A novel 30–40 GHz bandpass filter with sharp rejection using symmetrical notch cavity

Sharp-rejection characteristics are brought to a 30-40GHz H-plane waveguide filter by adopting a symmetrical notch cavity in this paper. Apart from the conventional H-plane inductance-diaphragm structure, the symmetrical notch cavity is implemented to improve the suppression level at the unwanted frequency close to the passband. The notch cavities which are actually H-plane symmetrical step extensions on the fundamental-mode rectangular waveguide make the manufacturing and assembly process simple and reliable. The pass band is from 30GHz to 40GHz with a measured insertion loss of typically 1.2dB. The measured suppression is greater than 40dB above 40.8GHz with a maximum of 55dB at 41.0GHz, which is 0.8-1.0GHz away from the upper-edge frequency of the passband. The measured return loss is greater than 15dB over 30-40GHz. The experimental results agree well with the simulation, demonstrating that the symmetrical notch cavity is well suited for the design of sharp-rejection bandpass filters (BPFs).