Design of planar metal stripline filter and diplexer with frequency‐dependent couplings

In this letter, frequency‐dependent coupling (FDC) is introduced into the design of metal stripline filters and diplexers for the first time. FDC is a popular research method to introduce the finite‐position transmission zeros in the synthesis and design of filters in recent years. Finite transmission zeros are critical for filters to enhance the rejection ratios. To verify the realizability and effectiveness of FDC, a filter centered at 3.52 GHz and a diplexer operating at 2.55 and 2.65 GHz have been designed and fabricated. The planar filter and diplexer are compact in structure with multiple transmission zeros, whose resonators and cavity are integrated and formed, the cavity and cover plate are soldered together. We designed, assembled and tested the filter and diplexer, the synthesis results, the simulation results, and the tested results are well matched with each other. Compared with the dielectric waveguide filter, the planar stripline filter has better insertion loss, better harmonic suppression, and higher reliability, which is expected to be widely used in 5G base station filters.