i am doing my project in rcs reduction of frequency selctive surfaces..
i am using hfss 13 for simulation
i selected a unit cell and upo validating i gt an error as


HFSSDesign1 (DrivenModal)
[error] Port 'FloquetPort1': The exterior boundary of the faces for a Floquet Port must consist of pairs of parallel and equal length segments, displaced from each other by an integer linear combination of the lattice vectors. (1:00:37 PM Aug 08, 2011)
[error] Port 'FloquetPort2': The exterior boundary of the faces for a Floquet Port must consist of pairs of parallel and equal length segments, displaced from each other by an integer linear combination of the lattice vectors. (1:00:37 PM Aug 08, 2011)
[error] Simulation completed with execution error on server: Local Machine. (1:00:37 PM Aug 08, 2011)\

anybody please help me..pls