Chu Sphere in HFSS for the quality factor of an antenna

My objectif is to get the quality factor of a small antenna. First , i've to get the stored electric and magnetic energies around this structure. Second, i've to calculate the radiated power. For the stored energies, i've drawn a Chu sphere in HFSS (vacuum) which encompasses exactly the greatest dimension of the antenna (it is not a radiation box) but i've different simulation results when the antenna is simulated with Chu sphere or without it. I'm so confused and i need help thanks.