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. Copyright © 1996 Published by Elsevier Science Ltd L Introduction Cylindrical microstrip transmission lines with multilayer dielectrics operating in the quasi-TEM mode have recently received much attention in the microwave literature (1-5). Using flexible dielectrics, it is possible to construct non-planar transmission lines that can be placed around conducting cylindrical surfaces. With the application of smaller and denser circuit dimensions, the coupling between circuit connections limits the performance of the cylindrical microstrip transmission lines (CMSTL). It is very important to find practical methods to accurately analyze and control the coupling between transmission lines. In this paper, attention is focused on the problem of reducing coupling between two cylindrical microstrip transmission lines. One possible technique is to employ a dielectric notch embedded between the two conducting lines as shown in Fig. 1. The reduction of coupling can be achieved by properly selecting the size of the notch and the relative permittivities of the overlay, substrate and notch regions. The CMSTL geometry shown in Fig. 1 is treated as a quasi-TEM mode problem.