Research On Passive Intermodulation Control Of High Power Radio Frequency Modules Of Communication Satellite

Passive intermodulation(PIM)caused by passive radio-frequency(RF)devices(like cable assembly,coaxial connector,waveguide assembly,duplexer,etc.)in the payloads becomes quite serious in designing mobile communication satellites.Especially,these passive RF components are quite easy to cause intermodulation distortion due to large current density flowing through them and fast temperature changes if they work in the high-power mode.Therefore,how to solve the PIM generated by passive RF components such as coaxial connectors and waveguide devices under the high-power working condition is a technical difficulty in the development of mobile communication satellites.In this thesis,the high-power RF cable of communication satellite is taken as the research object.The mechanism of PIM generated by passive cable connectors and waveguide flanges,the methods and technologies for PIM suppression are studied.Firstly,the basic theory and modeling evaluation methods of the PIM generated by high-power RF modules are introduced.Secondly,the design criteria,structural design methods and thermal analysis results of high-power cable module are introduced,A detailed study has been done to analyze the PIM generated by coaxial cables.including braiding material,coating,dielectric support,processing and assembly,system integration,etc.The control method of PIM is also presented,including cable test method and error control study.Finally,the property of PIM generated by waveguide flange is analyzed and a new waveguide flange of high performance in suppressing PIM is fabricated.The mian content in this thesis can be outlined as follows:(1)The major factor,i.e.,rubber ring that leads to severe PIM deterioration in the traditional coaxial connectors working in high-heat space environment has been found by using the analysis method of influence factors of coaxial cable.After the rubber ring is re-designed and optimized,the experimental results show that the PIM produced by the coaxial connector is significantly suppressed and intermodulation of the improved rubber ring is reduced more than 40dB;(2)It is found that the micromechanical superfluous material has obvious effects on PIM during the cable welding process.After the welding protection measures are taken,the measured PIM is improved more than 20dB;(3)A cable torque control method is proposed.Compared with the traditional torque connection method,the test level of PIM by using the new method achieves an improvement of about 25 dB,which has a valuable meaning in guidance to reduce the PIM caused by improper torque in the installation of coaxial devices.(4)A new Ku-band waveguide flange with a choke structure is designed.The measured third-order PIM of the flange is-140 dBm.Compared with the traditional flange,its PIM performance is improved up to 20 dB.The above methods and technologies remarkably reduce the PIM generated by passive coaxial connector and waveguide flange under high-power working conditions.The research results will promote the development of high-performance mobile satellite communication system.