Experts, I need your help here:

When I perform simulation in CST Studio suit on a metasurface unit cell intended to increase axial ratio bandwidth of a circularly polarized antenna, the reflection phase varies as long as the box height, h, increases or decreases. As depicted in the figures below, the location of zero degree reflection phase shifted left with increasing h. So how do we adjust the optimal height, h so that the zero degree reflection phase lies in the intended operating frequency [keeping in mind the dimension of unit cell plays key role in fixing the operating frequency.]

Thanks