Miniaturized branch-line coupler with wide upper stopband using novel synthesized microstrip lines with series LC tanks

Based on a new synthesized microstrip line, a miniaturized branch-line coupler with wide upper stopband is developed at 1.85 GHz. The new synthesized line consists of a pair of series LC tanks with two additional meander line inductors. The LC tanks introduce a pair of transmission zeros, which can be independently controlled to provide wide upper stopband from 3.5 to 8 GHz. Benefitting from the slow-wave nature of the synthesized microstrip lines, the proposed coupler also shows a very compact size, which is 28% the size of its conventional counterpart. The design methodology and experimental results are discussed in detail in the paper.