Miniaturised and harmonic-suppressed rat-race couplers based on slow-wave transmission lines

Abstract

In this study, a compact rat-race hybrid coupler with harmonic suppression based on slow-wave transmission lines (SW-TLs) is presented. Such artificial lines are implemented by periodic loading a host microstrip line with series meandered inductors and shunt patch capacitors. The presence of both loading elements has a twofold effect, i.e. phase velocity reduction (due to the enhancement of the effective inductance and capacitance of the periodic line), and the generation of a controllable stopband in the frequency response (due to the Bragg effect, inherent to periodicity). It is shown that by designing the unit cell of the periodic line with an electrical length of 45°, at least the first five harmonic bands of the rat-race coupler are efficiently suppressed, keeping the band of interest unaltered. Moreover, 79% size reduction, as compared to the ordinary coupler, is achieved in the reported SW-TL-based prototype.