Lets just clarify: sagar's problem is about axial transmission/propagation in a cylindrical waveguide. Regarding the described results: The dipole will not only emit into the single existing mode, the fundamental mode. It will also emit into the infinite number of freespace/lossy modes around the waveguide. With your simulation, you are measuring the coupling of a single dipole to the fundamental waveguide mode. This will change with the diameter of your waveguide, thats where the strongly random (actually I would guess oscillating) results come from.
If you want to study the axial propagation of a single mode, I suggest using the standard mode expansion source to excite this mode of interest.

My problem is about radial transimission/propagation in a planar waveguide. So instead of a planar mode front in a circular guide, I am interested in a circular mode in a planar guide.

So I guess there was a mix-up...