Design of Dual-Band-90°/+90° Transmission Lines for Miniaturized Dual-Band 1:4 Rat-Race Couplers

This paper presents a dual-band -90 degrees/+90 degrees transmission line for the miniaturization of dual-band rat-race couplers. This line was constructed using two diagonally end-shorted coupled-line sections tapped by open stubs at their center. The proposed structure has the advantages of -90 degrees/+90 degrees electrical lengths at two arbitrary frequencies and high equivalent characteristic impedances, enabling the creation of small couplers that have a large power-split ratio of up to 1:32. Using the proposed structure, a rat-race coupler operated at 2.4/5.1 GHz with a power-split ratio of 1:4 was fabricated for demonstration. This circuit occupies only 44% of the area of existing dual-band rat-race couplers.