Coplanar Waveguide Line with Signal width equal to $20$um and Signal$-$to$-$Ground width equal to $30$um is built on Alumina substrate $($er$=8\mathrm{.}8,$ fr$=1,0=0)$ with Zo$=74\mathrm{.}65$Q$.$ It is used for the feeding of a Parallel$-$Plate Waveguide filled with Alumina and plate$-$distance equal to $1\mathrm{.}2$mm$.$ The walls of the parallel$-$plate waveguide can be assumed to be perfect conductors.This topology is a part of a satellite transponder operated at $12$GHz$.$