Hello, I am working on a project  to recover the scattered field from a pec sphere. I am using a sphere of 75 mm radius illuminated by an incident plane wave and working in a large band ( 0.1 GHz to 5GHz with 0.1 GHz step frequency). Could you advise me on the right dimensions of the radiating box enclosing the sphere and based on which frequency if I apply the open region? I tried setting it on the lowest frequency but 0.1 GHz did not produce right results. I compare the results from HFSS with those I got analytically and they are not coherent when the frequency starts to rise (starting from 3GHz). Also is it possible to work on a frequency band starting from 10 MHz ? . 

Thank you