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November 2017 • The PCB Design Magazine 59 tures, propagating modes are very close to, but not identical to, those of the rectangular wave- guides. Each SIW has a specific lowest transmis- sion frequency. The cut-off frequency (fc) is pro- portional to the width (w) of the particular SIW where c is the speed of light and Er is the dielec- tric constant of the substrate material. The most distinguishing characteristic of the SIW is the current distribution of the vias. The surface current on a traditional waveguide can flow forward in any direction. However, the cur - rent on the SIW, via barrel surface, is limited to the vertical direction only. As the vias are discrete, the current cannot flow longitudinally across the regular intervals. Therefore, the electric field in the SIW can only travel in the transverse electric mode (TEM), perpendicular to the direction of propagation as shown in Figure 2 (top). Several types of transition from SIWs to mi- crostrip or CPW structures are possible but as mentioned previously, can be challenging to implement. They can be roughly divided into single substrate or multilayer substrate appli- cations. Dual-layered SIW transitions to mi- crostrip or CPW structures have been success- fully applied. But multilayer SIW circuits often suffer from alignment issues. Z-axis alignment, of the multilayer laminate book, has always been a major limitation of implementing any broadside coupled application. The requirement that the TEM field in the SIW be adapted to the fundamental mode of the transmission line is common to all transitions involving SIWs. However, due to the similar- ity between the traditional waveguide and mi- crostrip modes, the microstrip to SIW transition is undoubtedly the simplest to implement. NEXT-GEN PCBS—SUBSTRATE INTEGRATED WAVEGUIDES Figure 3: Microstrip to SIW transition and simulated electric field (source: Kumar [3] ).