[femm] source modeling

[Masoud Movahhedy <m7723200@xxxxxxxxxxxxx>]

I want analysing a surface Log-Periodic Antenna whit NEC-2 software .
for calculation of input impedance of this antenna , it should be
execuete whit a Voltage Source . but Voltage Source defined in wire and
antenna is surface . 
if the Surface Antenna defined as a Wiregrid model , and execuete whit a
Voltage Source , obtained input impedance is very sensetive to
position of Voltage Source .
please guide me for Analysis of this antenna.

Thank you in advance .