Forward-wave Directional Coupler based on Slow-wave Coupled Microstrip Lines

This study presents the design of a forward-wave directional coupler based on slow-wave coupled microstrip lines realised in a multilayered printed circuit board technology. Thanks to the concept of slow-wave, a high difference between the even- and odd-mode phase velocities can be inherently achieved. Hence, by choosing adequate even- and odd-mode characteristic impedances close to 50 Ω, reduced-length forward couplers can be designed. As a proof of concept, a 0 dB forward coupler operating at 4.5 GHz is proposed. It exhibits a very high coupling of 0.56 dB at the centre frequency and a very large bandwidth of 43%, corresponding to 10 dB of isolation. A length miniaturisation of 84% is obtained as compared to the standard microstrip, leading to a total surface area of 0.132 λ g 2 , the smallest one reported so far.