A Compact Wideband Millimeter-Wave Quadrature Hybrid Coupler Using Artificial Transmission Lines on a Glass Substrate

Abstract

In this letter, a compact branch-line-based quadrature hybrid coupler using a line miniaturization technique for a 28-GHz application is presented. Coplanar waveguide-based artificial transmission lines are used in the miniaturization, and T-equivalent circuit models are exploited to elaborate on the optimization method. A multistage T-model is utilized to eliminate the need for lumped elements, improve bandwidth performance, and simplify the fabrication process. To validate the proposed approach, the designed quadrature hybrid coupler was fabricated on a glass substrate. The measured insertion loss and phase difference at 28 GHz were 3.86 dB, 3.94 dB, and 90.4°, respectively. The output amplitude imbalance was within a 1-dB range from 25.3 to 31.0 GHz, and the total device area, excluding the measurement pad, was $1.15\times0.98$ mm2.