Compact 4-pole BRF-based directional filter with even-mode matching circuit for sharp cut-off

The directional filter structure using λ/2 strip resonators with better filtering performance has been the focus of recent research activities. The band-reject filter based directional filter structure has been demonstrated as an effective way to achieve this goal. However, the prototype reported is cascading resonators in the lateral direction. When the order of the filer gets higher, the lateral cascading leads to rapid oscillation in the stop-band of S₄₁ and a lengthy structure. This paper demonstrated a 4-pole band-reject filter based DF using direct-coupled band-reject filter. It achieves the highest order of DF response reported without the rapid oscillation in the stop-band. Yet it is about 30% smaller compared to previous 3-pole directional filter prototypes.