Two- and Four-Pole Multilayer SIW Filter with High Selectivity and Higher-Order Mode Suppression

This letter presents an approach to design two-pole source-load coupling and four-pole cross-coupling substrate integrated waveguide (SIW) bandpass filters based on multilayer process. Utilizing the field distribution, the vertical magnetic and electric coupling of fundamental mode is designed by suppressing the first spurious mode. Then, source-load and cross-coupling schemes are realized with controllable features in two-pole and four-pole filters, respectively. The harmonic passband produced by TE102 mode can be suppressed by proper coupling technique enabling the connection with TE102 mode in two- and four-pole filter designs, respectively. Three transmission zeros, which are derived from source-load coupling, are introduced around the passband of two-pole filter to improve its selectivity. In the four-pole filter design, a six-order cross-coupling scheme including source and load produces four transmission zeros around the passband, leading to a sharp selectivity. In addition, another transmission zero is generated at the adjacent location of the passband to improve the out-of-band rejection. Compared with conventional horizontally coupled filters made of single layer, the proposed filters show a compact size. To demonstrate the proposed design method, a two-pole and a four-pole double-layered SIW bandpass filters are fabricated and measured. Measured results show that the proposed filters exhibit high selectivity and good out-of-band rejection, as well as a good agreement between simulated and measured results.