(HFSS)how to define a DC E-field in a PBC box?

I wanna add a DC E-field to the unit SRR cell, the vector of incident wave as the FIGURE(1).

I use the PEC and PMC to define the periodic boundry on x-z plane, wave ports applied on right and left facets, now, how can I add a uniform DC E-field whose direction is parallel to z-axis ?

Could you please tell me how to do?

my HFSS file

http://www.badongo.com/file/8090797

Could someone kindly give me some suggestion?

I inter a rectangle plane with voltage excitation along -z direction and excitation wave port on both side of right and left face, but I still can not get the uniform DC E-field.