The directional coupler is one of the earliest and fundamental junctions in microwave/millimeterwave frequency band, which is mostly one- or two-axis symmetry. The most important common property in all directional couplers is that the output arms are isolated from each other and the input arms are matched under the condition that the other arms are terminated by the matched loads.
Parallel coupled-line directional coupler has a 90 degree phase response between output signals and an advantage of wide bandwidth than hybrid-ring or branch-line directional coupler. However, it is very difficult to realize tight coupling due to a narrow transmission width and space.
In order to solve the above problem, in this paper, a directional coupler using 2-stage parallel coupled-lines has been constructed by two coupled-lines and transmission line. Therefore, 2-stage parallel coupled-line directional coupler has been designed using both sides of substrate or multilayer plane which has 1-axis symmetry while the conventional one has 2-axis symmetry.
The frequency characteristics of the designed couplers were analyzed and the optimum parameters were found by CAD. The fractional bandwidth of the proposed 2-stage parallel coupled-line coupler was broadened to 130 percents. Furthermore, it was clearly shown that the experimental results agree well with the predicted ones with microstrip-line type.