Synthesis of Four‐Coupled‐Line Based Wideband Coupler With Inherent Filtering Feature and Its Modification

ABSTRACT In the paper, a compact wideband filtering quadrature coupler is proposed, which utilizes the inherent filtering feature of the unequal‐width four‐coupled‐line (FCL) for the first time. Simply by assigning four terminals of the FCL as open‐circuited, the functions of power division, quadrature phase, and inherent filtering response are obtained simultaneously within a wideband. It also has the merits of high directivity and low insertion loss. Since the proposed FDC has the inherent filtering feature without adding extra resonators, the size is the smallest (0.29 λ g × 0.13 λ g ), and the structure is the simplest when compared with state‐of‐the‐art wideband FDCs. Moreover, based on the derived universal impedance matrix of the unequal‐width FCL, the FCL‐based coupler, as well as other multiport circuits, can be theoretically analyzed. For validation, a prototype is implemented. Measurements show that the proposed coupler exhibits a bandwidth of over 45% under the conditions of 10‐dB return loss, 10‐dB isolation, 0.5‐dB amplitude imbalance, and 5° phase difference error. Besides, the stopband rejection is more than 15 dB, and the rectangle coefficient (RC = |BW 20 dB /BW 3 dB |) reaches 1.74. Further, as a modification, the high‐frequency selectivity characteristic (RC = 1.11) can be obtained by simply inserting λ /2 stepped impedance resonators with less influence on in‐band performance and little increase in size.