| In order to cope with different conditions and environments,modern wireless communication and radar systems have an increasing demand for low-cost and easy-tointegrate tunable phase shifters and reconfigurable beam scanning antennas.Nematic liquid crystal(NLC),whose permittivity can be continuously adjusted by voltage,has good dielectric anisotropy in microwave band and has been a good candidate for reconfigurable microwave devices.Traditional phased array antennas usually use ferrite materials and varactor diodes to control the amplitude and phase of antenna unit and then archive the beam reconstruction in the microwave band.Compared with traditional piezoelectric and ferroelectric materials,NLC based microwave devices have the advantages of low loss,small size,easy manufacture,high operating frequency.In this paper,using the accurate numerical modeling method,NLC based microwave devices are studied,and the error between the simplified model(HOMI model)and accurate model(ACU model)also has been explored.These works provide a feasible method for the further design of NLC based microwave devices with high precision and good performance,and lay a foundation for the practical application of NLC based microwave devices.First,the numerical modeling method of NLC in the steady state under the action of electric field is analyzed and studied.In this paper,the director of LC are presented by the azimuth angle and polarization angle,based on the Ossen-Frank elasticity theory and the Friedrix transition theory,the numerical modeling of the reorientation of the director of NLC can be used in the microwave frequency is given.The potential distribution generated by the bias voltage in the LC layer is described,and the over-relaxation method is used to accelerate the calculation.The modeling calculation can be simplified by considering the strong anchoring and making every elastic coefficient equal,and the solution equation for the reorientation of NLC under different alignment and application environments.Secondly,a tunable NLC based phase shifter is designed by using an inverted microstrip line structure.Applying the NLC with different dielectric anisotropy,the working principle of NLC based phase shifter is given more clearly.In view of the errors caused by assuming NLC is uniform and isotropic in the design of NLC based microwave devices,considering the NLC is non-uniform and anisotropy,an accurate numerical modeling of NLC is conducted.The working mechanism of NLC under voltage is studied and analyzed.By comparing the ACU model results of NLC based phase shifter with that of the HOMI model,it can be got that the accurate modeling method provides a theoretical reference for improving design accuracy of NLC based phase shifters.Then,the working principle of NLC based reflective phased array antenna is studied and analyzed.Using the NLC based single-dipole and multi-dipoles as reflective elements,the effects of the thickness of NLC layer on the unit are studied.Applying the ACU model and ignoring the influence of small power signal on the NLC permittivity,the orientation distribution of the NLC under different voltages for each unit are given,and the S-parameter of each reflective unit is simulated.Moreover,a NLC based reflection phased array antenna is constructed utilizing the double-dipoles unit.Based on the ACU model,the radiation pattern of the reflectarray antenna is analysed and the error from that of the simplified model is given,which has important practical significance for improving the design accuracy of NLC based phased array antenna.Finally,the modeling and design of existing NLC based microwave devices usually ignore the influence of the microwave signal on the NLC permittivity,so only the bias voltage is considered when calculating the permittivity of NLC.Then,in this paper,the accurate modeling method of NLC molecules under the combined action of bias voltage and microwave signal is given,and the influence of the power of microwave signal on the orientation distribution of NLC molecules is also obtained,which provides a theoretical reference for improving the design accuracy of high-power NLC microwave devices.Moreover,the dielectric properties of NLC with different initial alignment under the voltage are also analyzed.To sum up,the research of this paper aims to improve the design accuracy of LC based microwave devices.Based on the study of numerical modeling methods of NLC,we deeply explore the physical mechanisms of NLC based microwave devices and carry out comprehensive scientific research.This research provides a theoretical basis for the research and application of microwave devices based on NLC,and highlights the academic significance and research value of this paper. |
