Browsing by Subject "mutual coupling"
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Item Open Access Antenna analysis(2009) Tunç, Celal AlpMultiple-input-multiple-output (MIMO) wireless communication systems have been attracting huge interest, since a boost in the data rate was shown to be possible, using multiple antennas both at the transmitter and receiver. It is obvious that the electromagnetic effects of the multiple antennas have to be included in the wireless channel for an accurate system design, though they are often neglected by the early studies. In this thesis, the MIMO channel is investigated from an electromagnetics point of view. A full-wave channel model based on the method of moments solution of the electric field integral equation is developed and used in order to evaluate the MIMO channel matrix accurately. The model is called the channel model with electric fields (MEF) and it calculates the exact fields via the radiation integrals, and hence, it is rigorous except the random scatterer environment. The accuracy of the model is further verified by the measurement results. Thus, it is concluded that MEF achieves the accuracy over other approaches which are incapable of analyzing antenna effects in detail. Making use of the presented technique, MIMO performance of printed dipole arrays is analyzed. Effects of the electrical properties of printed dipoles on the MIMO capacity are explored in terms of the relative permittivity and thickness of the dielectric material. Appropriate dielectric slab configurations yielding high capacity printed dipole arrays are presented. The numerical efficiency of the technique (particularly for freestanding and printed dipoles) allows analyzing MIMO performance of arrays with large number of antennas, and high performance array design in conjunction with well-known optimization tools. Thus, MEF is combined with particle swarm optimization (PSO) to design MIMO arrays of dipole elements for superior capacity. Freestanding and printed dipole arrays are analyzed and optimized, and the adaptive performance of printed dipole arrays in the MIMO channel is investigated. Furthermore, capacity achieving input covariance matrices for different types of arrays are obtained numerically using PSO in conjunction with MEF. It is observed that, moderate capacity improvement is possible for small antenna spacing values where the correlation is relatively high, mainly utilizing nearly full or full covariance matrices. Otherwise, the selection of the diagonal covariance is almost the optimal solution.MIMO performance of printed rectangular patch arrays is analyzed using a modified version of MEF. Various array configurations are designed, manufactured, and their MIMO performance is measured in an indoor environment. The channel properties, such as the power delay profile, mean excess delay and delay spread, are obtained via measurements and compared with MEF results. Very good agreement is achieved.Item Open Access Development of closed-form Green's functions to investigate apertures on a pec circular cylinder covered with dielectric layer(s)(2009) Akyüz, Murat SencerClosed-form Green’s function representations for magnetic sources, which is in general used to represent aperture type antennas on conducting surfaces, are developed for a cylindrically stratified media. The resultant expressions are valid for almost all possible placements of source and observation points including the cases where ρ = ρ 0 and φ = φ 0 . Hence, they can be used in a Method of Moments solution procedure. In the course of obtaining these expressions, the conventional spectral domain Green’s function representations for magnetic sources are reorganized in order to handle relatively large cylinders and the axial line problem. Available acceleration techniques that exist in the literature are implemented to perform the summation over the cylindrical eigenmodes efficiently and to handle some numerical problems along the kz integration path. Then, 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 two-level approach is used. It should be noted that a similar methodology has recently been developed for electrical sources and very accurate results have been presented. In this work, its magnetic source counterpart has been developed. Numerical results are presented in two different forms: (a) ρ 6= ρ 0 ; the magnetic source is on the conducting cylinder, which forms the innermost layer of the dielectric coated cylinder. This is a typical scenario for the radiation problem of aperture type antennas. (b) ρ = ρ 0 ; both the magnetic source and the observations points are on the conducting cylinder which forms the innermost layer. There is a singledielectric layer on the top of them. This is a typical scenario for the mutual coupling between aperture type antennas.Item Open Access Method of moments analysis of microstrip antennas in cylindrically stratified media using closed-form Green's functions(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.