monopole antenna return loss

Hi all, i am attaching a S11 Vs frequency plot of microstrip monopole antenna. The pot shows different S11 curves as you vary the patch width from 10mm to 38mm. My question is, why the first resonance dip is happening at a constant frequency whereas the second goes on shifting?

The resonance of the antenna would not shift due to the the length of the antenna being static in your parameter sweep. Width of the monopole dictates its input impedance and therefore explaining why resonance at 1.6 Ghz gets sharper as the input impedance of the antenna gets closer to the characteristic impedance of your system. Traditional microstrip antennas are single band components.

The second dip suggests that the input impedance of the antenna is low varying across frequency and allows for wide band operation.