3D-FDTD non-uniform mesh

hi.I have a problem.
I want to use from non-unifrom FDTD mesh for my antenna meshing.I want to write a 3D fdtd matlab code with non-uniform mesh...I know how write a code with uniform mesh,but I don't know how start to write a non-uniform mesh.in fact my problem is that Idon't know how use from cells...my antenna geometry sizes is specified.
in case of uniform mesh,I will define my antenna geometry sizes in a matlab program and then with truncating of domain and divide to dx or dy or dz,I find number of cells in x,y,z direction (nx,ny,nz) respectively.
but in case of non-uniform mesh how do I work?my antenna is a monopole antenna above a Groun box with dimensions: 30*30*5 cm.in fact I want to use from smaller cells in antenna feeding region and gradually towards PML,cell size incresed.
does anyone an idea for me?
is there any free software or matlab code for only non-uniform meshing,if I import nx,ny,nz and antenna geometry size?
please help me...
thanks alot...