Novel tunable hexaferrite bandpass filter based on rectangular waveguide coupled shielded coplanar transmission lines

In this paper a novel tunable high isolation bandpass filter is presented. The filter consists of two shielded coplanar transmission lines coupled by a rectangular waveguide. Hexagonal ferrite spheres are used to cover a tuning frequency range from 50 to 80 GHz. Measurement results up to 69 GHz are shown. The measured insertion loss of the filter lies between 7.1 and 9.3 dB. An off-resonance isolation better than 60 dB is achieved what is extraordinarily high for a two spheres filter. The typical 3 dB-bandwidth is about 250 MHz. As planar transmission lines are used, the filter can be integrated into a coplanar or microstrip environment easily what is advantageous for the construction of highly integrated modules. The operating frequency range can be expanded up to 80 GHz by replacing the 1.85 mm-connectors by 1 mm-connectors or using other suitable waveguide transitions, thus covering considerably more bandwidth than rectangular waveguide based coupling structures.