Image field of a charge on top of a conducting plane

Hi,

I was interested in finding image force on a charged particle near a conducting rod in comsol. Therefore as an initial check i simulated a charge near a conducting plate and compared the resulting image field to the field from analytic solution of charge on a infinite conducting plane. The results in these two case doesnt seem to match even if i increase the size of the conducting plate so that it appprox. an infinite plate.

Can you help me solve this discrepancy? Am i doing something wrong here?
thanks
-Emil