Mode-Selective Transmission Line—Part I: Theoretical Foundation and Physical Mechanism

Mode-selective transmission line (MSTL) has been studied and realized in support of ultrabroadband and ultrafast electromagnetic signal guidance. Its operation is based on the fact that its dominant mode behaves as a quasi-TEM microstrip mode at low frequencies and is gradually converted into a quasi-TE
₁₀
waveguide mode at higher frequencies notwithstanding its longitudinal uniformity. In the first part of this work (Part I), the physical mechanism of the frequency-dependent modal behavior is presented. From the viewpoint of mode coupling, MSTL is heuristically considered as a coupled system consisting of one microstrip line and two half-mode substrate-integrated waveguides (HM-SIWs). Coupled-mode theory and normal-mode analysis are combined to sequentially analyze HM-MSTL, a relatively simple case, and MSTL. The coupled-mode theory provides an approximate representation for the normal modes of each coupled system, thus gaining in-depth physical insight into the MSTL mode-guidance behavior. A comparison between approximate fields derived from the theoretical analysis and exact fields extracted from the rigorous numerical simulation is made to support the modal analysis. A parametric study is then carried out to examine the effect of geometrical parameters on the MSTL characteristics and performance, thereby providing meaningful guidance in the MSTL design and optimization. A further discussion of mode selectivity is given at the end of this article.