Numerical Modelling of 180° Hybrid Ring Couplers Operating in the GSM 1800 Band

This numerical study is developed with Ansys HFSS software to evaluate the performance of ten conventional rat-race coupler models at a centre frequency of 1.85 GHz by using FR4, Duroid 5880, Duroid 6006 and TMM4 substrate types. We investigated the combined effect of the dielectric constant, dielectric loss tangent and substrate thickness upon the S-parameters variation. Duroid 5880 substrate with thickness of 0.79 mm, dielectric constant of 2.2 and loss tangent of 0.0009 had the lowest dielectric loss, lowest propagation delay time and the highest Q-factor. The coupler model based on this substrate presented the lowest deviation from the desired - 3dB value for the insertion loss coefficients and the highest isolation value at 1.85 GHz. The phase imbalance between signals at the output ports was also close to the desired value of 180% for this Duroid 5880 based model. This phase difference value was about 0.2 degrees higher than the value attained for the model with FR4 substrate.