Tri-band rat-race coupler using resonators

This paper presents a novel method to design a tri-band rat-race coupler by using resonators in π-shaped structure. The proposed structure demonstrates tri-band performance and a compact size due to π-shaped structure. In order to achieve tri-band operation, we use resonators and stubs with conventional rat-race coupler structure. By sharing the stub with resonator of two adjacent π-shaped circuits and making stubs inside the tri-band rat-race coupler, compactness is well kept. Compare to the size of 1 GHz conventional rat-race coupler, 84% reduction is archived by our proposed structure. We demonstrate that insertion losses are better than 4 dB, the reflection coefficients are better than 10 dB, and the isolations are better than 22 dB. In phase difference is less than 2.3°.