Hello
Dear ferdows,Substrate is the material that is used between two conducting layers where superstrate is the layer that is on top of the conducting layer to increase the gain,bandwidth etc.so there are two different things.
May be you should look into the below papers
D. R. Jackson and N. G. Alexopoulos, “Gain Enhancement
Methods for Printed Circuit Antennas,” IEEE Trans. Antennas
Propagat., vol. 33, no. 9, Sept. 1985, pp.976-987.
2.H. Y. Yang and N. G. Alexopoulos, “Gain Enhancement Methods
for Printed Circuit Antennas through Multiple Superstrates,” IEEE
Trans. Antennas Propagat., vol. 35, no. 7, July 1987, pp.860-863.
and attached is a paper for your reference. please have a look at it and see if its helpful,if not we can look in further.
Best Regards
Sajid Mohammed.