A Compact Filtering 180° Hybrid

A novel compact filtering 180 hybrid utilizing coupled resonators is presented. The equivalent-circuit structure is just like a bandpass filter, thus the frequency response is filter-like. The sum and difference port operations respectively correspond to the symmetric and antisymmetric states; therefore, isolation between sum and difference ports is accomplished by geometrical symmetry. The synthesis method of the filtering 180 hybrid is illustrated. The proposed structure is decomposed into one power divider and one balun, which are designed separately. A second-order filtering 180 hybrid prototype centered at 2.4 GHz with 0.2-dB Chebyshev equal-ripple response and 10% fractional bandwidth is demonstrated. The passband insertion loss is about (3.0 + 0.7) dB, and the isolation between the sum and difference port is greater than 20 dB. Magnitude imbalance is less than 1 dB and phase imbalance is within 2 degrees. Good agreement between simulation and experimental results is achieved.