On Oct 1, 12:45 am, Owen Duffy wrote:
Obviously, the model is sensitive to soil type, and different
soil types can be expected to yield different response... but it would
seem that just 3 radials at 1 to 3m height give similar system efficiency
to 16+ radials shallow buried for a range of common soil types.

The two links below may be of interest in comparing NEC modeling with
empirical results.

The first link is a clip from the "benchmark" 1937 I.R.E paper of
BL&E, showing that the radiated fields measured 3/10 of a mile from
monopoles ranging from about 45 to past 90 degrees in height, and
using 113 each 0.412-lambda buried radials is within several percent
of the theoretical maximum for a perfect monopole of those heights
when driven against a zero-ohm connection to a perfect ground plane.
The BL&E tests were conducted in the sandy soil of New Jersey, where
earth conductivity was/is 4 mS/m or less. Those measured results
indicate those systems were radiating 90% or more of the applied
power, and that the conductivity of the earth in which those radials
were buried is relatively unimportant.

The second link is a NEC model of a 1/4-lambda monopole driven against
four, elevated counterpoise wires with no antenna system connection to
a perfect ground plane, showing that its peak gain is 5.15 dBi --
which is the theoretical maximum for a perfect 1/4-lambda monopole
driven against a zero-ohm connection to a perfect ground plane.

Adding an ohm or two in the connection from the source to the four
elevated radials reduces the gain/field of the NEC model such that it
is approximately what was shown in the BL&E study, indicating that a
similar value must have been present in their buried radial ground
system consisting of 113 each 0.412-lambda wires.

Using NEC-4 to incorporate buried (or elevated) radials into the model
should show groundwave fields within 1 km of the monopole that are
very close to the theoretical maximum for the applied power when
radiated along a perfect ground plane, if the model is optimal, and
accurate.

The theoretical maximum inverse distance voltage field intensity at 1
km for 1 kW of radiated power from a perfect 1/4-lambda monopole
system is about 313 mV/m.

http://i62.photobucket.com/albums/h85/rfry-100/G.gif

http://i62.photobucket.com/albums/h8...tedRadials.jpg

RF