We consider semi-infinite open-ended cylindrical waveguide with uniform dielectric filling placed into collinear infinite vacuum waveguide with larger radius. The radiation produced by a point charge or gaussian bunch moving along structure's axis out off the dielectric waveguide into the vacuum one is analysed. Modified residue-calculus technique is used. Analytical expressions for mode excitation coefficients in each sub-domain of the structure are obtained. We also perform direct numerical simulation and compare the results.