Compact Broadside Coupled Directional Coupler Based on Coplanar CRLH Waveguides

Abstract

A compact broadside coupled (BSC) backward directional coupler based on composite right/left-handed (CRLH) coplanar waveguide (CPW) is proposed. The design is symmetric on both sides of the substrate and comprises of series gap capacitor, broadside-coupled capacitor, and shunt meandering short-circuited stub inductor. Compared to existing directional couplers, the proposed coupler has much smaller size, broader bandwidth, phase balance and easy fabrication. An equivalent circuit model including series inductance, capacitance, shunt inductance and capacitance tank has also been established. Good agreement between the full-wave simulation and equivalent circuit model verifies the effectiveness of the proposed circuit model. Measured bandwidth from 9.9 ∼ 14.7 GHz centered at 12 GHz (40%) is observed with the device size of 1.8 × 1.3 cm².