Tightly Coupled Directional Coupler Using Slotted-Microstrip Line

This paper presents the theory and implementation of a novel microstrip directional coupler (DC) with a high coupling coefficient that can achieve the value of 0 dB. The coupling region is a slotted-microstrip line that supports two modes, even and odd. The geometry of the slotted-microstrip line facilitates the phase velocity compensation, which improves the isolation and directivity. A 3-dB coupler is fabricated and characterized. Measurements and electromagnetic simulations are in a good agreement with the theoretical predictions. They show that the proposed DC has 52% ±1 dB coupling coefficient fractional bandwidth centered around 2.4 GHz with isolation and return loss better than 25 dB.