Problem with FEMM

["zifera <emiliano.menghi@xxxxxxxxx>" <emiliano.menghi@xxxxxxxxx>]

Dear David and Femm users, i've a big problem with Femm and i hope 
that you want help me.
I use FEMM to computate the shielding effectiveness (low frequency: 
50-60 hetz) of plane aluminium screen (tickness 3 mm, lenght 2 m). 
The source is a triphase cable system with simmtric current (500 A). 
The distance between the conductor is 25 cm. The position of the 
screen is simmetric respect the conductors (the conductors are in 
plane) 
The problem is that near the screen the magnetic field (calculated 
along a line parallel to the sceen) is not simmetric!! For distance 
bigger than 50 cm magnetic field return to be simmetric. Why?
I've tested the same configuration with a commercial fem (Ansoft) and 
the result is equal.
Thanks.

Emiliano