This letter presents a novel and compact rat-race coupler design that can operate at two widely separated frequency bands. It is accomplished by modifying the length and impedance of the branch lines in the conventional structure, and the insertion of two additional shunt stubs. The proposed circuit also features planar structure and low insertion loss. For verification, the measured performance of a microstrip coupler operating at 900/2000MHz are shown.
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Cheng et al. (2005) studied this question.
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