Realisation of compact quasi-elliptic bandpass filters based on coupled eighth-mode SIW cavities

A novel basic quasi-elliptic response bandpass filter in quadruplet based on coupled TE101-mode-based eighth-mode substrate integrated waveguide (EMSIW) cavities is proposed and analysed firstly. The negative cross coupling is achieved by the nature of coupled fringe electric-field components of the first and fourth cavities. Then, based on the proposed basic quadruplet structure, two methods using a step-impedance resonator and meandered feeding lines, respectively, are introduced and analysed to realise S-L coupling to improve the upper stopband performance further. Finally, two possible structures with quasi-inline and quasi-folded layouts, respectively, are proposed to realise sixth-order EMSIW filters. Five EMSIW filters are designed, fabricated, and measured to demonstrate and verify the proposed structures. The proposed filters have the advantages of compact size, good out-of-band rejection and realisation of higher-order filtering function. The size of the realised 4th-/6th-order filter is less than $0.13/0.17\lambda _g<^>2 $0.13/0.17 lambda g2. The out-of-band rejection higher than 28 dB can reach 2.32f(0).