Help in design

Im trying to simulate an antenna with a inveted U patch and W shape gnd plane...i have drawn the design in ie3d....and simulated it...but im getting a change in the resonant frequency....the author has got it at 2ghz but i have got it at 2.3Ghz...can any one please help me out with the simulation..please help mee...im including the paper....i did it for the best values...alpha=40,beta=90,Wf=60mm...