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The so-called mode-selective transmission line or simply ‘‘MSTL'' is studied theoretically and experimentally. This low-loss and low-dispersion transmission line operates with a frequency-dependent mode-switching behavior. This self-adaptive mode-selective guided-wave structure begins with the propagation of electromagnetic waves over the lower frequency range in the form of a quasi-TEM fundamental mode similar to the microstrip line case, then followed by a fundamental quasi-TE10 mode with reference to rectangular waveguide over the higher frequency region. To gain insight into the physical mechanism and fundamental features of this mode-selective transmission line, a detailed semi-analytical hybrid-mode analysis is developed through the application of a method of lines. This method allows accurate and effective modeling of MSTL guided-wave properties. Propagation characteristics of this proposed mode-agile structure in terms of dispersion, modal, and loss properties are examined, which leads to the establishment of some basic MSTL design guidelines. Numerical results confirm the expected mode conversion and low-loss behavior through the observation of field evolutions along the structure. For experimental verification, a set of MSTL prototypes are fabricated on two different substrates through dissimilar fabrication processes. Measurements are carried out from dc-to-500 GHz using a vector network analyzer. Excellent agreement between theoretical and experimental results is observed. It is confirmed that the low-dispersion and lowloss behavior of MSTL makes it an outstanding integrated waveguide in support of high-performance superbroadband signal transmission and/or ultra-fast pulse propagation in a fully-integrated platform.

A class of longitudinally uniform transmission lines with low loss, low dispersion, and high-field confinement, called mode-selective transmission lines (MSTLs), has been proposed for ultra-broadband and ultra-fast electromagnetic signal guidance and processing. Their operation is mainly based on the concept of frequency-enabled mode selectivity. This paper presents our latest research results on this emerging MSTL, including its operating mechanism, propagation characteristics, higher-order modes, and transition design. Throughout the detailed discussion, two MSTL structures operating in distinct frequency ranges (DC to 60 GHz and DC to 500 GHz as showcased here) are considered. First of all, a comparative study among MSTLs and several conventional transmission lines is made, illustrating significant differences in structural features, wave guidance, field distributions, and frequency characteristics. Second, the phenomenon of mode selectivity occurred in MSTLs is examined by means of identified physical evidence (i.e., field distributions in connection with modal behavior) and theoretical foundation. It is verified that, with increasing frequency, the dominant modes of MSTLs are converted from a quasi-TEM microstrip mode to a quasi-TE 10 waveguide mode over a certain frequency range. Following this thread, a more rigorous analysis is carried out by defining and formulating three characteristic frequencies based on the observed inherent physical dispersions, and the operating frequency ranges of MSTLs are thus divided into several distinct frequency regions associated with the frequency-related variable dominant mode. In addition, a general analysis of the attenuation characteristics of MSTLs and higher order modes in MSTLs is conducted. To facilitate practical measurements and to expedite the integrated applications of MSTLs, we propose a low-loss and ultra-broadband transition between MSTL and microstrip line, through which undesired higher order modes