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 Z_A, 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 0.23λ_g × 0.23λ_g. Moreover, the FBW of the developed crossover is the 59% 1-dB insertion-loss relative bandwidth corresponds to the S₃₁ parameter, whereas the 55% 15-dB one refers to the S₂₁ parameter. In addition, four bandstop filters can be added at four ports of the proposed crossover to achieve a wide stopband performance.