Adding geometry, keeping mesh.. Help

Hi everyone!

I am pretty new in Comsol world, so I hope you can help me..
I am currently doing some RF-simulations of the electrical field in a 3D setup with different geometries of different materials. Every time when I add/or change a geometry in my "main"-domain, the mesh changes. I really want to keep a constant mesh, so that I can easily compare the values of the electric field with different geometries/materials in Matlab.

So I my question is: How do I keep a constant mesh when adding/or changing geometries/materials?

I hope you can help.

- Kim