Research On High Frequency Characteristics Of Planar Traveling Wave Tube

In the millimeter and sub-millimeter(THz)frequency bands,miniature vacuum power amplifiers are widely used in electronic countermeasures,radar,communications and other fields.Slow wave structure(SWS)is the key component of such vacuum power amplifiers like traveling-wave tube(TWT).However,as the operating frequency increases to W-band or above,traditional machining methods cannot meet the required precision because the dimensional parameters of the SWS become smaller and smaller.Microstrip meander-line(MML)SWS,which seems easy to be fabricated and mass produced,is catching an extensive concern.According to the available technology,two kinds of the high frequency system of W-band microstrip meander-line slow wave structure are designed and fabricated in this paper.The main contents and research results of this paper are as follows:(1)According to the technological requirements,the U-shaped MML with conformal dielectric substrate TWT is designed.Firstly,the coupling impedance calculation program is written in CST,and the coupling impedance of a certain point and the average coupling impedance a certain region inside the slow wave structure is calculated,in order to analyzing the slow wave structure coupling impedance variation tendency and the influence of the main structural parameters of the designed SWS on the high frequency characteristics.Secondly,In order to solve the problem that the medium substrate is too wide due to machining,which can easily excite the higher order mode,the traditional input-output structure was improved in this paper.Finally,the designed TWT is simulated by CST Particle Studio.The simulation results show that the proposed structure can provide an achievable gain up to 23.8d B and a maximum output power of 29W at 96GHz.The 3d B bandwidth is 7GHz.(2)Combined with the process improvement,a U-shaped MML TWT is designed.Firstly,the optimal high-frequency structure is obtained through simulation.The distribution of high frequency field of the slow wave structure is studied,and the variation law of coupling impedance is also given in this part.Secondly,using microstrip probe-waveguide as the coupling structure between the slow wave structure and the input-output waveguides,the transmission characteristics of the U-shaped microstrip meander-line high frequency system with two kinds of transition forms are analyzed.Finally,the beam-wave interaction of this U-shaped microstrip meander-line TWT is investigated in detail.The simulation results show that the peak power of the traveling wave tube can reach 30.6W at 96GHz,and the corresponding gain and electronic efficiency are 24.07d B and 4.7%,respectively.The 3d B bandwidth is 9GHz.Compared with the U-shaped TWT designed according to unit A,the TWT designed according to unit B has higher output power,higher electronic efficiency and wider bandwidth.(3)Machining the designed high frequency systems and carrying out the corresponding experiments.Firstly,the fabrication process of the two kinds of microstrip meander-line slow wave structures are described in detail.Secondly,this paper describes the processing of the input-output structure,which is matching with the slow wave structure.Also,and the U-shaped MML high-frequency system is obtained by micro-assembly technology.Finally,a vector network analyzer was used to test the transmission characteristics of the U-shaped microstrip meander-line high frequency system.The test results show that the transmission coefficient S₂₁is greater than-4.87d B and the reflection coefficient S₁₁is less than-19.74d B over 90-102GHz.