Design of Three-Dimensional Ultra-Wideband Spatial Bandpass Filter Based on Slotline-Microstrip Structure

A novel design of an ultra-wideband spatial bandpass filter based on a slotline structure combined with a microstrip line is proposed in this letter. The unit cell of this design effectively transforms spatial waves into guided waves within the slotline, subsequently creating a multi-mode resonant cavity via the slotline-microstrip-slotline transition structure, thereby facilitating a wideband filtering effect characterized by multiple transmission poles. The design methodology and simulation results are discussed in detail, demonstrating the effectiveness of the proposed filter in achieving the desired wideband filtering characteristics. The obtained ultra-wideband filter can achieve insertion loss below 0.3 dB within the frequency range of 3.5-5.5 GHz, with a relative bandwidth of 44.4% and angular stability of up to 60°. The unique design of the slotline to microstrip transformation lays a solid foundation for combining three-dimensional filter structures with microstrip circuit designs.