This paper presents a dual-band -90°/+90° 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°/+90° 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.
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