New modeling of three-coupled-lines and its application to wideband filter

The paper presents a new modeling of three-coupled-lines by considering the cross-coupling between non-adjacent-coupled lines. A simulation based method is proposed to extract the cross-coupling. A synthesis method is then proposed to design the wideband filter composed of three-coupled-lines. As an example, a two-section three-line filter is synthesized, simulated and experimentally realized. The results obtained by our synthesis theory compare very well with those by simulation and experiment, while the conventional model neglecting the cross-coupling between non-adjacent lines exhibits a 17% error in bandwidth estimation.