Butterfly-shaped slot coupled microstrip 90° hybrid couplers for K- and Ka-band millimeter-wave radar applications

Design of butterfly-shaped slot coupled microstrip 90° hybrid coupler for millimeter-wave (mmWave) applications, which is operating between 18 and 40 GHz frequency range for K-Band, and Ka-Band radar applications, is presented in this paper. The proposed design utilizes multilayer printed circuit board (PCB) technology that allows having a compact and broad bandwidth coupler. The design uses butterfly-shaped broadside coupled patches and an elliptical-shaped slot etched in a common ground plane between two identical dielectric substrates. The proposed butterfly-shaped patches show good coupling and matching performance in addition to be easily modified to cover different coupling values from tight to loose coupling. The proposed coupler has good return losses and isolations, acceptable insertion losses with minimum transmission phase imbalance across the frequency band of interest.