All‐Port‐Reflectionless Planar Wideband Filtering Directional Coupler Using Two Groups of Four‐Coupled‐Lines

ABSTRACT In the paper, a planar wideband filtering directional coupler (FDC) with all‐port‐reflectionless behavior is presented by merely using two groups of four‐coupled‐lines (FCLs) and resistors. One group is the FCL with open‐circuited at the diagonal terminals, which forms the quadrature coupler with the feature of wideband filtering. The other group is the FCL with connection at the adjacent ports to construct the resonator. Wideband reflectionless behavior at all ports can be achieved when it cooperates with two resistors. Design equations are derived based on the Z ‐matrix of the FCL, and parameter investigations are performed. For validation, a prototype operating at 2 GHz is modeled and optimized in Ansoft HFSS, then fabricated on Rogers 4350b. Measurement results indicate that the prototype exhibits the all‐port‐reflectionless bandwidth of more than 320%, the passband bandwidth of larger than 70%, and the stopband rejection of over 19 dB. Moreover, it is also the smallest (occupying an area of only 0.27 λ g × 0.11 λ g ) when compared to the state‐of‐the‐art reflectionless FDCs, which is rather suitable for the highly integrated communication system applications.