High-Directivity and Low-Loss Directional Couplers Based on Empty Substrate Integrated Coaxial Line Technology

High-directivity and low-loss directional couplers based on Empty Substrate Integrated Coaxial Line (ESICL) technology are presented in this article. The proposed coupled line directional couplers are based on the combination of a high-isolation section, a tapered transition based on splines between the access lines and the high-isolation sections, coupled lines of reduced width and inclined arms. The design evolution has justified the combination and precise adjustment of these elements, proving to be a solution to increase directivity, improve return losses and achieve a more stable coupling coefficient. The use of ESICL technology shows great promise in terms of electrical performance, bandwidth, seamless integration with other planar circuits and ease of manufacture. For comparison, two directional couplers with the same coupling coefficient but different operating frequencies have been designed and fabricated. A microstrip coupler was also designed and fabricated for comparison. The simulated and measured results clearly show that the ESICL directional couplers outperform their microstrip counterparts in terms of losses and directivity, without the need for additional lumped elements or complex geometries.