传统环形耦合器只能工作在单一基频，且具有奇数次谐波．为此，文中提出了一种新型三频环形耦合器的设计方法，利用有4 种电路拓扑结构的Π 型阶梯阻抗变换器( ΠSIT) 替代传统环形耦合器中的1 /4 波长阻抗变换器，使得环形耦合器可以同时工作在任意两个频率及其中心频率．考虑到实际传输线阻抗实现的范围( 20 ～ 150 Ω) ，该环形耦合器的第3 个工作频率和第1 个工作频率的比值范围取为1. 70 ～ 4. 10 和6. 60 ～ 12. 30．文中给出了解析的设计公式，基于不同的Ⅱ型阶梯阻抗变换器电路结构，设计并加工了分别工作在2. 0 /3. 9 /5. 8GHz 和0. 9 /3. 5 /6. 1GHz 的3dB 微带三频环形耦合器．测量结果与仿真结果吻合良好，验证了文中三频环形耦合器设计方法的可行性．

As the conventional rat race coupler can only operate at the fundamental frequency and may cause odd harmonics,the design method of a novel tri-band rat race coupler is presented. In this method,the quarter-wavelength impedance transformers in the conventional rat race coupler are replaced by the Ⅱ-shaped stepped impedance ones ( ΠSITs) with four circuit topologies,which enables the proposed coupler to operate at two arbitrary frequencies and their center frequency. Then,by taking into consideration the practical realization range of transmission line impedance ( 20 ~150Ω) ,the ratio of the third operating frequency to the first one of the proposed coupler is selected from 1. 70 to 4. 10 and from 6. 60 to 12. 30. Moreover,the corresponding analytic design equations are deduced,and two microstrip 3-dB tri-band rat race couplers based on different ΠSIT circuit topologies,which respectively operate at 2. 0 /3. 9 /5. 8 GHz and 0. 9 /3. 5 /6. 1 GHz,are designed and fabricated. Simulated results accord well with the measured ones,meaning that the proposed design method is feasible.