Design of a Broadband Quadrature Directional Coupler

The design of an edge-coupled quadrature directional coupler, which has a broadband performance and a relaxed coupled-line spacing, is presented. A slotted ground plane is used underneath the coupled region in order to relax the requirement for a narrow slot between the coupled lines. The simulated results show that the designed coupler exhibits a coupling of 3plusmn1 dB across the band 3.8 GHz to 9.8 GHz, when the spacing between the coupled lines is larger than 0.1 mm. Without the slot in the ground plane, the spacing should be less than 0.01 mm in order to achieve the same value of coupling across that band. This broadband coupling is accompanied by an isolation and return loss in the order of 14 dB or better. The designed device has a compact size with a dimension of 30 mmx20 mm.