Research On K-Band Broadband Low Sidelobe Multimode Horn Antenna

Multi-mode horn antenna has compact structure,low sidelobe level and axisymmetric pattern,but its working frequency band is very narrow(relative bandwidth is about 6%),which limits the application of multimode horn antenna.Therefore,this article improves the multimode horn antenna to expand its working frequency band without weakening its excellent performance and complicating its simple structure.Firstly,a multi-step circular multi-mode horn antenna is proposed,which structure is based on the traditional Potter horn and Picket-Potter horn,through loadong multi-level metal ladder or dielectric ladder inside it to expand bandwidth.Finally,the multi-step circular multi-mode horn antenna works in 17?25GHz,whose VSWR parameters are lower than 1.5 and the sidelobe level is lower than-24d B in the whole frequency band.At the same time,the axisymmetric performance of its pattern is very good,and the relative bandwidth is about 38%.Secondly,according to the basic circular waveguide theory,the internal modes of circular multimode horn antenna are theoretically deduced and numerically analyzed.The following conclusions are drawn:when the TE₁₁ mode and TM₁₁ mode in the multimode horn are mixed according to a certain power ratio and phase difference,it can be considered that the electric field amplitude of the horn aperture presents Taylor distribution in the radial direction and uniform distribution in the circumferential direction.According to the relationship between aperture electric field and pattern,the pattern will be low sidelobe and axial symmetric.Thirdly,three kinds of circular polarizers are used to transform the proposed multi-step circular multi-mode horn antenna into circularly polarized multi-mode horn.The three kinds of circular polarizers are traditional dielectric insert circular polarizer,metal septum circular polarizer,and a noval ultra wideband spiral circular polarizer of which relative bandwidth is approximately equal to 75%.For these circularly polarized horn antennas,the VSWR parameters are less than 1.5,the sidelobe levels are less than-26d B,and the axial ratios are less than 3d B in the band of 17?25GHz.And the axisymmetric performance of the patterns is excellent.Finally,a broadband rectangular multimode horn antenna is realized by substituting the metal multi-step structure into the rectangular horn antenna,and the low sidelobe and axisymmetric pattern are also obtained..