| Terahertz imaging radar can obtain more details than microwave due to its higher frequency,larger bandwidth,higher target resolution and higher measurement accuracy.Due to the high frequency of terahertz wave,high Doppler bandwidth,Doppler effect is obvious,with good Doppler resolution,higher velocity measurement accuracy;At the same time,the scattering characteristics of terahertz wave are sensitive to the details of the target shape,so the ability of multi-target resolution and target recognition and image quality can be improved.At the same time,the wide spectrum characteristics of terahertz wave can also suppress the radar multipath effect and noise effect,and eliminate the mutual interference between the systems.The same physical size gain is higher,the system can achieve more miniaturization and integration,strong anti-interference ability.It is more sensitive to the surface shape and material properties of objects,so it has better perception ability and can recognize the shape and material of objects.Compared with laser,terahertz wave has a stronger ability to penetrate smoke,dust and sand,and it is insensitive to aerodynamic optical effect and thermal environment effect of high-speed moving targets in space.Therefore,Terahertz radar can be used for combat in complex environment and detection of high-speed moving targets in space.Compared with microwave radar,terahertz radar transceiver systems are easy to miniaturize and suitable for space-borne and missile-borne platforms.Since the terahertz band is generally difficult for stealth materials to achieve effective stealth,the terahertz radar is highly anti-stealth when used for detection and guidance.In addition,terahertz radar has the ability to penetrate plasma detection,so it also has good detection and tracking capabilities for hypersonic vehicles.Compared with traditional radar,terahertz radar has the above obvious advantages.Therefore,terahertz imaging radar is of great significance for national defense security.Monopulse technique is a kind of precise Angle measurement technique,which mainly uses the principle of monopulse Angle measurement to measure the target position.Since two beams receive echo signals at the same time,monopulse Angle measurement technology can obtain the target Angle error information in a very short time.Theoretically,it only needs to analyze one echo pulse to determine the distance and angular coordinate information of the target,which greatly speeds up the speed of extracting the target Angle information,so it is called "monopulse".Monopulse antenna is the core component of monopulse radar system,which has been widely used in various precision guidance radar systems,such as land,ship and airborne fire control tracking guidance radar.Range precision tracking and measuring radar;Long-range tracking radars to track intercontinental missiles,satellite communications,and spacecraft.Focusing lenses and mirrors can be designed by using quasi-optical technology to make electromagnetic beams beam propagation in free space,forming guided wave structures with low loss,easy processing and high power capacity.Radar systems,deep space exploration and radio astronomical telescope systems designed by quasi-optical technology have been widely reported at home and abroad.Therefore,based on quasi-optical technology,the study of terahertz monopulse antenna feed system will have very important significance.The main contents of this paper are listed as follows:1)A terahertz real-time focal plane array imaging system operating at 850 GHz was designed.A 170 mm aperture Cassegrain antenna with free-form surfaces on the main and secondary surfaces is designed to achieve ±1.75 degrees field of view and good aberration control with an angular resolution of 0.146 degrees.A scanning imaging structure with a scanning range of 30 degrees is designed.The zemax and integral equation method is proposed to jointly complete the fast iterative design of the super electric large antenna.The equivalent source simulation technique is proposed to solve the difficult problem of coupling efficiency calculation of reflecting surface antenna and focal plane detector unit system.The simulated evaluation of terahertz imaging is realized.Due to the limitation of the number and size of detector chips,the focal plane imaging is undersampled,and the image algorithm processing using kernel ridge regression improves the image pixel and signal-to-noise ratio under undersampling.The real-time terahertz focal plane imaging system with32*32 pixels at 850 GHz is realized for mounting and testing,and the imaging matches the actual measurement and simulation.The developed imaging system was tested for angular resolution and target imaging at 1.6m,5m,and 10 m distance from the target,and met the design requirements.The longest target detection distance of 70 m or more was achieved with only 0.5mw power,which is the first time to realize the long-range imaging radar prototype and experimental verification in this frequency band at home and abroad.2)The design of terahertz quasi-optical one-dimensional comparator and one-dimensional monopulse antenna improvements are studied.The quasi-optical one-dimensional comparator with different beam waist parameters are analyzed.By introducing the lens combination transformation,a one-dimensional monopulse antenna is designed with a 4 dB reduction of the sidelobe compared to the previous study [134].A new broadband one-dimensional comparator is systematically proposed and investigated.By deriving the equations,the dispersion effect of the conventional comparator is analyzed,and the new broadband comparator design structure is innovatively proposed,which significantly reduces the phase dispersion effect of the comparator by introducing the path compensation structure.Compared with the literature [91-93][98][134-135],if the 3 dB beam splitter characteristics do not change with frequency,the operating bandwidth of the new comparator can be extended from Δf to 22Δf.Full-wave simulation analysis is performed,and the simulation is consistent with the theory,and good phase dispersion characteristics of the output port are achieved,and the bandwidth is correlated with the beam splitter amplitude performance.3)A dual-polarized 3 dB beam splitter is proposed,an equivalent circuit model is proposed,and an analytical calculation formula is derived.Based on the proposed new broadband quasi-optical one-dimensional comparator,a new two-dimensional sum and difference comparator is designed,and its characteristics are simulated and analyzed,and the results show that the two-dimensional sum and difference comparator can be well achieved and the bandwidth is greatly increased,and the azimuthal difference of 14 GHz and the pitch difference of 20 GHz bandwidth can be achieved.The input and output correspondence of the two-dimensional sum-and-difference comparator with different polarizations is analyzed,and a fast analysis method of the input and output characteristics of the sum-and-difference comparator based on the matrix operation of the symbolic code is proposed.4)A Based on the previous work,a broadband quasi-optical 2D monopulse antenna feed system is designed and fabricated,and the conversion of the 2D quasi-optical comparator output to the monopulse antenna feed source and the matching with the reflecting surface antenna are realized in a compact space by using the beam transform method.Using the equivalent source simulation technique,the complex unsolvable system with tens of billions of meshes is disassembled into several sub-modules,which can be optimized individually and designed by system-level simulation to improve the design efficiency.In addition,metal wire grids and ferrite circulators were designed.The fabrication of the antenna feed system was completed and tested for assembly,and the test results matched with the simulation.Compared with literature [98][134][135],the bandwidth of azimuthal difference is widened by 6.5 times to 13 GHz,the elevation difference is widened by 10 times to 20 GHz,and the beam sidelobe level is below-18 dB,the null depth at center frequency is better than-30 dB,and the in-band null depth is better than-25 dB.The performance of bandwidth,gain,aperture efficiency,subflap,and null depth are all beyond the previous studies,achieving the breakthrough of monopulse antenna feed from 1D to 2D and excellent system performance. |
PDF Full Text Request