Frank wrote:
Ground planes above ground can approximate the results
from buried radials. The wires should be several wire diameters
above the ground, and not 10^(-6) wavelengths -- providing
that a finite ground, Sommerfeld/Norton method, is used.
The reflection coefficient approximation will produce large errors.

Here's what the EZNEC manual says: "Horizontal wires should not
be placed exactly on the ground, but should be at least 1/1000
wavelength above (and in the case of EZNEC/4, also below) the
ground."
--
73, Cecil http://www.qsl.net/w5dxp