In this paper, a classical rat-race coupler based on coplanar stripline is presented. It was built using BiCMOS 55nm technology. Parametrical analysis targeting the position of additive compulsory dummies from selected high quality coplanar stripline is carried out. Two versions of the rat-race couplers are compared: without and with metallic dummies. Simulation results show that isolation and reflection coefficients are preserved under 20 dB. With dummies considered, insertion loss value is 4.2 dB at 118 GHz. With the addition of metallic dummies coupler performance is down shifted in frequency with value of 2 GHz. Maximum amplitude imbalance is 0.4 dB reported at 140 GHz while phase imbalance difference between both cases doesn't exceed 3.1° for the whole bandwidth of interest.