A 6-4 Waveguide Bandpass Filter With Extremely Small Negative Couplings

This letter presents the design and implementation of a sixth-order waveguide bandpass filter (BPF) with four finite transmission zeros (TZs), in which a negative coupling as small as 0.000452 is required. Such an extremely small negative coupling typically poses significant manufacturing challenges when using conventionally negative coupling structures, such as capacitive irises. To address this, a negative dispersion-less coupling structure (NDLCS) is proposed based on a partial-height post and a pair of partitioning walls. This design increases the minimum feature size from tens of micrometers to a few millimeters compared to capacitive irises, thereby significantly reducing the manufacturing difficulty. To demonstrate its advantages, the 6-4 BPF incorporating the proposed NDLCS is also compared with BPFs using capacitive iris and a single post. Full-wave simulations and experimental measurements show excellent agreement in passband performance and TZ placement.