Sharif Digital Repository

In this study, the authors perform a complete survey on S-parameter method applications to compensate the coupling effect in circular arrays. The authors illustrate that, despite the simplicity and ease of measurements, the S-parameter algorithm has an adequate performance in comparison with the external calibration techniques in real environments. Based on the simulation and experimental results, for unmatched arrays and low-frequency applications [e.g. high-frequency (HF) radars], the S-parameter method performance is demonstrated to be comparable in accuracy to the external calibration techniques. Despite multipath effect and structural scattering, the direction of arrival can be... 

A closed-form spatial Green's function for a truncated dielectric slab is derived by using a combination of the characteristic Green's function (CGF) and perfectly matched layer (PML) method. The original structure is terminated by PML that is backed by perfect electric conductor (PEC) in semi-infinite layer at the top and/or bottom. The eigenmodes of the closed structure by PML construct the continuous spectrum contribution of the original structure efficiently. Derived Green's function is exact for separable finite structure while it is approximate for nonseparable one especially in the corners. Generalized scattering matrix (GSM) of truncating surface which contains possible conversions... 

In this article, a novel approach based on differential transfer matrix method is proposed for fast analysis and optimization of wideband planar tapered directional couplers. In this method, both even and odd modes can be analyzed independently to obtain overall scattering matrix. The new method accelerates the optimization time considerably in planar structures. An 8.34 dB tapered coupler in the frequency band of 1-12 GHz is designed and optimized in the form of offset stripline structure. The experimental result shows good agreement with theory  

An analysis is presented of the effective electromagnetic parameters of high-permittivity, anisotropic artificial dielectrics which are built by stacking arrays of metallic elements and conventional dielectric films, with adjacent arrays shifted with respect to each other. The effective parameters of the artificial dielectric are extracted from the scattering coefficients of plane electromagnetic waves which are normally or obliquely incident on a slab of the artificial material with finite thickness. These coefficients are derived from the generalised scattering matrix of a single layer of metallic elements which is computed using the integral equation technique. Both two-dimensional and... 

This article applies a systematic approach based on the normalized determinant function (NDF) theory to analyse stability in multi-loop circuits and to design the required stabilization network. Presenting several provisions, the return ratios are extracted by employing immittance or hybrid matrices (Z, Y, G or H) of active two ports. Using these matrices, instead of the S-parameters, facilitates the selection of an appropriate stabilizer network. As a practical case, a non-uniform distributed amplifier (NDA) is designed and inspected for potential instabilities. The presented procedure detects instability associated with one of the NDA circuit's loops, and an appropriate stabilization... 

In this paper a wave approach has been introduced to study the performance of a distributed amplifier. The time domain response of a FET is obtained by means of the fully distributed model. By applying the procedure to a pi-gate GaAs MESFET, the S matrix is computed from time domain results over a frequency range of 2-20 GHz. Scattering parameters of gate and drain lines are extracted from three-dimensional FDTD simulation. The result obtained by this wave approach is compared with device lumped model and Quasi static approach of transmission line. © 2007 IEEE  

A uniform and closed-form spatial Green's function for finite dielectric structures is derived by using a combination of the characteristic Green's function (CGF) and rational function fitting method (RFFM). Employing the concept of quasi leaky waves, CGF-RFFM represents both of the discrete and continuous spectrum contributions efficiently by using the modified VECTFIT algorithm. The method is examined for 2-D truncated dielectric slab while it can be implemented for 3-D structure straightforwardly. An error of less than 0.2% is achieved compared with the direct numerical integration of the spectral integral. The derived Green's function is exact for separable structures while it is... 

The Telegrapher's equations for a general nonuniform transmission line (NUTL) is analytically solved when the nonuniformities are of sub-wavelength scale. The proposed solution is based on an approximation of the chronological ordering operator for the Telegrapher's equations in an arbitrary NUTL. The proposed approximation is quite accurate when the transmission line has sub-wavelength nonuniformities. The scale of transmission line nonuniformity is assessed by using the concept of minimum resolvable length of nonuniformity (MRLN), which is based on the physically intuitive idea that electromagnetic waves are not affected by spectrally mild nonuniformities. The MRLN is inferred by using the... 

A new method is presented for the extraction of the bulk and surface parameters of a periodic artificial medium which uses the eigenvectors of the generalized transfer matrix of a unit layer. These eigenvectors correspond to the Bloch modes of the periodic structure. The eigenvector related to the propagating Bloch mode directly yields an expression for the effective, intrinsic wave impedance of the medium. Moreover, the interface between the artificial material and a surrounding, conventional (dielectric) region is described by an interface impedance matrix which accounts for the excitation of higher order, nonpropagating Bloch modes at the interface. Although these modes do not propagate... 

A millimeter wave H-plane duplexer using E-plane filters has been designed. T-junction is used for connecting the channel filters compensated by cylindrical post and analyzed by integral contour method. Moreover the junction discontinuity effects have been taken into account. The combination of filters and compensated T-junction allows for low-cost manufacturing. The E-plane filters are analyzed separately, by mode matching method. The scattering parameters of the combination of filters and T-junction are obtained. The diameter and position of the compensating rod and length of T arms are optimized to achieve desired performance. This procedure saves time and memory in computations. The 38... 

In this study, an auxiliary damping controller based on a robust controller considering the active and reactive power control loops for a doubly-fed induction generator for wind farms is proposed. The presented controller is able to improve the inter-area oscillation damping. In addition, the proposed controller applies only one accessible local signal as the input; however, it can improve the inter-area oscillation damping and, consequently the system stability for the various working conditions and uncertainties. The oscillatory modes of the system are appointed using the linear analysis. Then, the controller’s parameters are determined using the robust control approaches (H∞/ H2) with the... 

A novel UWB MIMO antenna with band-notched characteristic operating in the frequency range between 3.1 GHz and 10.6 GHz is presented. The designed MIMO antenna has two ports and the size of 32×14 mm2 which is fabricated on an FR-4 printed-circuit-board. The feeding system of the proposed antenna is a microstrip line. The two antennas are positioned face to face but laid reversely upon the substrate in order to have a good isolation, less cross polarization, high gain and good envelope correlation coefficient. S-parameters describe the input-output relationship between ports (or terminals) in a MIMO antenna. The effects of the strip feed line width on the variations in reflection coefficient...