Mode-Selective Transmission Line—Part II: Excitation Scheme and Experimental Verification

Abstract

In Part I of this two-part work, we have presented the theoretical foundation and physical mechanism of the mode-selective transmission line (MSTL). This part is devoted to its excitation scheme, practical transition design, and experimental verification. The MSTL excitation is first explained by comparison with a conductor-backed coplanar waveguide (CBCPW). To produce the specific mode excitation, an ultrabroadband transition between CBCPW and MSTL is proposed. It has a tapered structure operating as a tapered-mode coupler, which supports smooth evolution from the fixed CPW mode of CBCPW to the frequency-dependent dominant mode of MSTL. This modal evolution along the transition is analytically treated based on a local normal mode theory. Couplings between the local normal modes are negligible as a result of gradual tapering, thereby effectively suppressing higher order modes in MSTL and sustaining a mode purity over an ultrabroad frequency range. For experimental demonstration purposes, back-to-back CBCPW-to-MSTL transition circuits are physically realized. The measured results are in agreement with the prediction, showing a return loss of 15 dB from 3 to 109 GHz; the single transition introduces a relatively low insertion loss of 1.84 dB at 100 GHz. The propagation characteristics of MSTL are also extracted from the measured S-parameters, which provides an excellent validation of the proposed structure.