Multi-aperture couplers between microstrip circuitry and substrate-integrated waveguides are presented. The design procedure is described, and couplers formed by single-aperture and dual-aperture configurations are presented. It is demonstrated that the coupling can be as tight as 1 dB but that weaker coupling leads to more broadband and flatter coupler performance. Designs are carried out in an HFSS environment; comparison with CST validate the design procedure. Dimensions and parameters of all couplers are presented as guidelines for general design purpose. © 2011 IEEE.