Air-Filled and Slow-Wave CNT-Based Substrate Integrated Waveguide

This paper presents an air-filled substrate integrated waveguide (SIW) internally including a forest of carbon nanotubes (CNT) to create a slow-wave effect and thus, to decrease the dimensions of the structure. For 400-$\mu \mathrm{m}$ of CNT height, the simulation shows a reduction of the surface up to 68%, as compared to a structure without CNT, for an impact on the unloaded quality factor reduced by 19% at 28 GHz. A comparison is also made with copper blind vias instead of CNT to create the slow-wave effect: simulation shows twice more losses than the solution with CNT. By considering $200-\mu \mathrm{m}$ CNT height, measurement results allow obtaining a 25%-reduction of the cut-off frequency as compared to a non-slow-wave waveguide with the same width, a good agreement with a theoretical expression being obtained for the phase constant. These first results represent a proof-of-concept of a low-loss and compact SIW for 5G applications. Attenuation constant can be improved with higher density of CNT.