how can i do nec simulation of dipole in free space?

check forums with this topic FDTD. you can find some source codes that simulate dipole anntena in 1D , 2D and 3D
you this page is one of them. you can find more pages
https://www.edaboard.com/viewtopic.p...032608#1032608

I suggest downloading the demo version of WIPL-D Microwave from http://www.wipl-d.com and making a wire model. This tool has wire building elements just like NEC and is therefore similar for this kind of antennas.

And for the dipole the demo is enough so its free!