Planar antenna radiation efficiency >1 in HFSS

Hi,
I've done a model in HFSS for an inverted planar F antenna for 868 MHz on a FR4 substrate of 100x85 mm^2 and a ground plane 100x65 mm^2.
I've fed the antenna with a microstrip line designed for 50 Ohm impedance.
I've also used as radiation boundary a vacuum sphere of radius 270 mm, more than 3/4 lambda at 868 MHz.
I've assigned to the antenna and ground, copper finite conductivity to define a model as real as possible.
I've strongly seeded the mesh on the radiation boundary knowing the problems related to antenna parameters accuracy but however I've obtained radiation efficiency major than 1 all over the frequency sweep from 700MHz to 1.1 GHz.
Why?Could anyone help me?
Thanks in adv.

Please search the forum, this question has been asked many times... most likely due to inadequate meshing on the airbox boundary. Please take a look at this tutorial to make sure you setup everything correctly/optimally:

http://www.emtalk.com/tut_1.htm

and this

http://www.emtalk.com/forums/viewtopic.php?t=4

for information about the meshing.