Creating a region with a constant gradient field

I want to create a region of air with a static magnetic field in the z direction that has a constant slope (gradient) in that direction as well, for example H(x,y,z)=(0,0,1+z/10). Once I have this, I'm planning to put an object in it, but thus far I've not figured out to to make this sort of initial field. I tried creating a cube with the H field on the boundaries set to 0,0,1+z/10, but the calculated result within the cube wobbles away. It's even different along the boundaries! Very puzzling.