Research On Millimeter-Wave And Terahertz High-Gain Antennas And Its Planar Near-Field Measurement Techniques

High-gain antenna is the essential component for millimeter-wave and terahertz wireless communication systems.With the rapid development of millimeter-wave and terahertz wireless communication,radio astronomy,imaging,etc.,it requires an antenna not only to have a high gain with compact size but also a better beam scanning capability.Therefore,high-gain,high-efficiency,compact size,multi-beam millimeter-wave and terahertz antennas have become one of the hotspots for the current antenna research field.Additionally,due to the small wavelength and high atmospheric loss at these bands,accurate testing of high gain antennas faces many challenges,including 1)The output power and signal-to-noise ratio of millimeter-wave and terahertz test instruments are significantly lower than counterparts at microwave band,which lead to limited measurement performance and high time cost.2)The beamwidth of high gain antenna is quite narrow,the test result is extremely sensitive to measurement errors.3)Conventional far-field test method often faces the dilemma of long test range and low test signal level.In this dissertation,an in-depth study is given on the topic of“Research on Millimeter-Wave and Terahertz High-gain Antennas and its Planar Near-field Measurement techniques”,and the following progress is made:（1）An indoor antenna measurement system that can perform high precision planar near-field and far-field measurement tasks for millimeter-wave and terahertz antennas,is set up here.All the control and post-processing software is developed by the author.This system mainly focuses on the test chal enges of high-gain antennas in the terahertz band,and has achieved good results in high-precision beam alignment,fast test sampling,and fast near-to-far field conversion:1)The beamwidth of the high-gain antenna is extremely narrow,which leads to a large beam alignment error during the test.In response to this problem,a low-cost,high-accuracy beam-alignment method is presented,which can be directly performed through post-processing,which requires no actual displacement or rotation operations,therefore the system cost and workload for error correction can be drastically reduced.2)Since the planar near-field measurement for terahertz high-gain antennas is an extremely time-consuming process,a low-cost and high-precision dynamic scanning method is proposed in this dissertation.By combining this method with other methods,such as enlarging the sampling step or using adaptive rectangular spiral scanning,both the sampling scale and measurement time per point can be significantly reduced,hence the measurement efficiency can be greatly improved.3)Due to the low dynamic range of the terahertz room temperature measurement system,the instrument sampling time and the displacement time for the translation stage are comparable.Therefore,the near-field data obtained by the dynamic scanning method is non-uniform distributed,and cannot be used by regular near-field to far-field transformation process.In order to address this problem,a low-cost high efficient solution is proposed,which uses software commands to record the position for every sampling point.The position data along with near-field data are both processed by non-uniform fast Fourier transform algorithm,thus fast and accurate near-field to far-field transformation can be realized.（2）The dynamic range of the terahertz vector measurement apparatus is far worse than the instruments at microwave and lower part of millimeter-wave bands,so the signal level is quite low during tests,and easy to be affected by various external factors,which make it harder to perform accurate vector field measurement at terahertz bands.In response to these challenges,the plane to plane diffraction（PTP3）algorithm is thoroughly studied in this dissertation,with new solutions are proposed in terms of calculation accuracy,sampling method and noise reduction method,etc.,which can effectively enhance the measurement accuracy,and reduce the time and instrument cost.The specific research progress is summarized as follows:1)In the diffraction step of PTP3,a modified effective angular spectrum region is introduced to constrain the angular spectrum components involved in the calculation.This area is determined by the near-field scanning area and distance between two scanning planes.Simulation results show that this modification can significantly enhance the accuracy of forward diffraction operation.By introducing this modified angular spectrum into the PTP3 algorithm,it can speed up the convergence rate and improve the convergence accuracy.2)Due to the difficulty of determining the actual position of the aperture plane,a concept of equivalent aperture plane is introduced,and a new method to estimate the initial guess of aperture field is presented.This method measures one row and one column of near-field data,which is centered in the vicinity of the maximum intensity field point inside the equivalent aperture plane.Then these two lines of data are used to fit near-field distribution inside the equivalent aperture plane.The simulation result shows that this method is basically equal to the initial value of constant amplitude and phase.However,by using this initial value estimation method,the aperture constraint range,which is a key parameter of the PTP3 algorithm,can be properly defined with little time,therefore the convergence probability of PTP3 can be improved.3)Two methods are proposed to deal with the noisy measurement environment at terahertz band:a)reduce the noise by properly set a hybrid input-output（HIO）parameter inside the PTP3 algorithm.b)before using PTP3,use Block matching and 3D filtering（BM3D）algorithm to denoise measured near-field data at two sampling planes.In order to verify these two noise reduction methods,two different noise sources are introduced,which correspond to two different measurement scenarios.A series of experiments have proven the effectiveness and shown the applicable scenarios for both methods.（3）A W-band high gain planar slot array antenna based on high-order mode substrate integrated cavity is designed in this dissertation.By adopting a substrate integrated cavity operating at quasi TE₅₆₀ mode,a total number of 30 radiation slots are simultaneously excited at the top surface of this cavity,which can support high gain with a very simple feed structure.Compared with other high-order cavity-backed slot antennas reported in previous literature,this proposed antenna can support higher gain while maintaining considerable aperture efficiency.Two arrays with different scales based on this antenna are designed and manufactured.By using the staggered distribution at the sub-array level,the grating lobe along the staggered direction can be significantly reduced Both arrays are verified with the antenna measurement system illustrated in chapter 2.The maximum measured gain of these two arrays are 26.39 d Bi and 30.36 d Bi,respectively.Benefit from the advantages of high gain and simple feeding of high-order mode cavity-backed slot unit antenna,the scale and design difficulty of the feeding network of the proposed millimeter-wave antenna array can be significantly reduced,hence the feeding loss is reduced,and the overall efficiency is improved.（4）Two high gain reflector antennas with two-dimensional electromechanical scanning capability,are designed for terahertz wireless communication and active imaging applications.The design center frequencies of these two reflector antennas are 400GHz and 650GHz,respectively.The 400GHz reflector antenna is consists of two cylindrical reflectors and a large size planar mirror.The cylindrical sub-reflector and the plane mirror are both connected to a servor,which can achieve20 degrees and 120 degrees in elevation and horizontal plane,respectively.This antenna has a measured gain of 44.2 d Bi at 400GHz.Compare to conventional parabolic reflector antennas such as Cassegrain and Gregorian reflectors,this antenna is much easier to manufacture;has a compact overall structure,and can provide wide-angle beam steering.As a result,this dual cylindrical reflector is suitable for high-speed point-to-point wireless communication scenarios at terahertz band.The main structure of the proposed 650GHz antenna is similar to the confocal near field Gregorian reflector,with two-dimensional beam scanning achieved by using motors to control the orientation of a small size plane mirror.The measured gain of this antenna is 46.49 d Bi at 650GHz.The function of far-field transmission and near-field imaging can be switched by simply changing the main reflector type.In addition,the radiation and beam scanning characteristics of these two antennas are fully measured and compared by using the developed measurement system and three different test methods.Wherein,the validity of the proposed phaseless PTP3 test method is verified based on the measured near-field amplitude data.As for the proposed dynamic scanning method,the experiments have shown that the far-field results are consistent with the results obtained by the point-to-point scanning method,but the measurement time is reduced by 56%,hence the measurement efficiency is significantly increased.Part of the above work has been included in two SCI-indexed papers,both are published in IEEE Transactions on Antennas Propagation,an authoritative journal in the antenna field,as well as three EI-indexed papers that are published in the International Wireless Conference（IEEE IWS2019）,Asia Pacific Antenna and Propagation Conference（APCAP2019）and European Microwave Conference（Eu MC2016）.