Waveguide - A Stripline - Coaxial Directional Coupler

The directional coupler may be regard as one kind of the directive power divider, and was widespread applied in the microwave system. For example, It can be applied for power surveillance in the microwave signal generator, the frequency surveillance, the power synthesis and the microwave receiver mixer and so on. The directional coupler has been studied in the fields of microwave project. Structurally speaking, the coaxial line, stripline, microstrip lines, rectangular waveguide, circular waveguide can constitute directional coupler. Therefore, the directional coupler range varies greatly.The waveguide-stripling-coaxial line directional coupler is a simple, small size and low-cost new directional coupler, it is very applicable to high power, limited space for installation of microwave engineering systems. It has been applied in many microwave systems. For the design, this kind of directional coupler, the solution of the field distribution in the stripline, is a essential difficulty problem. A approximate solution is that regard the sub-line of this coupler as the same waveguide with the main-line. Obviously, the results obtained by this method have large error and can not satisfactorily meet the performance requirements of engineering practice. In this paper the theoretical research and numerical analysis for the waveguide-stripline-coaxial line directional coupler are presented.The main work of this paper are as following:First, the directional coupler has analyzed in theory, and the formulate are derived for calculating coupling strength from the diffraction theory of small coupled holes, which has provided a theoretical basis for the following research. Secondly, with the Schwarz-Christoffel transition function twice, the perfect stripline in the waveguide-stripline-coaxial line directional coupler has transformed into plate capacitor. So, the field distribution in stripline can obtained from the plate capacitor. Third, the calculation formula of the coupling coefficient and directivity of the directional coupler was derived by the diffraction theory of small coupled holes, and the approximate calculation formula was given of which the coupled hole has a certain thickness. Finally, the relationship between technical parameter and the structure size of coupler was analyzed in numeral by computer program. And the parameters of manufactured couplers are also measured and the results show good agreement with the calculated results.The calculation formula in theoretical and the result of numerical analysis in this paper indicated that, In the design of waveguide - stripline - coaxial line directional coupler, to get the satisfaction of coupling coefficient and directivity, besides may change the size of the small hole and rotate the hand as in the waveguide - waveguide directional coupler, but also the size of stripline as well as the coupler position. In brief, this paper gives the theoretical calculation formula and the correlation analytic curve, will provide a good reference for after people in such directional coupler design.