Research and development

Abstract

This paper presents expressions for the transverse electromagnetic (TEM) parameters of high-frequency shielded and coupled cylindrical striplines that can support a variety of compact high-power RF/microwave circuits, including filters and directional couplers. In the electrostatic domain, the problem is to solve for the even- and odd-modes the Laplace equation of homogeneous cases, with fixed boundary conditions. The calculation is made using the finite element method in two dimensions (2D FEM) under freeFEM environment. Our realized CAE program based on the 2D FEM was adapted for characterizing the EM-parameters of the general configuration of the shielded and coupled cylindrical stripline structure. The FEM-results are very close with those obtained by other methods of the scientific literature. In the case of our studied structure, comparisons between the 2D FEM-results and those we obtained under LINPAR environment using our program based on the method of moments (2D MoM) show a good agreement between the results. When the TEM-parameters are determined it is possible to deduce and propose rigorous analytical expressions based on curve-fitting techniques done under Origin50 graph environment. Our proposed models, with relative errors less than 1 percent, cover all the practically used substrate materials for those TEM-coupled cylindrical striplines with a wide range of geometrical parameters (r2/r1) and (θ) varying respectively between 1.5–9 and 5–179°. Using these analytical models, an analysis can be readily implemented in modern CAE software tools for the design of high-power microwave and wireless components. For instance, we show our design results of a 50 Ω-cylindrical stripline TEM-directional coupler with 20-dB of coupling and operating at 2 GHz, obtained with our proposed expressions. The obtained scattering parameters (S-parameters) show excellent performance in terms of directivity (45 dB) and isolation (65 dB). The S-coupling of the TEM-directional coupler obtained under MATPAR environment was validated by our result obtained using CST MICROWAVE STUDIO software and confirm the accuracy and the usefulness of our accurate concise closed-form expressions.