AFSIW-to-Microstrip Directional Coupler for High-Performance Systems on Substrate

Very recently, the high-power handling capability of the high-performance air-filled substrate integrated waveguide (AFSIW) technology has been experimentally demonstrated and reported in literature. This important milestone confirms the interest of this novel technology for the design of low-cost, highperformance and self-packaged transmitters. In the purpose of designing such AFSIW sub-systems with a required integrated power monitoring capability, an AFSIW to microstrip directional coupler is proposed. The introduced coupler topology takes advantage of the multilayer feature of the technology by coupling some of the high power signal propagating in the inner substrate AFSIW with the top layer, implementing a microstrip line, on which a surface mounted MMIC power detector and termination can be implemented. To the authors' best knowledge, it is the first AFSIW to microstrip coupler reported in literature. For demonstration purposes, a wideband 25 dB coupler operating from 25 to 35 GHz has been designed, fabricated and measured. It achieves a measured insertion loss and coupling of 3.53 ±0.57 dB (taking into account the GCPW-to-SIW and SIW-to-AFSIW transitions required for interconnection with laboratory instruments) and 26.55 ±0.85 dB, respectively. The measured return loss and isolation are better than 11 dB and 33 dB, respectively.