Research On Wideband Orthomode Transducer And Miniaturized Planar Couplers

In modern communication and microwave systems,in order to make full use of the increasingly crowded spectrum resources and satisfy the requirements of various practical applications,it is usually desirable for microwave systems to be broad in band,high in performance and integration.Therefore,as a key device in the microwave system for isolation frequency signal,the research on broadband orthomode transducer is urgent.Directional coupler is an important part of microwave system,and the investigation on its design is always the focus in this field.This thesis focuses on the design of a broadband circular wave guide coupler and a multiband planar directional coupler with compact size.Two types of couplers are designed: a broadband orthomode transducer and a branch-line directional coupler.Firstly,a novel triband orthomode transducer is proposed,which is designed to be wide in band with the gradual transition between the circular wave guides.A circular wave guide with four symmetrical ridges are used to suppress the higher-order modes,leading to a wider working bandwidth and better isolation.The coaxial feeding technology is used for better matching between the probe and the circular wave guide,so that most of the energy can be transmitted into the wave guide.The high impedance of the circular wave guide can be smoothly transformed to the low impedance of the ridge wave guide by choosing the ridge curve appropriately.In addition,the cone structure at the end of the OMT is designed to achieve better matching results.The working frequency band of the proposed design is 4.6-13.8 GHz,and the voltage standing wave ratio is no more than 1.3(the voltage standing wave ratio curve is smooth).Port isolations are better than 28 d B and the insertion losses are less than 0.2 d B.Secondly,two miniaturized multi-band branch line couplers are designed.One of them is miniaturized by using dual transmission lines instead of the traditional branch-line.To further miniaturize the coupler,an open stub are loaded in the middle of the dual transmission line.Two working frequencies 0.92 GHz and 2.32 GHz are obtained.The other coupler is designed by using the folded T-type impedance stubs at each port of the traditional branchline coupler.The introduction of the T-type impedance stubs increases the design freedom.By changing the vertical size of the T-type impedance stubs,it can be folded to reduce the size.So a single multiband branch-line coupler operating at 0.35 / 1 / 1.78 / 2.36 GHz is acquired,which has high isolation and coupling performance.Compared with the traditional branch-line couplers,the sizes of the proposed couplers are reduced by 70.2% and 71.9%,respectively.Couplers are tested and results agree well with simulated ones,validating the designs.