A Folded Substrate-Integrated Waveguide Quadrature Backward Filtering Coupler

This letter presents, for the first time, a folded substrate-integrated waveguide (FSIW) quadrature backward filtering coupler. The proposed structure consists of two identical rows of three FSIW resonators, where each resonator is coupled to adjacent ones. The design employs both positive and negative couplings. A positive coupling is realized through conventional coupling windows, while a negative coupling is introduced via a fully enclosed rectangular slot within the FSIW structure, effectively eliminating radiation loss. A detailed design procedure is provided, followed by the fabrication and measurement of a prototype. The experimental results demonstrate good agreement with simulations.