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A compact 60-GHz band branch-line coupler using cpacitively loaded lower-ground coplanar-waveguide (LG-CPW) lines has been successfully demonstrated in a glass-substrate integrated passive device technology. The fabricated coupler has a size reduction of more than 83% compared to that of a conventional CPW branch-line coupler. The capacitive loading is achieved by utilizing the signal layer and the LG of the LG-CPW structure to form microstrip open-circuited stubs. The measured results show a phase error of less than 0.5° between the coupler's output ports and an amplitude imbalance of less than 1.2 dB over the frequency band 57-64 GHz. The measurements also show that both the return loss and isolation are better than 25 dB at 60 GHz and better than 15 dB over the 57-64-GHz band. The proposed coupler is well suited for low-cost high-performance system-on-package V-band radio front ends for high data rate applications.