Research On Dual-band Helical Cavity Bandpass Filter

With the rapid development of the microwave communication technology, thelimited frequency content becomes more and more crowded. Modern communicationsystem requires bandpass filters with higher performance. The use of dual-bandmicrowave filter in communication system can improve the integration level andreduce the cost. In the communication system with low frequencies, such as VHF andUHF, helical resonator is characterized by small size, light weight and high Q value.Thus, it is widely used in mobile communication system. Theoretically, dual-bandhelical filter will takes the advantages of high integration level, low cost, small sizeand light weight. Thus, it has promising application potential in the communicationsystem with VHF and UHF.This thesis focuses on the structure of dual-band helical bandpass filter.Emphasis is given to the design methodology of dual-band helical resonator. In orderto design a resonator with controllable dual-frequency, the electromagnetic fielddistribution of the traditional helical resonator is firstly investigated. Then, theeffects of the helical radius and pitch on the frequency of the helical resonators areconcerned. The results indicate that the dual-frequency of the helix resonator can betuned by changing the helical radius and pitch of the open end and short-circuited ofthe helix resonator. Several curves of the ratio of two frequencies are obtained. Onthe basis of the investigations, two frequency-controllable dual-band helical cavitiesare designed. One of them is designed based on the stepped impedance resonator, andthe other is designed based on the resonator with non-uniform pitch. By using theresonator with non-uniform pitch, a second-order dual-band bandpass filter withnon-uniform helical cavity resonator is designed. The S-parameter curve is given forthe case of using the coupling tap and coupling probe simultaneously. The effects ofthe height and width of the coupling window on the S-parameter curve areinvestigated. Finally, the designed filter is processed and tested by using theoptimized parameter. The tested results are consistent with simulation results.