HFSS- problem with flat plane

Hi
i'm italian and i try to use HFSS but i' ve got some problems.

I try with a wellknown object: a flat plane
the result aren't correct.

From Skolnik "Radar Handbook" the expression for RCS is:

4 pi A^2
RCS = --------------
lambda^2

use 3 GHz (lambda =0,1), A= 0,25 m^2;

if i watch the result of RCS normal to the plane i should find the max but this is very different respect the expression from Skolnik.

i attach the project, please let me know where is the error
thanks
MArco