2D-FEM Characterization and Design of Inhomogeneous Coupled Cylindrical Microstrip Line for Microwave Applications

In the recent years, there has been a growing interest in the use of cylindrical transmission lines in a variety of applications. This work presents analysis and study of inhomogeneous coupled cylindrical microstrip line (CCML), convenient for microwave and millimeter-wave circuits mounted on cylindrical substrate applications. Firstly, the influence of the geometrical parameters on the electromagnetic parameters (EM) by using finite element method (FEM) analysis in two dimensions (2D) under FreeFEM environment is studied. We have focused mainly on calculating the even- and odd-mode characteristic impedances, the even- and odd-mode effective dielectric constants, and the coupling coefficient, as a function of the geometrical parameters. As validation, we have compared our results with those published in the literature and excellent agreement is found. Later, the typical design-curves results are used to design and build several high-performance microwave directional cylindrical microstrip couplers (CMC), of 20 dB, 30dB and 15 dB, operating respectively at 6, 2 and 1 GHz, and adapted to 50 $\Omega$, useful for microwave measurement systems. Finally, we have made a comparison between simulation results in terms of directivity and isolation performances.