Research On Key Technologies Of Broadband Stealth For Integrated Model Of Platforms And Antennas

Considerable attention is now paid to stealth performance of aircraft as the performance of radar system is highly improved.Antennas on platforms are special scattering sources and impact the original radar cross section(RCS)of the platform drastically.RCS reduction for antennas is now a hot topic.Radiation performance of antennas on platform has been widely studied,but further study is still needed for scattering properties of antennas on platform.Multi-scale and broadband are the two main characteristics of this kind of problems.This dissertation focuses on this problem,and a series of researches are carried out for RCS calculation and RCS reduction.For the calculation of RCS,the advantages and disadvantages of the traditional finite difference time-domain(FDTD)method is briefly introduced.To overcome its disadvantages,a nonstandard FDTD method is proposed,in which the electric field and magnetic field are updated utilizing two different finite difference schemes.Numerical properties of the nonstandard FDTD method is studied and compared with their corresponding standard higher order counterparts both for the explicit and implicit versions of the method.Numerical results show that the proposed method is more efficient compared with standard method and the same dispersion accuracy can be achieved.In addition,the higher order scheme and subgridding scheme are combined together to handle multi-scale problems.Different finite difference schemes can be used in the higher order region and substep in time domain is used in the subgridding region to reduce computational burden.This hybrid method is applied to analyze a finite frequency selective surface(FSS)structure.For the RCS reduction of antennas mounted on platform,grazing incidence and normal incidence are taken into consideration according to the locations of the antennas mounted on the platform.In the grazing incidence case,to avoid the effect of the feeding coaxial line and to take the coupling between the antenna and platform into consideration,an integrated model of a small stealth platform and the mounted antenna is used to analyze the scattering property of the microstrip antenna.According to the surface current distribution,sawtooth-like edge is used in the design of the microstrip antenna to reduce its RCS in the concerned angles.Prototypes have been fabricated and measured to verify the corresponding design.The idea of shaping is also used to design a discone antenna and RCS reduction can also be obtained.In the normal incidence case,passive cancellation is used to reduce RCS of antennas.The design rule of thecheckerboard surface is comprehensively studied,and based on this,the RCS reduction is achieved for dipole antennas with reflector.The RCS reduction performance of the checkerboard surface and its influences on the radiation performance of the antenna are investigated.Results show that RCS of the antenna can be well reduced by the checkerboard surface while no big influence on radiation performance is observed.Moreover,the profile of the antenna can be reduced at the same time.Passive cancellation is also used to reduce the RCS of a microstrip antenna array.It is noticed that additional structure is usually used in passive cancellation design for microstrip antenna and the unit space must be increased when the antenna is used to form an array,which may not be suitable for wide-angle-scanning arrays.Two types of antenna units are used to design a hybrid array antenna to achieve passive cancellation.Slots on the patch is used to control the resonant frequency for cross polarization of the antenna and nearly opposite phase can be realized between the scattered field of the original patch antenna and the one with slots,thus cancellation can be realized.To verify the design,prototype is also fabricated and measured.Some related researches is also carried out for FSS radome,which is a commonly used method to reduce out-of-band RCS.Equivalent circuit model of frequently-used FSS unit cell is studied and the method to extract the values of capacitor and conductor from S parameter is given.Given the single-layered FSS,full wave method is used to obtain the S parameter and subsequently the values of circuit parameters can be extracted.Interpolation and circuit model are then used to analyze the multilayered FSS unit efficiently.Based on the method,a second order FSS unit with good transmission property for circularly polarized plane wave is optimized.Prototype is fabricated and measured to verify the analysis.Based on the understanding of equivalent circuit model of the FSS unit,interdigital capacitor and folded inductor structures are used to design a miniaturized FSS unit,which is used to build a rooftop-like FSS radome.The FSS radome is applied to reduce out-of-band RCS of microstrip array antenna.When this method is combined with the passive cancellation technique for in-band RCS reduction,both in-band and out-of-band RCS reduction can be achieved.The whole design is also fabricated and measured.