NEC-2 and NEC-2 based programs like EZNEC do a good job of modeling
Beverage antennas. (The EZNEC demo isn't adequate for this.) The only
trick is making the ground connection. In NEC-2 use Sommerfeld ground;
in EZNEC it's called Real, High Accuracy ground. At each end of the
antenna, create a semi-circle of radials, a few inches above the ground.
For wire length, a free-space quarter wavelength is more than adequate
-- the only requirement is that the effective resistance should be
considerably less than the termination resistance. The semi-circles
should be facing away from the antenna, so none of the wires extend
under the antenna wire. Ground "connections" are made to the center of
the semi-circle. This of course assumes that there are reasonably
effective ground connections at both ends of the real antenna.

Roy Lewallen, W7EL