Theoretical design of wideband filters with attenuation poles using improved parallel-coupled three-line units

A theoretical filter-design-scheme is proposed for a wideband filter consisting of an compound parallel-coupled three-line unit. After revealing the work mechanism of the proposed three-line unit, the design formulae are derived correspondingly. To validate the proposed techniques, a wideband filter with a bandwidth of 3–5GHz (FBW:50%) is designed as an example. The measured frequency responses of the filter agree well with the predicted ones, which validates the effectiveness of the newly-proposed three-line unit and the corresponding design scheme.