Design of Broadband CPW Branch-Line 3-dB Couplers

This paper presents a design technique of a broadband CPW branch-line 3-dB coupler with open-circuited quarter-wavelength series-stubs added at each port as a matching network. After describing the design concept, a coupler with series-stubs constructed inside the central strip of each CPW port is designed and a good hybrid performance over a fairly wide bandwidth of 31 percent or more is demonstrated by em-simulations and experiments. In addition, downsizing is attempted by the use of reactive loading onto the series-stubs and the ring line sections, and successfully achieved without destroying the broadband properties.