How to simulate DR in CST?

You can look at the program's example - "Dielectric Resonator Filter".

I construct an air box and insert the DR in it.
set the background material is PEC,and all boundary condition is Et.
I don't it right or not.
Durning the simulation if the air box's dimension is changed, the DR resonant frequency is changed.
I don't why.
using the eigenmode solver! maybe we can discuss it without the master instructing!