Design of a compact branch-line coupler with high performance

In this paper, a branch-line coupler based on the planar simplified dual composite right/left-handed (SD-CRLH) transmission line (TL) structure is presented. Based on the parametric analysis of the planar SD-CRLH TL structure, a branch-line coupler operating at 1 GHz is investigated and fabricated. In the frequency range of 0.92-1.08 GHz, the return loss and isolation are better than 15 dB, the insertion loss S₂₁ and S₃₁ are around -3.5 dB, and the phase difference between S₂₁ and S₃₁ is 90°±2°. At 2 GHz, S₂₁ and S₃₁ are lower than -20 dB. The proposed coupler can reduce the area to 44% of the conventional one, and it has high second harmonic suppression performance.