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.

中文翻译：

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模式选择传输线-第一部分：理论基础和物理机制

为了支持超宽带和超快电磁信号引导，已经研究并实现了模式选择传输线（MSTL）。它的运行基于以下事实：其主导模式在低频下表现为准TEM微带模式，并逐渐转换为准TE ₁₀尽管具有纵向均匀性，但在较高频率下仍具有较高的波导模式。在这项工作的第一部分（第一部分）中，介绍了频率依赖的模态行为的物理机制。从模式耦合的角度来看，MSTL被试探性地认为是由一个微带线和两个半模衬底集成波导（HM-SIW）组成的耦合系统。结合了耦合模式理论和正常模式分析，可以依次分析HM-MSTL（一种相对简单的情况）和MSTL。耦合模式理论为每个耦合系统的正常模式提供了近似表示，从而获得了对MSTL模式指导行为的深入物理了解。为了支持模态分析，对从理论分析得出的近似场与从严格数值模拟中提取的精确场进行了比较。然后进行参数研究，以检查几何参数对MSTL特性和性能的影响，从而为MSTL设计和优化提供有意义的指导。本文末尾将进一步讨论模式选择性。