what is the relation between substrate size and patch size for good simulation?

I about to simulate a patch antenna. What is the relation between the patch size and the substrate size? For example, if the patch I want to try is 12x12 cm and the substrate thickness is 1.6 mm, what the is substrate size?

hi
this is wrong question its better u define u antenna frequency
but u find u answer by read the chapter of micro strip of balanis or every antenna book and use patch formula

Thanks for your reply. I more or less have the dimensions of W and L for the patch. The question is, how far do I extend the substrate? Al pictures on the book show the substrate larger than the patch but I don't see a calculation about the width and length of the substrate.

usually for rectangular patch length of substrate is (length of patch + 6 times height of substrate) and same for width

In the hfss design MIMO antenna how to do mathematical analysis? please anybody help me?