Slow-Wave Artificial Transmission Lines based on Stepped Impedance Shunt Stub (SISS) Loading: Analysis and Stopband Bandwidth Enhancement

摘要

This paper is focused on the analysis of slow-wave transmission lines implemented by periodic loading a host microstrip line with stepped impedance shunt stubs (SISS). Due to SISS-loading, two main effects arise: (i) the phase velocity of the artificial line is reduced, due to the enhancement of the effective capacitance of the line (slow wave effect); (ii) owing to periodicity, stop bands and pass bands in the frequency response appear (Bragg effect). Therefore, these SISS-loaded lines are useful for size reduction and spurious/harmonic suppression in microwave circuits. For design purposes, an accurate circuit of these lines, able to provide the parameters of the host line and SISS (i.e., characteristic impedances and electrical lengths) at the operational frequency, is required. With such circuit, it is also possible to optimize the bandwidth of the first stop band, relevant for spurious/harmonic suppression in microwave circuits based on these artificial lines. This latter aspect constitutes the main original contribution of this paper. The design, fabrication and characterization of a SISS-loaded slow-wave transmission line with roughly 43% length reduction, as compared to the equivalent ordinary counterpart, and exhibiting a wide stop band, are reported as an illustrative example.