As far as I know, non-model objects do not affect generation of the mesh.
When simulating antennas in free space, I add a 'virtual box', which is just a box (material: vaccuum, no boundary conditions) about halfway between my object and the radiating boundary box. This helps to improve the meshing, without affecting the results. This box does not intersect any other object and is included in the model.
Maybe you can could use a similar technique for your model ?