| With the rapid development of modern radar and communication technology,the number of microwave devices mounted on various target carriers has increased dramatically,the weight of the loads has increased,and the cost of the whole system has also increased.Moreover,with the increasing density of microwave functional devices,the electromagnetic coupling interference between devices becomes very serious,even the whole communication system can not work properly.These phenomenas are very harmful in terms of reducing the overall cost of communication system,reducing weight,reducing radar cross section,and achieving good electromagnetic compatibility characteristics.They have become difficult problems restricting the development and application of modern communication system towards integrated information system.As a pioneering hotspot technology in the 21st century,electromagnetic metamaterials technology is of great help to improve communication systems and solve the above problems.At the same time,the development of new microwave dielectric materials can also effectively solve the performance and miniaturization of microwave devices.Therefore,aiming at the key and difficult points in the research of microwave communication system.In this thesis,a series of studies have been carried out from the electromagnetic metamaterial technology and the application technology of liquid crystal?LC?materials in microwave devices.The main works of the thesis are listed as follows:1.Analysis and study of microwave nematic liquid crystal materialsIn view of the fact that the electromagnetic properties of domestic liquid crystals can not meet the requirements of microwave applications at present,liquid crystal compounds and nematic liquid crystal materials which suitable for microwave band are designed,synthesized and purified independently.In view of the physical properties of K-band microwave nematic liquid crystals,the phase states and dielectric properties of liquid crystal compounds and their liquid crystalline compositions for K-band are tested and analyzed.Based on these,a high dielectric anisotropy,low viscosity,high resistivity and fast response nematic liquid crystal material suitable for microwave devices at K band has been developed.The variable dielectric range of the material is??r>1.12,and the loss tangent tan??r max<0.02.This research fills the gap of liquid crystal materials for microwave in China,completes the system of independent intellectual property rights,and plays a great role in promoting the research and development of liquid crystal microwave devices in space,national defense,military and civil communications.2.Study on the mechanism of metamaterials and microwave nematic liquid crystalmetamaterialsIn order to overcome the shortcomings of existing planar metamaterials that can not simultaneously achieve large-angle multi-band polarization conversion,a new type of dual-band circular polarizer based on metamaterials is designed,which has a simpler structure.It converts obliquely incident linear polarized wave into left-handed circular polarization?LCP?wave or right-handed circular polarization?RCP?wave at two resonance frequencies,and the difference between left-handed circular polarization?LCP?wave and right-handed circular polarization?RCP?wave is greater than 15 dB at two resonance frequencies.Then,aiming at the problem that metamaterials have higher control freedom and more functions,microwave nematic liquid crystal materials are introduced into the design idea,and an arbitrary base digital coding metamaterial based on electronically controlled microwave nematic liquid crystal is designed.The coding is realized by loading bias voltage on the liquid crystal to make the exit wave phase shift.The validity of the coding mechanism is verified by theoretical derivation,numerical simulation and preliminary experiments.Then two representative coding metamaterials'applications are proposed for this liquid crystal metamaterial,i.e.beam steering and radar cross section reduction.The results show that the liquid crystal metamaterial deflects the output beam from 0 to 27 degrees at 54 GHz frequency,and reduces the radar cross section from 51 GHz to 56 GHz by at least 10 dB.The results prove that the new liquid crystal digital coding is feasible.Rationality of metamaterials and the coding freedom of digital metamaterials based on microwave nematic liquid crystal has been proved very well.3.Research on Nematic Liquid Crystal Microwave Functional DevicesIn order to improve the performance of reconfigurable microwave antenna and the radio frequency miniaturization technology in Ku-band and Ka-band broadband satellite communications and internet of things communications,a special research on the manufacturing technology of miniaturized K-band liquid crystal phase shifter is proposed to solve the key problems in the complete and autonomous preparation of reconfigurable antenna.The structure and encapsulation method of LC microwave phase shifter based on special-shaped substrate structure are proposed to solve the encapsulation and uniformity of LC phase shifter devices with large cell thickness,and to meet the guided wave transmission conditions of microwave microstrip line.The sealing test method based on LC encapsulation is combined with the quality evaluation method of LC material to solve the leakage test of microwave LC phase shifter.Through process optimization,the problems of collapsing and uneven coating of frame glue caused by large cross-section area of sealing glue are solved.Distributed point-by-point control technology is proposed to control bias voltage of liquid crystal phase shifter in order to realize high precision microwave phase shifting function.The prototype output of a miniaturized K-band liquid crystal phase shifter with 360 degree phase shift in K-band is realized effectively.Then,in order to achieve the requirements of miniaturization and multi-function of antenna,and the original tunable components can not work perfectly in the high frequency band.The miniaturized K-band liquid crystal phase shifter is used to design reconfigurable antenna,and a reconfigurable antenna array based on the frequency and pattern of liquid crystal technology is designed.Through design and theoretical analysis,the antenna array can dynamically change its operating frequency between 14.5 GHz and16.4 GHz,and the beam direction can also be dynamically adjusted between–20°and20°. |
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