Design of a 6db Directional Coupler Using Defected Ground Structure

This paper presents the design of 6dB directional coupler in C-band. A defected ground structure 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 6±0.1dB across the band 4.0 GHz to 4.5 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 0.04 mm in order to achieve the same value of coupling across that band. The designed device has a compact size with a dimension of 20mm x 9mm.Optimization of the design is carried out in Ansoft Designer V6.0 to meet the required specifications. Simulated results show 11% bandwidth for a return loss of better than 15dB and amplitude imbalance of 6±0.1dB over 4.0-4.5GHz. Isolation of more than 16dB has been simulated.