Analysis and Design of Compact and Wide-Passband Planar Crossovers

Compact planar crossovers with a wide passband are proposed. By cascoding two quarter-wavelength branch-line couplers, a wide passband can be obtained. Moreover, the even-odd mode and the fourfold symmetry are adopted to derive the analytical equations. According to the derived formulas, the transmission lines' impedances Z1, Z2, and ZA, which may influence the performance of the operating fractional bandwidth (FBW), should be properly selected. In terms of experimental verification, the circuit size of the fabricated crossover is 0.23λg× 0.23λg. Moreover, the FBW of the developed crossover is the 59% 1-dB insertion-loss relative bandwidth corresponds to the S31parameter, whereas the 55% 15-dB one refers to the S21parameter. In addition, four bandstop filters can be added at four ports of the proposed crossover to achieve a wide stopband performance.