Research And Design Of The Overmoded-waveguide Directional Coupler

The directional coupler is a kind of commonly device used in high power microwave system,which can couple a small portion of microwave power from the system to monitor and isolate high power microwave signals.The directional coupler is constituted by the rectangular waveguide,circular waveguide or microstrip transmission lines and so on.The main-waveguide and the sub-waveguide can utilize different transmission lines,which have many design patterns and various structure forms with its advantages and disadvantages,respectively.At present,the research of directional coupler is mostly based on the situation of single mode.However,with the development of electronic technology and increasing power requirements for microwave systems in industry,the power capacity of waveguide based on single mode transmission is difficult to meet the requirements in some cases.Therefore,adopting overmoded-waveguide with multiple modes becomes a novel kind of solution and the design of corresponding overmoded-waveguide directional coupler becomes a difficult problem.In this dissertation,the overmoded-waveguide directional coupler is studied,the relationship between structural parameters with coupling parameters is analyzed,and the design of coupler is carried out.The main work can be summarized as follows:1.The principle of hole coupling is described.The commonly used types of main-waveguide and sub-waveguide of the directional coupler are summarized and their advantages and disadvantages are compared.The main technical indicators of the directional coupler,including the degree of coupling,isolation,directivity,VSWR(Voltage Standing Wave Ratio),and the distribution of electric field in waveguide which are all studied.2.The directional coupler working at the frequency of 2.4 GHz is studied and analyzed.When using small size coupling holes,it's easy to spark and the coupling is not stable.To avoid these problems,the design of broadside coupling with a transition section is proposed.A rectangular waveguide BJ22 with larger size is used in the main waveguide,and the standard rectangular waveguide BJ22 is applied in the transition section.Besides,in order to reduce the size of coupler,the microstrip line is used in the sub-waveguide.The field distributions of the main-waveguide and the sub-waveguide are studied as the main waveguide has three different propagating modes(TE10,TE20,and TE01).The structure parameters of coupler influence on the coupling degree and directivity are also studied.3.In this paper,the number and proportion of higher-order modes are discussed by increasing the size of the rectangular waveguide.The coupled modes between the narrow side of overmoded-waveguide and the microstrip line are researched.The feasibility of the field directional coupled to the microstrip line through the hole is verified by analyzing the field distribution around the hole.Therefore,a kind of overmoded-waveguide directional coupler based on narrow side coupling which can withstand high power is designed,and the structure parameters of coupler is optimized.4.In the process of simulation and analysis,it's hard to avoid standing waves existing in the waveguide.However,the parameters such as the coupling degree and directivity of the directional coupler are theoretically defined in the traveling wave state.Therefore,in order to avoid the influence of the standing wave in the waveguide on the design of the electrical parameters of the directional coupler,a method of coupling coefficient calculation using the poynting vector integral to calculate power is proposed in this dissertation,which can correct the effect from the standing wave.Finally,the effectiveness of this method is analyzed.