A Compact 94 GHz Image Substrate Integrated Non-Radiative Dielectric (iSINRD) Waveguide Cruciform Coupler

This letter presents a study of image substrate integrated dielectric (iSINRD) guide arms at 94 GHz, from which a cruciform 3 dB coupler is conceived by superimposing their respective reflection coefficients. Four configurations are analyzed by investigating the boundary value problem in light of the electric and magnetic symmetries of the iSINRD guide. Mode conversion, and thus double bends, is totally avoided thanks to the PEC image plane. Thus, in addition to being planar, the coupler is very compact which makes it easily integrable in millimeter-wave circuits. The lateral dimensions of each arm are 0.413 × 0.635 mm². Perforation is done using square air-vias, each having a side length of 0.35 mm. The total size of the circuit is 4.413 × 4.413 mm². Measured results agree well with simulated ones. A coupling level of 3.5 dB and isolation better than 17 dB are obtained for a bandwidth of 15%.