A harmonic suppression and size-reduced rat-race hybrid coupler using dual coupled-lines

Abstract

A newly proposed coupled-line low-pass filter structure is firstly integrated into rat-race coupler design in this paper for size reduction and harmonic suppression. This low-pass filter is composed of dual parallel coupled-lines with one open stub at the end. By properly substituting this low-pass filter unit cell (LUC) for each one of the six λ/4 sections in the conventional rat-race coupler, significant size reduction and harmonic suppression can be achieved while the property of this proposed coupler remains comparable to the conventional one for the passband (f₀=0.9GHz) with 33% relative size. Transmission is suppressed below -20dB from 2.5GHz to 5.6GHz, including the third, fourth, fifth and sixth harmonics.