If you are modeling the microstrip antenna (MA) using transmission line model, then first calculate Zin. Then using conventional formula, calculate VSWR, BW, Qt etc. Plot several graphs for different imag part of Zin. For microstrip antenna, the difference will be very small as it has no variation along the thickness of the height. In case of DRA, you will find huge change for a fixed mode (mnp). Unfortunately, transmission line modeling is not available for DRA.
The imaginary part of antenna impedance represents the energy stored in the near field of the antenna, which is a non-radiated power. It is for this reason that it must be canceled for a good matching.