Forward‐wave directional coupler based on slow‐wave coupled microstrip lines

Abstract

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 0.132λg2, the smallest one reported so far.

2.3 GHz Microstrip Rectangular Structured Hybrid Coupler for WiMax applications

“…Deviating from the rectangular structured couplers, ring hybrid coupler with different slit orientations is shown to provide stable results in terms of coupling factor and insertion loss as well as improved return loss and isolation [15]. Additionally, a concept of slow-wave coupled design of microstrip coupler is implemented to improve isolation and high coupling factor of the directional couplers [16]. Another advantage of such design is reduction in overall size of the coupler [16].…”

“…Additionally, a concept of slow-wave coupled design of microstrip coupler is implemented to improve isolation and high coupling factor of the directional couplers [16]. Another advantage of such design is reduction in overall size of the coupler [16]. Asymmetric directional coupler with a cascade structure can be used in different applications related to motion detection due to the relatively miniature size, reduced cost and acceptable range for S parameters with an emphasis on the increased isolation factor [17].…”