FDTD problem:coax feeding and transmission line feed model

hi.
I have a question in FDTD and antenna feeding.
I want to feed my DRA with coaxial cable and transmission line feed model.I wrote a code in matlab.
do I have to use from thin-wire modeling,to model inner conductor of coaxial cable?
how model outer conductor of coax?
I'm confused really!
please help me...