| Radar is one of the most common detection methods.During the detection process,the electromagnetic waves emitted by the radar will be reflected if they encounter obstacles.Radar detects the outside world by receiving and processing the information in the reflected electromagnetic wave.The radar cross section(RCS)characterizes the ability of the target to generate echoes under the irradiation of radar detection waves,and is an important indicator of the stealth characteristics.In stealth-designed platforms,takng the aircraft as an example,the air intake,cockpit and antenna are the three scattering sources that contribute the most to the entire system.The antenna needs to ensure its radiation performance,so the traditional RCS reduction method is difficult to be used in antenna RCS reduction.Antenna loaded electromagnetic metasurface is a common method to reduce RCS.Different electromagnetic metasurfaces can change the properties of electromagnetic waves and reduce RCS based on different principles.The polarization conversion metasurface(PCM)can realize the conversion of electromagnetic wave between different polarizations,and can be combined with the antenna through reasonable design,so as to achieve the effect of reducing RCS without damaging the radiation performance of the antenna.this thesis explains how the PCM realizes the polarization conversion of electromagnetic wave and the working mechanism of RCS reduction,proposes several antenna design methods based on high-performance polarization conversion surface to achieve low RCS characteristics.The work of this thesis mainly includes the following aspects:1.A design method of low RCS microstrip antenna based on broadband PCM is proposed The RCS reduction effect of the polarization conversion surface is directly related to its own performance.In order to reduce the RCS of the antenna in the broadband range,this thesis first designs an ultra-wideband PCM.The PCM has more than 90% polarization conversion rate and 131% relative bandwidth between 6.1-29.5GHz.The gain of the designed antenna is 7.7d Bi,which is 0.5d B higher than that of the reference antenna,and the average RCS reduction of single station is 12.6d B in the frequency band of 6-30 GHz.2.A design method of reconfigurable low RCS circularly polarized antenna array is proposed.The designed antenna unit can switch between the radiation state and the polarization transition state by controlling the pin diode on and off.Firstly,the antenna element is analyzed based on the characteristic mode theory,and the characteristic current distribution and the amplitude and phase of the mode weighting coefficient corresponding to the characteristic current under the probe excitation and plane wave excitation are concerned respectively.The working principle of the designed antenna under the active state and the passive state is explained.A rotating feed network is designed to feed the checkerboard antenna array.Finally,a reconfigurable low RCS circularly polarized antenna array which can switch between radiated circularly polarized electromagnetic wave and low scattering state is obtained.In the radiation state,the antenna array has good matching in the 7.8-11.8GHz frequency band,with 40.8% impedance bandwidth,the axial ratio is less than 3d B in the maximum radiation direction between 9.5-10.8GHz,with 12.8% axial ratio bandwidth.The antenna array has a right-handed circular polarization gain of 19.7d Bi in the maximum radiation direction at 10 GHz.In the scattering state,the antenna array has an average RCS reduction of 11.1d B in the range of 8.6-13.2GHz,with a bandwidth reduction of 42.2%.3.A design method of wideband low RCS antenna array with antenna cladding is proposed.A cladding layer is designed above the reconfigurable low RCS antenna.Firstly,a PCM unit is designed.When the metal floor is under the PCM unit,it has good polarization conversion characteristics between 4.4 GHz and 16.9 GHz,and the relative bandwidth is 117%.When there is only the upper dielectric substrate but no metal floor,the co-polarization reflection coefficient of the PCM is less than-10 d B in the 8-11.6GHz frequency band,and the copolarization transmission coefficient is greater than-3d B in the 7.8-12 GHz frequency band,which has good frequency selection characteristics.Secondly,a band-pass frequency selective surface with resonant frequency of 10 GHz is designed,and the metal floor of the designed PCM is replaced to ensure the wave transmission performance of the cladding at the working frequency,and a "rote-transmit-rote" PCM is obtained.Finally,the designed PCM is arranged in checkerboard to form an antenna cladding and loaded on the top of the reconfigurable low RCS antenna.The simulation results show that when the designed antenna cladding is loaded on the top of the reconfigurable low RCS antenna,the radiation performance of the original array antenna will not be damaged,and an extra out of band RCS reduction band can be provided for the antenna array. |
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