In order to provide a means for measurement and to allow the complete integration of SIW components with active circuits, these SIW components must be interconnected with planaf structures by means of SIW-to-planar transitions.
Typical to all transitions involving SIWs is the requirement that the TE10-mode-like field of the SIW is adapted to the fundamental mode of the transmission line it is interfaced with. Due to the similarity between the fundamental waveguide and microstrip modes, and to the fact that the microstrip line is widely used in planar circuits, the SIW-to-mictrostrip transition was the first one proposed [DW01a], and design guidelines of the traditional microstrip taper are well understood [Des10]. This transition covers the complete bandwidth of the SIW, and the performances are better when compared to other microstrip [JK01,YCH+05,kYHS+07, AESFD07] or coplanar transitions [DW01b,DW05,TDBW12] (see figure). However, the microstrip transition does not necessarily have to vary linearly [MEB+11].