Cylindrical Transmission Line Research Articles

Using the EC-S-FDTD Scheme to Approximate Eddy Currents Induced by Z-Gradient Coils

Eddy currents are induced by rapidly switching currents of the gradient coils in an magnetic resonance imaging (MRI) scanner, which is typically cylindrical in shape; hence, the use of cylindrical coordinates is more numerically efficient. We develop the energy conserved splitting finite-difference time-domain (EC-S-FDTD) scheme in the cylindrical coordinates to estimate the eddy currents. The important feature is that this scheme conserves energy and is unconditionally stable. The transformation results in numerical singularities are formed at the internal boundary, and we formulate methods to handle these singularities and efficiently solve the equations. The accuracy of the scheme and energy conversation property are tested, and the performance of the proposed method will be evaluated using two numerical simulations, cylindrical transmission lines, and eddy currents induced by Z-gradient coils. Our numerical results indicate that current density distribution reflects the differences in the tissues by their dielectric properties and has second-order convergence in time and space while preserving energy conservation.

Modeling, analysis, and simulation of millimeter-wave Graphene Nano-Ribbon (GNR) cylindrical coupled transmission line

This paper discusses an analytical formulation for extracting electrical parameters of the Graphene Nano-Ribbon cylindrical Transmission Line (TL), and compares the results with numerical ones obtained through analysis of the same structure using the TLM method in the mm-wave frequency range. To assess the validity of this model, our proposed numerical approach and the simulated models have been compared, and a good agreement has been obtained in this comparison.

Reduced-order modelling of transient flow in transmission lines using distributed lumped parameters

AbstractDeveloped in this paper are mathematical models capturing the one-dimensional underdamped dynamics of confined fluid flow within cylindrical transmission lines. The resulting models are rational transfer functions with coefficients that are explicit functions of the fluid properties and line geometry. Unlike a traditional lumped-parameter approach, the accuracy of the fluid resonant frequencies predicted by the proposed models is precise and not a function of transmission line axial discretisation. Therefore, model order (complexity) is solely a function of the number of desired modes, which in turn influences pressure and flow predictions. The results are applicable to both laminar and turbulent flow. To develop the models, a distributed lumped-parameter approach is employed. Specifically, a quasi-steady state friction approximation is used within the governing partial differential equations. The solution to the linearised ordinary differential equations produces three transcendent transfer funct...

Quasi-TEM Analysis of Electromagnetic Parameters for Different Conducting Bodies

Accurate and efficient computation of electromagnetic parameters for different conducting bodies represents an essential part of spacecraft modern integrated circuits. In this paper, we will illustrate modeling of inhomogeneous shielded rectangular, circular, triangular, and cylindrical transmission lines using the finite element method (FEM). We specifically determine the capacitance per unit length, inductance per unit length, and characteristic impedance of shielded transmission lines. Also, excellent agreement with some results obtained previously using the analytical formulas and methods, the approximate method, and numerical method is demonstrated.

Complex Permittivity Frequency Variations From Multioffset GPR Data: Hydraulic Concrete Characterization

The objective of this paper is threefold. First, a parametric study is performed to evaluate the absorption–dispersion effect on ground penetrating radar (GPR) waveform spectra. Reference data for this evaluation correspond to synthetic GPR signals generated by one of the frequency power-law variants. The result of this paper justifies the need for processing within the spectral domain in order to calculate either propagation velocities or complex permittivities. Second, an innovative technique for extracting the electromagnetic (EM) dispersion of hydraulic concrete using GPR is presented. This technique is based on a coupling between modified forms of the 2-D Fourier transform and estimation methods of quality factors; moreover, it allows identifying dispersion phenomena as a variation in the complex permittivity as a function of frequency. For each method, an analytical validation is carried out on simulated propagation signals. For the third objective, an experimental study is conducted in order to correlate the EM dispersion indicator with both physical and hydrical characteristics of the various concrete mixtures. The complex permittivities obtained according to this “intermethod coupling” (applied to GPR measurements) are compared with those obtained from semidestructive measurements on cores using a cylindrical transmission line considered as the reference. This procedure makes it possible to derive trends that provide information on the conditioning state of the studied media.

Comments on “Ensuring safety of implanted devices under MRI using reversed polarization”

In the work titled ‘‘Ensuring Safety of Implanted Devices Under MRI Using Reversed RF Polarization,’’ the authors propose a method based on reversed polarization, which can be used as a prescan technique for patients with implants. In their work, to visualize qualitatively the induced radiofrequency (RF) currents on a metallic lead inside the patient, they propose to reverse the RF polarization for the transmit and/or the receive procedure. They performed simulations and experiments to investigate the performance of this technique and correctly identified the dangerous coupling currents that may cause risk to the patients. They formulated the RF field that is generated by an induced current on a straight wire, and they calculated its effect on the image intensity in both the forward and the reversed polarization cases. They compared these analytical test results with the experiments and showed that the results accurately match. They claim that the coupling currents can be visualized with better sensitivity when reversed polarization is used. According to the authors, the transmit polarization reversal provides better background suppression, making it easier to distinguish signal variation in the vicinity of the lead from variations due to anatomy. With polarization reversal, they claim that better wire-signal accuracy is provided. Although using reversed polarization to visualize the coupling currents is an innovative idea and could enhance the safety of patients with implants who are undergoing MRI, with this letter we would like to note that there may be certain situations where the proposed method may not work efficiently to accurately visualize the induced currents. In these specific cases, the RF safety of the patient could still be under question.

Geometry- and size-dependence of electrical properties of metal contacts on semiconducting nanowires

We report on a theoretical study of the geometry-and size-dependent effects of metal-semiconductor contacts to understand the origins of significant contact effects on nanowire (NW) devices. The difference in metal-semiconductor barrier width and height for a planar and NW device is calculated based on the differences in depletion-layer width and image-force barrier-lowering effects. We calculate the ratio of the specific contact resistance of a NW contact to that of a planar contact assuming the two systems have been made of the same material set. Using a modified cylindrical transmission line model, we compare the effects of contacts in the two systems and discuss the reasons for larger contact effects in NW devices. The study suggests that the formation of a higher doping concentration in the contact regions is essential for better NW transistors.

Dispersion analysis of multilayer cylindrical transmission lines containing magnetized ferrite substrates

Recently, there has been a growing interest in using cylindrical transmission lines that contain magnetized ferrite material in a variety of applications. In this paper, the finite-difference time-domain (FDTD) method (in cylindrical coordinates) and the spectral-domain analysis (SDA) are used to calculate the propagation characteristics of cylindrical transmission lines that contain magnetized ferrite material. The magnetization can be either in the longitudinal or azimuthal directions. Specifically, the cylindrical microstrip line, and the cylindrical coplanar waveguide printed on magnetized ferrite substrate are analyzed. Both the FDTD and SDA results are in very good agreement. In addition, the results are compared to those of planar structures by taking the radius of the substrate to be large enough such that the curvature effect is negligible.

Spectral domain analysis of cylindrical microstrip transmission lines on ferrite substrates

This paper finds the propagation constant of general 2-D cylindrical microstrip and coupled microstrip transmission lines, with and without dielectric overlay, that are printed on ferrite substrates. The ferrite substrate is excited by an axial DC magnetic field. The accuracy of our results is verified by comparisons with other full-wave and quasi-static results. The proposed method is versatile, accurate and rapidly convergent.

Two-dimensional finite difference time domain analysis of cylindrical transmission lines

Recently, there has been a growing interest in the use of cylindrical transmission lines in a variety of applications. In this paper, the finite difference time domain (FDTD) method is used to calculate the propagation characteristics of cylindrical transmission lines. An efficient two-dimensional (2D) FDTD algorithm is developed by projecting the three-dimensional FDTD cell in the cylindrical coordinates on to the r−φ plane. An effective absorbing boundary condition is employed to truncate the mesh at its outer radial boundary. In addition to a detailed description of the developed 2D-FDTD algorithm, extensive numerical results are derived for different cylindrical transmission lines and compared with data available in the literature. Specifically, the newly proposed cylindrical coplanar waveguide is studied both theoretically and experimentally.

Finite difference time domain analysis of cylindrical coplanar waveguide circuits

There is a growing interest in using cylindrical transmission line structures in microwave applications. In this paper, the finite difference time domain (FDTD) method is used to characterize several three-dimensional cylindrical coplanar waveguide (CCPW) geometries. Specifically, a CCPW series stub and a three-section CCPW filter are studied both theoretically and experimentally. Moreover, CCPW gap is characterized and the FDTD results are compared to those obtained using conformal mapping techniques. Effective absorbing boundary conditions are employed to truncate the FDTD mesh at the end walls and the outer radial boundary.

Analysis of cylindrical transmission lines with the finite-difference time-domain method

In this paper, the finite-difference time-domain (FDTD) method is used to calculate the propagation characteristics of cylindrical transmission lines. An efficient two-dimensional FDTD algorithm is developed by projecting the three-dimensional FDTD cell in the cylindrical coordinates onto the r-/spl phi/ plane. An effective absorbing boundary condition is employed to truncate the mesh at its outer radial boundary. Numerical results are derived for different cylindrical transmission lines and compared to data available in the literature. Specifically, the newly proposed cylindrical coplanar waveguide is studied both theoretically and experimentally.

Efficient analysis of propagation characteristics of printed cylindrical transmission lines using non-uniform FDTD technique

A two-dimensional cylindrical, non-uniform finite difference time domain technique is employed in conjunction with an unsplit, anisotropic perfectly matched layer for boundary mesh truncation, for the analysis of printed cylindrical transmission lines. The computed propagation characteristics for both shielded and open structures are found to be in good agreement with the results obtained via the uniform cylindrical FDTD technique. However, the requirements of computation capacity are significantly reduced. It is found that the Gibb's phenomenon corrupts the results, but can be eliminated by using the Blackman-Harris window function to truncate and modulate the time domain signatures.

Finite difference analysis of cylindrical two conductor microstrip transmission line with truncated dielectrics

The characteristics of the quasi-static TEM mode of a cylindrical microstrip transmission line are investigated using the finite difference technique. The transmission line consists of two perfectly conducting strips located between two different layers of dielectric materials, and a dielectric notch embedded in the substrate between two strips. The dielectric overlay and substrate are truncated for practical purposes. The formulation of the problem is based on the solution of Laplace's equation subject to appropriate boundary conditions and the use of Taylor's expansion to approximate the first and second order derivatives in Laplace's equation. To truncate the finite difference mesh, two artificial boundaries have been considered. The goal of this research is to study the effects of the parameters of the multi-layered cylindrical transmission line on the odd and even mode phase velocities, and to present several techniques to minimize the coupling and distortion between the two conductors.

Analysis of cylindrical transmission lines with the method of lines

Cylindrical transmission lines are important for a variety of applications. To calculate their propagation characteristics, the method of lines in cylindrical coordinates has been adopted. By discretizing the angular space direction with radial lines, the two-dimensional (2-D) Helmholtz equation reduces to a set of ordinary one-dimensional (1-D) differential equations, which can be solved analytically in radial direction after an orthogonal transformation. To improve the accuracy of the cylindrical method of lines from second-order to fourth-order, neighboring lines are used to eliminate second-order discretization errors not only in the Helmholtz equation but also in the continuity equation and in the edge condition. The method is suitable for the analysis of asymmetric cylindrical homogeneous and inhomogeneous guided wave structures.

Radiation characteristics of cavity backed aperture antennas in finite ground plane using the hybrid FEM/MoM technique and geometrical theory of diffraction

A technique using the hybrid finite element method (FEM)/method of moments (MoM) and geometrical theory of diffraction (GTD) is presented to analyze the radiation characteristics of cavity fed aperture antennas in a finite ground plane. The cavity which excites the aperture is assumed to be fed by a cylindrical transmission line. The electromagnetic (EM) fields inside the cavity are obtained using finite element method (FEM). The EM fields and their normal derivatives required for FEM solution are obtained using: (1) the modal expansion in the feed region and (2) the MoM for the radiating aperture region (assuming an infinite ground plane). The finiteness of the ground plane is taken into account using GTD. The input admittance of open-ended circular, rectangular, and coaxial line radiating into free space through an infinite ground plane are computed and compared with earlier published results. Radiation characteristics of a coaxial cavity-fed circular aperture in a finite rectangular ground plane are verified with experimental results.

VLSI interconnections with a MMTL in a multilayered dielectric media

In this paper, an analysis of VLSI interconnetions of a cylindrical multiconductor microstrip transmission line (MMTL) in a multilayered dielectric media based on finite and infinite element method...

A Prolate Spheroidal Uniform Isotropic Dielectric Lens Feeding A Circular Coax

ABSTRACT In launching the TEM mode on a coaxial circular cylindrical transmission line at high frequencies, one can use coaxial circular cones as a wave launcher. Characteristic impedances are matched at the boundary of the conical waveguide and the cylindrical waveguide, and in the usual lens sense rays on the conical structure travel with equal time to an aperture plane perpendicular to the axis of the system. To accomplish this the conical region is filled with a uniform, isotropic, dielectric with frequency-independent dielectric constant (lossless and dispersionless). While the lens is not perfect in that there are small reflections at the lens surface, the high-frequency performance can be quite good for a large range of lens parameters.

New method for analysis of electromagnetic coupling to an angled two-strip transmission line

The electromagnetic coupling to an angled two-strip transmission line is analysed to constructing an angled two-plate transmission line equivalent to the original geometry, and then solving the cylindrical transmission line equations of the original source analytically. The results agree with the numerical values of the circuit-concept approach. >

Electromagnetic coupling to a nonuniform transmission line consisting of two angled wires

AbstractA new method is developed to analyze electromagnetic coupling to an angled two‐wire transmission line. It consists of solving the cylindrical transmission line equations analytically, and then adjusting the involved parameters to support the cylindrical TEM mode along its equivalent model of the angled two‐plate transmission line. The validity of the derived solution is assured by showing its convergence to the numerical value obtained by using the circuit concept approach. © 1992 John Wiley & Sons, Inc.