Actually, I had used dipole as a source inside a cylindrical waveguide and placed a mode monitor to select a particular mode, and tracked the T_forward of a power monitor placed at the edge of the cylinder to compare with the total power emitted through the monitor, to get an idea out of the total power the contribution of the selected mode. But getting very wired results. As for a particular diameter of the cylindrical waveguide, there would be only fundamental mode, so T_forward of the fundamental mode should be equal to the total T, transmitted through the power monitor.  But for the diameter sweep of the cylinder the T_forward value is fluctuating in between 0.0001 to 0.4, and at lower diameter the T_forward value is very less.