A compact 0-dB coupled-line forward coupler by loading with shunt periodic stubs

A wideband coupled-line forward (co-directional) coupler with tight coupling-level is presented. In this coupler we loaded two coupled microstrip lines with periodic shunt stubs that show capacitive and inductive nature in the even and odd modes, respectively. These periodic stubs, lead to increase the difference between the even and odd mode propagation constants of the coupled-line coupler, and also to decrease the structure length. Compared with the conventional forward coupler, this symmetrical structure, achieves wider operation bandwidth and smaller size with larger coupling level. The designed prototype is a 0-dB microstrip coupler over a wide bandwidth of 66% from 2GHz to 4GHz centered at 3GHz. The length of the coupled line is nearly λ_g/2 and length of the stubs is 2 mm, which make it more compact than other forward couplers have been proposed. To characterize this structure, the equivalent circuit model is established and verified by simulation. Moreover, we present some curves for selecting dimension of the proposed coupler for indicated coupling-level on FR-4 substrate.