The Research On Design Method Of Key Components In Quasi-optical Network System

At the frequency range of millimeter wave(MMW)and sub-millimeter wave(Sub-MMW),benefiting from two important transmission spectrum characteristics with the atmospheric absorption and atmospheric window,quasi-optical network system(QONS)is an important equipment in the front-end transceiver of antenna system,which can change the propagation direction of gaussian beam(GB)and beam radius to separate or synthetic different channel signals for realizing light path transformation,so it has been widely used in remote sensing,radio astronomy,radar detection,missile guidance,biomedical science,airport security and other application fields.Although foreign quasi-optical technology has persistently developed for a long time,they have accumulated a lot of theoretical and engineering achievement.However,the fact is that it has been restricted by many precision machining conditions and testing environment,the development of MMW and Sub-MMW quasi-optical technology still needs to be improved in China.In particular,the design and testing of key devices in QONS need to be studied in detail.In this context of demand,combining with some space projects that I participated fortunately in my laboratory,a series of QONS researches among theoretical analysis,three key components design,and near-far-field testing techniques have been carried out during my postgraduate study from 2012 year to 2018 year in the BUPT.The specific research contents and common achievements mainly contain five aspects as following.First of all,a fast-electromagnetic analysis and calculation method for dual-reflection mirrors is finished.Based on the physical optics(PO)and geometrical optics(GO),the GB analysis method has been proposed and verified to realize continuous beam tracing in double-mirrors system as an electromagnetic analysis technique,including the Gabor expansion method and point matching method(PMM)in pattern expansion of input field,the use of astigmatic GB on mirror surface by parabolic reflection model and Q-matrix for obtaining reflection parameters of GB,the output field analysis of QONS in two different elliptic or parabolic mirrors by GB continuous tracing technique.The electromagnetic calculation results show that the output near-field by continuous tracking technology of GBs is accurate to-60dB,and the corresponding far-field pattern can be accurate to-50dB.Secondly,Gaussian mode transformation is realized by quasi-optical mode converter for improving quality of output beam.Based on the irradiance moments method,the QO mode converter has been designed with double shaped mirrors system.It can convert mixed higher order GB modes into fundamental GB(FGB)mode of GB(0,0),at the same time,and complete the correction function of output amplitude and phase simultaneously,which solves a problem of beam distortion in multi-mirrors QONS.After application of third order,fourth order,and five order moments method,comparing transformation ability of algorithm from seven aspects in the first 15 hybrid high-order GB modes,it proves that the fourth-order irradiance moments method is the most effective,and it is able to restore mixture modes with a leastest fundamental GB(0,0)mode ratio of 40%into a pure FGB.The furst advantage of this electromagnetic analysis technology has fast computation speed that can converge to 10-32 only after 6?14 iterations.Another by means of linear fitting of dual-shaped mirrors reduces the difficulty of processing and has broad working bandwidth characteristic.Thirdly,two kinds of GB horn feed with high performance gaussian radiation characteristic are designed.The aperture field distribution of TE and TM modes in circular waveguide has been studied.Then the modes decomposition about HE1n and EH1n of aperture filed in an ultra-Gaussian feed horn is analyzed by using spherical wave expansion method(SWE).What's more,the mode matching method(MMM)is learned for analyzing discontinuity structures in circular waveguide,so the corrugated horn and linear cone horn with gaussian radiation characteristics are designed and optimized.Thereinto,the Tanh-profile corrugated horn in the center frequency of 322GHz has the low side-lobe and cross-polarization level that reach-40dB and-58dB,respectively.Another linear conical horn with three sections can also obtain a good gaussian radiation characteristics,where the side lobe level and cross polarization level reach below-30dB between 310GHz and 340GHz.In addition,it can reduce the difficulty of manufacture and be suitable to work at one case of horn array.Fourth,the beam separation components with good performance using two wire grid polarizers(WGP)are optimized,manufactured and measured.The two different kinds of wire grid polarizer(WGP)are designed by simulation working at 1GHz to 500GHz.There are free-standing WGP and dielectric plate WGP.The simulation process has shown that the pitch-to-width ratio of g/2a should be selected as one range at[3.1,3.25]for both good polarization isolation effect.After the study of different location relationship of dielectric layers,the best reflection and transmission results can be obtained in the periodic structure with backward anti-reflection layer(ARL).More importantly,the machining parameters are determined by optimal simulation structures,and then the WGP is fabricated and measured from 75GHz to 325GHz.At the dielectric WGP,the time domain analysis technology has been applied into its test of transmissive polarization isolation curve,where the spurious signals and multi-path effects from environment are eliminated during whole test process.Typically,transmission polarization isolation at 183GHz and 324GHz is-25.33dB and-19.06dB,respectively.Finally,the overall work performance of QONS is assessed by different near-far-field antenna measurement and anslysis techniques.It relies on the design of dual-channel QONS and the development of near-far-field test software.After using the dielectric plate WGP as beam separation components,the dual-channel QONS realizes the reflection of horizontal-polarization signals at 183GHz channel and the transmission of vertical-polarization signals at 324GHz channel.At the situation of planar near-field measurement,it obtains the three kinds of different reflection field by using polarizer,copper plate,or aluminum plate,and the two different transmission fields by polarizer or direct air.Further,by means of three different far-field analysis methods,which are near-far-field transformation theory,ideal quiet zone(QZ)integration principle and direct far field test in CATR,the output beam quality of near-and far-field in QONS is compared,where the differences of output field in energy loss,plane pattern symmetry,beam radius,cross polarization isolation and side lobe level are analyzed in detail.Under the condition after using polarizer,a series of compared test results have shown that the reflection and transmission channels of QONS can output the field with circular-symmetry planar pattern,high cross polarization isolation,low energy loss and side lobe level.From what has been discussed above,the researches on three kinds of key components about Gaussian modes converter,Gaussian feed,and polarizer,and the application analysis of antenna near-far-field test technology of QONS can further promote their application in domestic aerospace industry and scientific research,meanwhile provide technical reference for related radiometer and other quasi-optical systems in China.