Just model a vertical dipole. The presence of a coaxial cable, within the
lower conductor, will not effect its performance.

But the presence of the coax below the antenna sure will.

As an experiment I modeled a #14 AWG, 133 segment,134 ft free space dipole.
The input impedance at 3.575 MHz is 74.01 - j 0.749 ohms. Adding an
additional 25 ft wire at one end -- ends separated by 1.2" -- the input
impedance becomes 74.155 - j1.97 ohms. There is no discernable effect on
the radiation pattern.

While this test is not precisely equivalent; the presence of a, non
connected, axial extension appears to have only a minimal effect on the
antenna parameters.

Regards,

Frank