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 80GHz. Measurement results up to 69GHz are shown. The measured insertion loss of the filter lies between 7.1 and 9.3dB. An off-resonance isolation better than 60dB is achieved what is extraordinarily high for a two spheres filter. The typical 3dB-bandwidth is about 250MHz. 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 80GHz by replacing the 1.85mm-connectors by 1mm-connectors or using other suitable waveguide transitions, thus covering considerably more bandwidth than rectangular waveguide based coupling structures.