Problem in simulating a coaxial-probe-fed microstrip patch antenna in IE3D

Hello,

I am trying to simulate a coaxial-probe-fed microstrip patch antenna to achieve some proven data.
I am using IE3D EM Design System , Version 14.04.
I used finite groundplane by defining No0. layer's conductivity to be 0, then drawing a rectangle as the metal ground.
I used the method from section 16.2 of the IE3D manual to specify and define my finite substrate layer. (i defined my own polygon to be the finite substrate instead of using the existing groundplane)

However, i got a VSWR vs Freq graph that differed from the proven data by around 3Ghz. (ie the whole graph shifted right by ~3Ghz)

I have spent 3 weeks trying to find out whats wrong, but to no avail.
Can someone enlighten me?

Thanks alot,
Kevin