In order to get the radiated field pattern, you have to consider the model you are using.Transmission line model would be the easiest, and the intergral equation would be abit rigorous.
for a coaxial fed antenna , you dont consider the current in the coaxial.in any case the antenna can be a transmitting or a recieving antenna.
The excitation fields could could be fields of a plane wave coming from infinity(recieving antenna), or the local fields created by a finite source located within the microstrip structure(transmitting antenna) .
the excitation fields induce surface currents on both sides of the mettallic patch and on the upper side of the ground plane. therefore model the patch as a sheet of current whose value at any point is the algebraic sum of the upper and lower currents.
Antenna designers have found that the most sensitive parameter in microstrip antenna performance estimation is the dielectric constant of the substrate material.this will come more clear when using the transmission line model because the constant will be appearing in the formulas especially when calculating the resonant frequency.