Browsing by Subject "Mutual coupling"
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Item Open Access Adaptive measurement matrix design in direction of arrival estimation(IEEE, 2022-09-26) Kılıç, Berkan; Güngör, Alper; Kalfa, Mert; Arıkan, OrhanAdvances in compressed sensing (CS) theory have brought new perspectives to encoding and decoding of signals with sparse representations. The encoding strategies are determined by measurement matrices whose design is a critical aspect of the CS applications. In this study, we propose a novel measurement matrix design methodology for direction of arrival estimation that adapts to the prior probability distribution on the source scene, and we compare its performance over alternative approaches using both on-grid and gridless reconstruction methods. The proposed technique is derived in closed-form and shown to provide improved compression rates compared to the state-of-the-art. This technique is also robust to the uncertainty in the prior source information. In the presence of significant mutual coupling between antenna elements, the proposed technique is adapted to mitigate these mutual coupling effects.Item Open Access Analysis of finite arrays of axially directed printed dipoles on electrically large circular cylinders(IEEE, 2004) Ertürk, V. B.; Rojas, R. G.; Lee, K. W.Various arrays consisting of finite number of printed dipoles on electrically large dielectric coated circular cylinders are investigated using a hybrid method of moments/Green's function technique in the spatial domain. This is basically an "element by element" approach in which the mutual coupling between dipoles through space as well as surface waves is incorporated. The efficiency of the method comes from the computation of the Green's function, where three types of spatial domain Green's function representations are used interchangeably, based on their computational efficiency and regions where they remain accurate. Numerical results are presented in the form of array current distributions, active reflection coefficient and far-field pattern to indicate the efficiency and accuracy of the method. Furthermore, these results are compared with similar results obtained from finite arrays of printed dipoles on grounded planar dielectric slabs. It is shown that planar approximations, except for small separations, can not be used due to the mutual coupling between the array elements. Consequently, basic performance metrics of printed dipole arrays on coated cylinders show significant discrepancies when compared to their planar counterparts. © 2004 IEEE.Item Open Access Analysis of input impedance and mutual coupling of microstrip antennas on multilayered circular cylinders using closed-form green's function representations(Institute of Electrical and Electronics Engineers Inc., 2014) Karan, S.; Erturk, V. B.Closed-form Green's function (CFGF) representations for cylindrically stratified media are developed and used in conjunction with a Galerkin method of moments (MoM) in the space domain for the analysis of microstrip antennas on multilayered circular cylinders. An attachment mode is used in the MoM solution procedure to accurately model the feeding of probe-fed microstrip antennas. The developed CFGF representations are modified in the source region (where two current modes can partially or fully overlap with each other during the MoM procedure) so that singularities can be treated analytically and hence, the proposed CFGF representations can be safely used in this region. Furthermore, accurate CFGF representations for the probe-related components (necessary for probe type excitations including the attachment mode) are obtained when the radial distance between the source and field points is electrically small or zero. Numerical results in the form of input impedance of various microstrip antennas and the mutual coupling between two antennas are presented showing good agreement when compared to the available published results as well as the results obtained from CST Microwave Studio.Item Open Access Closed-form green's function representations for mutual coupling calculations between apertures on a perfect electric conductor circular cylinder covered with dielectric layers(IEEE, 2011-06-07) Akyüz, M. S.; Ertürk, V. B.; Kalfa, M.Closed-form Green's function (CFGF) representations are developed for tangential magnetic current sources to calculate the mutual coupling between apertures on perfectly conducting circular cylinders covered with dielectric layers. The new representations are obtained by first rewriting the corresponding spectral domain Green's function representations in a different form (so that accurate results for electrically large cylinders, and along the axial line of a cylinder can be obtained). Then, the summation over the cylindrical eigenmodes is calculated efficiently. Finally, the resulting expressions are transformed to the spatial domain using a modified two-level generalized pencil of function method. Numerical results are presented showing good agreement when compared to CST Microwave Studio results.Item Open Access Decoupling of multifrequency dipole antenna arrays for microwave imaging applications(Hindawi Publishing Corporation, 2010) Saenz, E.; Guven, K.; Özbay, Ekmel; Ederra, I.; Gonzalo, R.The mutual coupling between elements of a multifrequency dipole antenna array is experimentally investigated by S -parameter measurements and planar near-field scanning of the radiated field. A multifrequency array with six dipoles is analyzed. In order to reduce the coupling between dipoles, a planar metasurface is placed atop the array acting as superstrate. The mutual coupling of the antenna elements in the absence and presence of the superstrate is presented comparatively. Between 3 and 20dB mutual coupling reduction is achieved when the superstrate is used. By scanning the field radiated by the antennas and far-field measurements of the radiation pattern, it is observed that the superstrate confines the radiated power, increases the boresight radiation, and reduces the endfire radiation.Item Open Access Driving mutually coupled gradient array coils in magnetic resonance imaging(International Society for Magnetic Resonance in Medicine, 2019) Ertan, KorayPurpose In contrast to conventional linear gradients, gradient coil arrays with arbitrary spatial dependency might experience strong mutual coupling. Although conventional gradient power amplifiers with feedback loop might compensate the effect of coupling, required voltages for the compensation are generally unknown and has to be considered beforehand to ensure that amplifier voltage limits are not exceeded. A first‐order circuit model is proposed to be used as a feedforward model which enables analytical formulas of required voltages to drive the mutually coupled gradient coil arrays. Theory and Methods A first‐order circuit model including the mutual couplings is provided to analytically calculate the input voltages and minimum achievable rise times for a given set of gradient array currents and amplifier limitations. Previously designed 9‐channel Z‐gradient coil array and home‐built gradient amplifiers (50 V and 20 A) are used in the experiments. Three sets of currents optimized for linear Z‐gradient, second‐order Z2, and third‐order Z3 fields are used in the bench‐top experiments. The current weightings for the linear Z‐gradient are also used as the readout gradient in the 3T MRI experiments. Results Current measurements for the example magnetic field profiles with minimum rise times are demonstrated for the simultaneous use of 9‐channel gradient coils and amplifiers. MRI experiments verify that a linear Z‐gradient field with a desired time waveform can be generated using a mutually coupled array coils. Conclusion Bench‐top and MRI experiments demonstrate the feasibility of the proposed circuit model and analytical formulas to drive the mutually coupled gradient coils.Item Open Access A high-frequency based asymptotic solution for surface fields on a source-excited sphere with an impedance boundary condition(Wiley-Blackwell Publishing, 2010-10-05) Alisan, B.; Ertrk V. B.A high-frequency asymptotic solution based on the Uniform Geometrical Theory of Diffraction (UTD) is proposed for the surface fields excited by a magnetic source located on the surface of a sphere with an impedance boundary condition. The assumed large parameters, compared to the wavelength, are the radius of the sphere and the distance between the source and observation points along the geodesic path, when both these points are located on the surface of the sphere. Different from the UTD-based solution for a perfect electrically conducting sphere, some higher-order terms and derivatives of Fock type integrals are included as they may become important for certain surface impedance values as well as for certain separations between the source and observation points. This work is especially useful in the analysis of mutual coupling between conformal slot/aperture antennas on a thin material coated or partially coated sphere.Item Open Access Investigation of planar and conformal printed arrays for MIMO performance analysis(IEEE, 2006) Tunç, Celal Alp; Ircı, Erdinç; Bakır, Onur; Aktaş, Defne; Ertürk, Vakur B.; Altıntaş, AyhanMIMO channel capacity of printed arrays with dipole elements is analyzed. A MIMO channel model based on electric fields is used. The effects of mutual interactions among the array elements through space and surface waves are included into the channel matrix using a full-wave hybrid Method of Moments (MoM)/Green's function technique in the spatial domain. MIMO capacity of printed arrays is then compared with that of free standing thin wire dipole arrays. Results show better performance of printed arrays.Item Open Access A model with electric fields for the inclusion of mutual coupling effects in the MIMO channel(IEEE, 2007) Tunç, Celal Alp; Ircı, Erdinç; Bakır, Onur; Aktaş, Defne; Ertürk, Vakur B.; Altıntaş, AyhanMultiple input multiple output (MIMO) wireless communication systems have been a focus of interest, due to their ability to increase the capacity in rich scattering environments by using multi-element antenna arrays both at the transmitter and the receiver sides. However, when dealing with multi-element antenna arrays, effects of mutual coupling among the array elements become significant and should be included in the channel matrix properly. These effects were included in the MIMO channel matrix mainly for free standing linear arrays (FSLA) of uniform thin-wire dipole antennas using coupling matrices obtained from the mutual interaction matrix and terminations. These matrices reduce to the identity matrix when the interactions are ignored, because of the scaling factors related with termination impedances. Therefore, in this paper we propose a partially stochastic full-wave electromagnetic model with electric fields (MEF), to evaluate the MIMO channel matrix accurately with and/or without including effects of mutual coupling. Effects of mutual interactions among the array elements through space and surface waves (when printed arrays are considered) are included in the channel matrix using a full-wave hybrid method of moments (MoM)/Green's function technique. The stochastic part of the model comes from a local cluster of uniformly distributed scatterers. Consequently, the proposed method is exact except the scatterer scenario, thus, besides achieving the accuracy to be used as a benchmark solution for other approaches, comparisons can be made among any kind of arrays.Item Open Access Optimization of linear wire antenna arrays to increase MIMO capacity using swarm intelligence(Institution of Engineering and Technology, 2007) Olgun, Uğur; Tunç, Celal Alp; Aktaş, Defne; Ertürk, Vakur B.; Altıntaş, AyhanFree standing linear arrays (FSLA) are analyzed and optimized to increase MIMO capacity. A MIMO channel model based on electric fields is used. The effects of mutual interactions among the array elements are included into the channel matrix using method of moments (MoM) based full-wave solvers. A tool to design an antenna array of superior MIMO capacity for any specified volume is developed. Particle swarm optimization is used as the main engine for the optimization tasks of the tool. Uniform linear arrays, uniform circular arrays and non-uniform arrays are analyzed and compared in terms of their channel capacity.Item Open Access Particle swarm optimization of dipole arrays for superior MIMO capacity(WILEY, 2009) Olgun, U.; Tunc, C. A.; Aktas, D.; Ertürk, V. B.; Altintas, A.The particle swarm optimization (PSO) technique is employed to design MIMO arrays for superior capacity. A channel model based on the method of moments solution of the electric field integral equation is utilized with PSO for arrays of dipole elements. Freestanding and printed dipole arrays are analyzed and optimized. Their adaptive performance in the MIMO channel is compared. Numerical results in the form of mean capacity, including comparisons with genetic algorithm results and measurements, are given.