axial ratio equation

Hi

I have problems regarding the axial ratio for a circularly polarized waveguide antenna in hfss.
As i have always done for waveguide slot antennas , i choose an airbox of λg/4 of height and use PML boundaries.
with this boundaries, the structure was optimized for an axial ratio of 0.1 dB.
but now, as i enlarge the radiation box, the Axial ration gets worth and worth...
some people tell me to enlarge the box, so that i can obtain a fixed value, but.. ?

could anyone help me ?

thanks
saras