IF your laptop has a VGA output, then something like this
http
/www.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=4779573&csid=
Would give you an RCA/Svideo output, then you would need an old VCR with RCA/svideo inputs in order to output a RF/Coax signal.
If you laptop doesn't have VGA, then you would need like an HDMI->VGA->Svideo->VCR->Coax
May or may not work. Kind of deal with old school type of things and a VCR is kind of the best way to do it.