Plane wave and probe in CST

In CST 2007, does anyone know how to adjust the position (height) of the plane wave. When I set the plane wave and the probe, by default they are all at the same height from the analyzed structure. Adjusting the height of the probe, the height of the plane wave also change too.
Anyway I have to construct the vacumm box on top of the structure to set the plane wave but it so inconvenient. Is there another method?

i guess u need to define the boundary condition
and controlling the frequency range u can control ur height of plane wave incidence