A novel design concept of dual-band 180° hybrid ring coupler is presented in this paper. Coupler is a key element in front-end building blocks of wireless transceiver systems such as industrial systems and consumer electronic devices. The proposed design is realized by combining multiple arbitrary length transmission lines operating at two frequencies with one dual-band 180° phase shifter. The even-odd mode method is applied to derive the design equations for proposed dual-band 3 dB 180° directional coupler. Based on the analysis, it is found that the realizable frequency ratio of the proposed coupler is very flexible (i.e. the ratio between the two operating frequencies). Moreover, the 180° phase shifter features arbitrary characteristic impedance (i.e. its characteristic impedance can be arbitrarily chosen), which further ensures the easy implementation of proposed structures. To prove the design concept, full-wave electromagnetic simulations are performed to design a dual-band ring hybrid coupler working at 0.9 and 1.98 GHz. An experimental prototype is fabricated on Rogers RT/Duroid 5880 board. The measurement results match well with the theoretical and numerical ones.
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