Commonly, resonators are coupled to each other by a variety of means such as irises, screws, windows, polarization and notches for example. When these coupling devices are used in narrow band filters, they act as barriers to a large portion of the resonator field thus allowing only a small portion of the resonator field to be coupled between resonators.
A useful coupling device is a below-cutoff section which does not propagate certain resonator energy due to the intrinsic limitations of the coupling device as disclosed in U.S. Pat. No. 4,692,723. However, these devices permit (pass) undesired modes about the desired frequency mode to be coupled since they are not far below cutoff. Thus, while these coupling devices generally provide adequate cutoff for the desired operating mode, undesired modes about the desired frequency mode and higher order frequency modes are permitted to leak through.
Tuning pins or mode filters may individually suppress modes as disclosed in U.S. Pat. Nos. 4,138,652 and 3,495,192. However, there is no general device which may suppress all the undesired higher order frequency modes and undesired modes about the desired frequency mode.