Research On Microstrip Line Planar Power Amplifier

With the advanages of broadband,high frequency and high power,traveling wave tubes(TWTs),are widely used in electronic countermeasures,unmanned aerial vehicle and satellite communications.However,with the rapid development of solid-state power amplifier in recent years,vacuum electron devices(VED)subject to more and more competitive pressure,which prompted the concept of microwave power module(MPM)to be put forward.MPM combines the advantages of miniaturized TWT and solid-state power amplifier,and has a great improvement in size,efficiency,weight and so on.Especially under the demand of miniaturization and integration of all kinds of military and civil equipments,MPM has important potential application value.Helix TWTs have advantages of wide band,high efficiency and low voltage.After decades of development,manufacturing process of the helix TWT is very mature.Therefore,vacuum electron power amplifiers utilized in the current reported MPMs are almost helix TWTs.On the other hand,MPMs need to develop towards miniaturization,integration and high frequency,but the helix TWT becomes one of the limiting factors.One is that the helix TWT is a three-dimensional structure,which is difficult to integrate with the solid-state power amplifier,and the other is that the helical TWT is difficult to be machined and assembled during the development to a higher frequency band(> 65 GHz).Therefore,it is of great practical value to obtain TWT amplifiers with simple processing technology,small volume and light-weight,which can promote the development of MPM to higher frequency.Finally,the concept of planar TWT is proposed.The planar TWT generally uses the microstrip meander line(MML)slow-wave structure(SWS)as the interaction circuit of the electron beam.In low frequency,the MML SWS can use the printed circuit board technology to carry on the large-scale production.In high frequency,the MML SWS can use the micromachining processing to fabricate.At the same time,the MML SWS is easy to integrate with the solid-state circuit.However,the planar TWT based on MML SWSs also faces several problems: 1)as a substitute for helix SWS in MPM,the operating bandwidth of the MML SWS is limited.2)The working mode of the MML SWS is surface wave,and the low coupling impedance leads to the low interaction efficiency of the MML planar TWT.3)The problem of charge accumulation.4)The electron beam interacting with the MML SWS requires a large focusing magnetic field,which is difficult to realize in the actual tube making process.Aiming at the problems existing in the MML SWS,the author takes the new plane SWS as the breakthrough point and studies the MML SWS from the computer simulation and experiment.The main work and innovation are as follows:1.In order to expand the operating bandwidth,a coplanar waveguide(CPW)SWS is proposed.The results show that the CPW SWS has a very flat dispersion curve.The 3-dB bandwidth of the designed Ka-band CPW planar TWT is 14 GHz,which increased 180% than the 3-dB bandwidth of the Ka-band MML planar TWT.2.In order to increase the coupling impedance of the MML SWS,a suspended MML SWS is proposed.The electromagnetic field of the traditional MML SWS is mainly concentrated in the dielectric substrate.The electromagnetic field in the vacuum cavity exists in the form of surface wave,which leads to the low coupling impedance of the MML SWS.The dielectric substrate of the suspended MML SWS mainly plays a supporting role,and the electromagnetic field is concentrated in the vacuum cavity on the upper and lower sides of the dielectric substrate,which can greatly improve the coupling impedance of the SWS.At 35 GH,the coupling impedance of the Ka band suspended MML SWS is 86.8% higher than that of the traditional MML SWS.In the range of 34GHz-38 GHz,the interaction efficiency of the suspended MML planar TWT is greater than 10.31%.3.To solve the charge accumulation problem,a dielectric embedded MML SWS is proposed.The proposed dielectric embedded MML SWS can avoid charge hitting the dielectric to a certain extent,thus providing a new idea for solving the charge accumulation problem.At the same time,the dielectric embedded MML SWS also has higher coupling impedance.In the range of 33 GHz to 37 GHz,the saturated output power of the dielectric embedded MML planar TWT is greater than 46.8 W,and the gain is greater than 21.66 dB.4.In order to decrease the focusing magnetic field,a winding MML SWS suitable for cylindircal electron beam(CEB)is proposed.The winding MML SWS has higher coupling impedance than the conventional MML SWS,and it is suitable for interaction with CEB.The focused magnetic field of the Ka-band winding MML TWT is only 0.4T,while the focused magnetic field of the Ka-band planar MML TWT is 0.6T.5.The application of 3D printing technology in VED has been discussed.Ka-band Sine waveguide SWS were fabricated utilizing selective laser sintering(SLS)and fused deposition modeling(FDM),respectively.Transmission characteristics and losses of the 3D printed Sine waveguide SWS are measured.The prospects and problems of the application of 3D printing technology in VED are discussed.