Dual-band Helical Filter Based on Helical Coaxial Stepped Impedance Resonators

A dual-band helical filter with two coaxial stepped impedance resonators (SIRs) is proposed in this paper. The helical filter can realize independent control of the frequency ratios of the first two resonant modes in a certain range by adjusting the impedance ratio of the helical SIRs. The prototype of the helical resonator is analyzed by referring to the equivalent transmission line models. In this case, the supporting portion of the helical structure is designed to be part of the SIR, which enhances the strength of the physical support and realizes the miniaturization of the filter. Meanwhile, the use of the helical resonator enables further miniaturization by bending the internal conductive pillars. The modified T-shape feeding probes are proposed to relative freely control the external coupling of different passbands. In addition, the coupling between the two cavities takes the form of a ridge coupling, which greatly enhances the coupling strength between the two cavities. It is worth mentioning that each passband has transmission zeros (TZs) on both sides to achieve the purpose of high frequency selectivity. Therefore, control-lable filtering response in a certain range can be achieved in this design. The dual-band helical filter can be fully compatible with 3D printing technology, which meets the merit of low cost, light weight, as well as complex structure implementation. For verification, the simulated results satisfy the requirements of the design.