A novel microstrip coupled-line directional coupler is proposed in this paper. It is based on the introduction of a complementary split ring resonator and dumbbell-like defected ground structure on the coupled lines to strongly enhance the designed backward coupling. The designed frequency band is from 1.2 to 1.5 GHz. The coupler is fabricated and tested. The insertion loss is less than 3.5 dB. The simulated and measured return losses are better than -13.5 dB, and the isolation is higher than 20 dB across the operating band. The overall size of the coupler is 80 mm×70 mm, which is about 0.36λ×0.32λ at the central frequency 1.35 GHz.
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