Design Of A Orthomode Transducer With Single Mode Transmission

With the continuous development of communication technology,people have higher and higher requirements for communication quality,which has also led to the rapid expansion of communication capacity requirements.In order to meet this demand,frequency reuse technology has become the focus of people's research.The concept of dual-polarized antennas has also emerged in microwave communication systems.Orthomode Transducer(OMT)is the core component of the dual-polarized antenna feeder network.It can achieve the coupling and separation of orthogonally polarized electromagnetic waves,which is the key to achieve dual-polarized signal transmission.Therefore,the design of an excellent OMT is also indispensable.This paper designs two different types of OMTs with simple processing and excellent performance by sacrificing operation bandwidth because of narrow commercial bandwidth.The first type of OMT achieved by two vertical coaxial line fed in a cylindrical waveguide.The OMT excited by the coaxial probe is designed to work in the 7.125-8.3GHz band.First of all,we introduced the feasibility of the dual polarized signal transmission through cylindrical waveguide excited by the two vertical coaxial probe.Secondly,we simplified a broadband OMT excited by coaxial probe by removing the ridge structure and the tapered structure at the bottom of the broadband OMT.We have added two metal pillars between the two coaxial probes.These two metal pillars are parallel to the higher probe inside the cylindrical waveguide.They can effectively stop the TE₁₁ mode excited by the upper coaxial probe from transmitting downward.It improves the isolation of the two coaxial ports,and these two metal pillars have little effect on the TE₁₁ mode excited by the bottom coaxial probe.Properly increasing the distance between the two coaxial ports will also improve isolation.Finally,a vector network analyzer was used to test the actual OMT.The return loss of the OMT is less than-18.46d B.The isolation is greater than 37.92d B.the short-circuit loss is less than 1.09d B.The cross polarization is greater than 28.94d B.The electrical performance of the OMT meets the requirements of the index,and the structure is simple and easy to process.The disadvantage is that the frequency band is narrow,and the use of metal pillars results in positioning errors.Regarding the OMT excited by the rectangular waveguide,the designed operating frequency band is 11.7-12.7GHz.Firstly we designed a Chebyshev impedance converter,which can realize the conversion from rectangular waveguide TE₁₀ mode to cylindrical waveguide TE₁₁mode.The simulation analysis shows that the TE₁₁ modes with different polarization directions in the cylindrical waveguide can be excited by rotating the rectangular waveguide port.In the same way,the mutually orthogonal rectangular waveguide ports can excite TE₁₁modes with orthogonal polarizations in cylindrical waveguide to achieve dual-polarized signal transmission.Secondly,an OMT with rectangular waveguide excitation is designed.By analyzing the transmission characteristics of the square waveguide,we find that the relative bandwidth of the single-mode transmission of the square waveguide is greater than the relative bandwidth of the single-mode transmission of the cylindrical waveguide,and that orthogonally polarized electromagnetic waves can be transmitted in the single-mode region of the square waveguide.A square waveguide is designed in the middle of the common waveguide.Compared with cylindrical waveguide,square waveguide is easier to achieve single-mode region transmission in a larger size range,which can suppress higher-order mode transmission.It is also easier to adjust impedance matching.The effects of the extended cavity in the OMT structure on two rectangular waveguide ports is analyzed by HFSS simulation.It was found that the extended cavity has the effect of adjusting impedance matching on one of the rectangular waveguide ports.Finally,the physical test results of the OMT are given:the return loss is less than-16d B,the short-circuit loss is less than 0.7d B,the cross polarization is greater than 35d B,and its electrical performance meets the requirements of the index.The two types of commercial OMT that are widely used at present are made by processing multiple cores,but this actual object of OMT is processed from one core block.It has no assembly error.the OMT can restore more accurately the design model.It avoids many structural reliability issues.