Compact Filtering Rat-Race Hybrid With Wide Stopband

This paper presents a novel method to design filtering rat-race hybrids with wide-stopband bandpass responses. Four ± K-inverters with bandpass functions are utilized to replace the one or three quarter-wavelength transmission lines in the conventional rat-race hybrid. The ± K-inverters consist of transmission lines and capacitors, which can reduce the area occupied and suppress the high-order harmonics. Furthermore, the isolation performance is enhanced by embedding the ± K-inverters. Theoretical analysis is carried out and design equations are derived. For demonstration, a filtering rat-race hybrid with bandpass responses is implemented. The center frequency is located at 470 MHz with the insertion loss of 1.2 dB. Good in-phase and out-of-phase characteristics are observed. Comparisons between the measured and simulated results are also presented to verify the theoretical analysis. The overall size is only 4.6% of the conventional hybrid and stopband is extended to 5f₀.