The distance between patch and superstate should be decided by first analyzing a single metamaterial unit cell which should show near zero refractive index in the frequency range of interest. Then the array of such unit cells which forms the superstate should be placed at one third of the wavelength (λ/3) above ground plane.

www.sersc.org/journals/IJHIT/vol8_no11_2015/17.pdf

thanks naveen but why the distance between the patch and superstate is lemda by 3

thanks for replying sir

In case of metamaterials, you should take the advantage of the structure that is-

in comparison to FSS or other periodic structures where the distance is typically half wavelength for superstrate, metamaterial structure should be placed at a spacing much less than half wavelength or quarter wavelength. So place your structure close to the antenna and then vary the spacing. use of metamaterials reduce the spacing.