A compact wideband 1800 hybrid ring coupler using a novel interdigital CPS inverter

This paper proposes a stepped-impedance wideband 180deg hybrid ring coupler with a novel frequency-independent 180deg phase inverter implemented by interdigital coplanar strip (CPS). Simple design equations for stepped-impedance lines and design curves for CPS/interdigital CPS are also presented. Due to the combination of a stepped-impedance structure and an ideal phase inverter, the proposed hybrid achieves size reduction, wide bandwidth, and excellent phase and amplitude performance. The size of this hybrid is about 13% of the conventional one. Experimental results show that this hybrid ring coupler centered at 2 GHz has a 10-dB return loss bandwidth of 97.5% from 0.98 to 2.93 GHz with 0.5 dB amplitude imbalance and 3deg phase imbalance for both out-of-phase and in-phase operation. Its isolation is better than 30 dB from 0.5 to 4 GHz.