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We use full 3D EM simulation to optimize the design of a 30 GHz branch line coupler using thin film microstrip lines (TFMS) in a commercial 120 GHz SiGe HBT BiCMOS technology. The effects of various ground plane connections are considered for designing the branch line coupler. With an improper design of the ground plane in the TFMS, the measured S21 shows 7 dB degradation at 30 GHz, in agreement with simulation. A carefully optimized coupler design shows that -4 dB S21/S31 with -15 dB reflection coefficient at 30 GHz should be achievable.