A 3-dB branch-line hybrid coupler with wide stopband responses is presented in this letter. An equivalent K-inverter with bandpass function is used instead of one quarter-wavelength transmission line, which will realize size reduction and wide stopband characteristic of the coupler. Prototype of a branch-line hybrid coupler, which divides the power equally with 90° phase difference between the output ports, is also fabricated and tested. Both the simulation and measurement results show that such a hybrid coupler exhibits an 83.2% size reduction and has transmission suppression of -20 dB or less within five-fold bandwidth.
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