An analysis of coupled transmission lines with tapered transitions

One of the factors responsible for cross-talk, that is, the unnecessary cou pling between transmission lines, are tapered transitions of coupled transm ission lines. This article presents an analysis and experimental data on th e transmission characteristics of the coupled transmission lines considerin g the transitions. Applying the traveling wave mode method, the distributed quantities for the even and odd modes of the traveling wave are determined , so that they indicate the unnecessary coupling in the transitions. Then, the four-port 5 parameters are determined for the coupled transmission line s, which represent the following characteristics: matching, coupling (near- end cross-talk), transmission, and directivity (far-end cross-talk). The re sult is compared to the results of the conventional method, where the coupl ing in the transition parts is ignored. It is revealed by these analyses th at the transmission characteristics depend on the taper angle of the transi tions and do not agree with the results given by the conventional method. I t is also shown that the tapered structure is responsible for the incomplet e directivity. An experiment is also performed, and the results agree well with the results of theory. In other words, the validity of the method pres ented in this article is demonstrated. (C) 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(3): 26-38, 2000.