help to find dielectric constant of micromachined antenna

hi,in my attachements ; can anyone help me to find the equivalent dielectric constant for the area under the patch which consist of dielectric material (15 mil) and air cavtiy (35 mil).
i try to use a capacitance model to find the equivalent dielectric constant and it gives the result (1.37254902), but when i simulate this value in HFSS by replacing the two part under the cavity with an equivalent box has material with dielectric constant = 1.37254902 as in second file, the result is different from the original case.

thnks for ur interest