A Filtering Rat-Race Coupler Based on Coupling-Enhanced Structure

This paper proposed a filtering Rat-Race coupler based on coupling-enhanced structure, and the principle is introduced in detail. Based on the existing coupling scheme of the filtering Rat-Race coupler, a coupling-enhanced structure has been introduced. This design method can significantly increase the relative bandwidth by utilizing a coupling topology network to design the filtering Rat-Race coupler. In order to verify the feasibility of the design method, a filtering Rat-Race coupler operating at 2 GHz was designed using microstrip technology. The designed filtering Rat-Race coupler replaces the coupling nodes with a $\lambda / 2$ stepped-impedance resonator (SIR). The simulation results indicate that the designed filtering Rat-Race coupler has an insertion loss of 3.31 dB at the center frequency, a return loss better than 22 dB, and an isolation better than 23 dB. The relative bandwidth reaches $20 \%$, while maintaining a compact size of approximately $0.25 \lambda \mathrm{~g} \times 0.25 \lambda \mathrm{~g}$.