and supporting structure parameters, including curvature effects, are discussed. (2) A discrete Fourier transform (DFT) based acceleration algorithm is used in conjunction with the generalized forward backward method (GFBM) to reduce the computational complexity and memory storage requirements of the aforementioned full-wave solution method for the fast analysis of electrically large finite phased arrays of microstrip patches. As a result both the computational complexity and memory storage requirements are reduced to O(N) (of order N), where N is the number of unknowns.Item Open Access Method of moments analysis of microstrip antennas in cylindrically stratified media using closed-form Green's functions(Bilkent University, 2012) Karan, ŞakirNumerical methods based on Method of Moments (MoM) have been widely used for the design and analysis of planar microstrip antennas/arrays and printed circuits for various applications for many years. On the other hand, although the design and analysis of similar antennas/arrays and printed circuits on cylindrical structures are of great interest for many military, civil and commercial applications, their MoM-based analysis suffers from the efficiency and accuracy problems related with the evaluation of the Green’s function representations which constitute the kernel of the regarding integral equations. In this dissertation, novel closed-form Green’s function (CFGF) representations for cylindrically stratified media, which can be used as the kernel of an electric field integral equation (EFIE) are developed. The developed CFGF representations are used in a hybrid MoM/Green’s function solution procedure. In the course of obtaining the CFGF representations, first the conventional spectral domain Green’s function representations are modified so that all the Hankel (Bessel) functions are written in the form of ratio with another Hankel (Bessel) function. Furthermore, Debye representations for the ratio terms are used when necessary in order to avoid the possible overflow and underflow problems. Acceleration techniques that are present in the literature are implemented to further increase the efficiency and accuracy of the summation and integration. Once the acceleration techniques are performed, the resultant expressions are transformed to the space domain in the form of discrete complex images (DCIM) with the aid of the generalized pencil of function (GPOF) method and the fi- nal CFGF expressions are obtained by performing the resultant space domain integrals analytically. The novel CFGF expressions are used in conjunction with MoM for the investigation of microstrip antennas on cylindrically stratified media. The singular terms in mutual impedance calculations are treated analytically. The probe-fed excitation is modeled by implementing an attachment mode that is consistent with the current modes that are used to expand the induced current on the patches. In the course of modeling the probe-fed excitation, the probe-related components of CFGF representations are also derived for the first time in the literature and MoM formulation is given in the presence of an attachment mode. Consequently, several microstrip antennas and two antenna arrays are investigated using a hybrid MoM/Green’s function technique that use the CFGF representations developed in this dissertation. Numerical results in the form of input impedance of microstrip antennas in the presence of several layers as well as the mutual coupling between two microstrip antennas are presented and compared with the available results in the literature and the results obtained from the CST Microwave Studio.