High-Q tunable bandstop filters with adaptable bandwidth and pole allocation

Tunable bandstop filters are demonstrated with wide tuning ranges, tunable bandwidth, and a variable number of dynamically allocated poles. The number of poles can be dynamically applied at different frequencies to maximize isolation for a given interference scenario. These filters are made with evanescent-mode cavities which use a new loading post geometry that results in greater external coupling for a given physical coupling aperture size. This geometry concentrates the magnetic field within the cavity structure near the coupling aperture, allowing for a smaller coupling aperture and reducing perturbation to the feeding transmission line. The design allows for wider bandwidth bandstop filter responses compared to previously reported geometries. Relatively low external Q values are achieved with minimal perturbation to the passband response. A relatively high Q (450 @ 3 GHz) allows for deep notches and good selectivity.