# Post-wall waveguide

${\displaystyle a_{RWG}=a_{SIW}-1.08(2r)^{2}/p+0.1(2r)^{2}/a_{SIW}}$
where ${\displaystyle p}$ is the distance between the posts in the post wall, ${\displaystyle r}$ describes the radius, and ${\displaystyle a_{RWG}}$ as well as ${\displaystyle a_{SIW}}$ are the widths of the rectangular waveguide and SIW respectively.