Research on the equivalent circuit of microstrip T-junctions and its application on branch-line coupler

Analysis of microstrip T-junctions based on their equivalent circuit model is introduced in this paper. Explicit design formulas are derived using the admittance-matrix. Improved T-junctions with convex and concave segments are investigated and applied to the branch-line coupler with center frequency of 12GHz. The return loss of the coupler is better than −36.3 dB in all ports with isolation of −43.8 dB at the center frequency, while the amplitude imbalance is below 0.12 dB.