Design of a Compact Wideband Easy-to-integrate Stripline Directional Coupler

In this paper, a wideband easy-to-integrate stripline directional coupler is proposed. The coupling structures are offset parallel-coupled striplines. Symmetrical multi-section coupled-lines are cascaded to expand the frequency bandwidth. In the design, stripline ports of the directional coupler are transferred to microstrip line ports, making it easy to integrate into microstrip circuits. The coupler has a compact size of
$\mathbf{51}\mathbf{mm}\times\mathbf{32}\mathbf{mm}$
. The frequency range is 8GHz-23GHz. The proposed coupler was fabricated and tested. According to the test results, within a 96% relative bandwidth, the coupling degree obtains 15 dB, the insertion loss is less than 5 dB and the return loss is better than 10 dB.