Longitudinally uniform transmission lines with frequency-enabled mode conversion

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 are effectively suppressed. The numerical and theoretical analyses of MSTLs are carried out with experimental verifications. At the end of this paper, different fabrication and measurement techniques for the two MSTLs of interest are briefly described.

Uncontrolled Keywords

Field distribution ; high-order mode ; mode-selective transmission line (MSTL) ; mode selectivity ; propagation characteristic ; quasi-TEM mode ; quasi-TE10 mode ; tapered transition