hi
here we can consider the ground plane as a field reflector
and it has a large effect on the radiation patterns and the resonance frequency
you cold notice that by using numerical methods for analyzing your microstrip
while most of the analytical methods assume that the ground plane is infinite in size
and hence no radiation fields appear in the back of microstrip(behind the ground plane)
as you could notice in many books like Bahl
i hope that i helped you:idea: