Broadband Stripline Directional Coupler Design

Modern electronic warfare systems, measurement and instrument systems and other microwave systems have more and more broad bandwidth, the trend raise a broadband demand to microwave components. As an extensive use in microwave components, directional couplers were challenged by this trend. Therefore, development of broadband and ultra-wideband directional coupler has urgent and practical significance. There are many ways to design directional couplers, coaxial line, rectangular waveguide, circular waveguide, stripline and microstrip line can use to design directional couplers. It is use stripline to develop directional couplers in this paper.We expounded detailed the design methods of two broadband stripline directional coupler in this paper. It is use multi-element coupled transmission line to design the former coupler, and use tapered couple transmission line to design the latter coupler. Stripline is base of stripline coupler, so we discussed many traits of stripline first. Because stripline carry TEM wave, we particularly concerned about the TEM characteristics of the stripline.The main method of analyzing coupled stripline is using odd and even mode characteristics of stripline. We analyzed three common coupled striplines, edge coupled stripline, broadside coupled stripline and offset coupled stripline. Calculation of even and odd mode impedances for these striplines is most important problem, because of the dimension parameters is decided by even and odd mode impedances. Calculation of even and odd mode impedances is a complex problem for broadband coupler, and it is a key problem of broadband coupler design. So this paper pays more attention on calculation of even and odd mode impedances when discussing the design method of two type broadband stripline couplers.For multi-element coupled transmission line, the base of analyzing method is odd and even mode method. Equaling multi-element coupled transmission line as multi-element stepped-impedance filter, we can gain attenuation function L_a and reflective coefficientΓof multi-element coupled transmission line. For the reason of equal-ripple coupling response, transforming reflective coefficient into Chebyshev form, and importing Richard's transform, we finally obtained reflective coefficient with Chebyshev polynomial form. Solving the numerator and denominator of the polynomial respectively, the roots are even mode impedance. This is a very complex synthesis procedure, we must use the computer to complete the synthesis procedure. The author designed a broadband stripline with 2~18GHz by using edge coupled stripline. Test results by microwave scalar network analyzer shown the broadband stripline directional coupler had excellent specifications in the bandwidth.In order to further expand the bandwidth, we must use tapered coupled transmission line. The basic method of tapered transmission line is taking a very small section of transmission as parallel coupled line, integrating it we can gain coupling response curve as desired. Interesting, the integral exactly is a Fourier transform. Inversing Fourier transform, the result of inverse transform is coupling coefficient k(x). We had adopted tapered coupled transmission line with high-pass Chebyshev form, and designed a broadband stripline directional coupler with 0.5～18.5GHz. Test results by microwave vector network analyzer shown the broadband stripline directional coupler had excellent specifications in the bandwidth.