Mode matching analysis of dimension for single-mode operation of shielded microstrip lines

Shielded microstrip lines are widely used in microwave and millimeter wave systems to connect devices while protecting circuits from environmental contaminants, preventing electromagnetic interference, and enhancing the mechanical strength. To avoid the influence of the metallic enclosures on the fundamental mode, the choice of dimensions for the housing is an important issue. In this paper, the influence of those dimensions on the fundamental mode is investigated rigorously using the mode matching technique. In addition, the relationships between the cutoff frequency of the second-order mode and those geometrical variables of the cross-section of the shielded transmission line are presented. With these relationships, a reasonable geometry can be efficiently determined, which is important to prevent performance degradation through higher-order mode excitations, e.g. in waveguide transitions. The analysis results are verified with finite-element simulations obtained with HFSS.