Design And Experimental Study On Coupling Structure Of Millimeter Wave Helix Traveling Wave Tube

Millimeter wave traveling wave tube(TWT)are a class of high-quality millimeter wave sources that have now become a key fundamental device in national defense construction and national economy.With the strong demand for high performance radar,electronic countermeasures and satellite communications,there is an urgent need for research into higher frequency bands,higher power and higher efficiency for millimeter wave TWTs.As one of the key components of TWTs,coupling structure directly affects the bandwidth and power level of the tube.Therefore,it is of great practical importance to study coupling structure.In this paper,theoretical analysis,design simulation and experimental research are carried out to study coupling structure of helix TWTs in Ka-band and E-band,and the following innovative work is carried out:1.Four types of coupler for Ka-band helix TWT are designed.One is coaxial coupler,and the rest are different types of coaxial-to-waveguide coupler,including coaxial-to-single ridge waveguide-to-rectangular waveguide coupler,coaxial-to-double ridge waveguide-to-rectangular waveguide coupler and coaxial-to-rectangular waveguide coupler.The results reveal that the overall voltage standing wave ratio(VSWR)of coaxial coupler with slow wave structure(SWS)is less than 1.33,while the ones of the coaxial-to-waveguide coupler with SWS are less than 1.43,1.88 and1.47 in the frequency of 26.5-40GHz,respectively,which accomplish the design goal of VSWR less than 2 in the operating band.2.Experimental on coupling structures of Ka-band helix TWT are carried out.The input end adopts coaxial coupler,which has superiorities of wide frequency band,low insertion loss and strong reliability.The cold test results demonstrate that in the band of26.5-40GHz,the VSWR of the coaxial coupler with SWS is less than 1.98.The output end adopts the coaxial-to-single ridge waveguide-to-rectangular waveguide coupler.The experiment results show that the structure with SWS has a VSWR less than 1.96 in the frequency band range of 26.5-40GHz.Problems in the experiment are analyzed in detail and corresponding solutions are obtained in the meantime.To summarize,the designed coupling structure exhibits the characteristics of simple structure,easy processing and small reflection in the band,which verifies the effectiveness and reliability of the design.3.The coupling structure of E-band helix TWT is optimized.Based on the analysis of the high frequency characteristics of the E-band TWT,the causes for the poor transmission characteristics of this type TWT are studied.The matching cylinder is designed via deducing the factors affecting the characteristic impedance of helix,which improves the transmission characteristics of the whole structure.After optimization,the E-band helix TWT S₁₁ decreases from-8d B to-14d B,and S₂₂ decreases from-6d B to-12d B.On this basis the PIC simulation of the entire tube is carried out.In the working frequency band,the output power reaches more than 36W,the maximum output power reaches 52W,and the gain is 41d B.Lastly,a well-matched coupler structure is designed and optimized for the helix TWT.The simulation results indicate that when operated in the 80-87GHz band,the VSWRs of the input and output structures with SWS are less than 1.43 and 1.55,respectively,which meet the design requirements.