As GP pointed out, the problem is that if the source is inside the can, the walls are near the source - where the EM field diverges - whereas with an EMP the source would be far away (hopefully) so the EM field lines would be roughly parallel over the surface of the cage
As GP pointed out, the problem is that if the source is inside the can, the walls are near the source - where the EM field diverges - whereas with an EMP the source would be far away (hopefully) so the EM field lines would be roughly parallel over the surface of the cage