A Novel 3-dB Trans-directional Coupler with DGS and Conductor-backed Asymmetric Coplanar Waveguides

Abstract

A novel 3-dB trans-directional (TRD) coupler with a defected ground structure (DGS) and conductor-backed asymmetric coplanar waveguides (CBACPWs) is proposed. To operate at TRD mode, coupled lines are loaded by periodical shunt capacitors. However, even- and odd-mode phase velocities of coupled microstrip lines are different and capacitances of commercial lumped capacitors usually don't match with theoretical values, which lead to more than 10% drift of operating frequency with maximum coupling level and poor isolation. To solve this problem, DGS is etched under coupled microstrip lines to adjust even- and odd-mode phase velocities, and CBACPW is inserted into each port of the coupler to improve isolation. The proposed coupler exhibits a return loss of better than 20 dB, isolation of better than 20 dB, balanced output power distribution (3.25 ± 0.35 dB) and consistent 90° (± 2°) output signals phase difference over a fractional bandwidth of 15.4% (1.501–1.752 GHz) with a maximum isolation of about 53 dB at 1.635 GHz.