Maurizio,

To return to your original question about MF antenna modeling with
ground effect (surface wave) included: Both NEC-2 and NEC-4 have the
capability to include "ground wave" in the Far Field calculations by
using an "RP 1" instead of an "RP 0" radiation pattern card. However,
you can also see the ground wave effect by doing Near Field
calculations, even if the distance of interest is not physically
"near" the antenna.

The MultiNEC program allows you to calculate the electric field
strength in millivolts per meter (or volts/m) using both far field and
near field algorithms, and then allows you to plot both on a familiar
polar diagram.

I have uploaded a few illustrations. The first shows an elevation
pattern for a quarter wave vertical over Sommerfeld ground, including
radials that are very close to, but slightly above, the ground level.
The frequency is 1.832 MHz, in the amateur 160 meter band.
Calculations were done with a NEC-2 engine.

www.qsl.net/ac6la/adhoc/nfversusff.png is a full screen capture and

www.qsl.net/ac6la/adhoc/nfversusff.gif is just the plot itself. You
can see that at low elevation angles the field strength that is
calculated using normal far field methods (RP 0 card) does not include
the surface wave, whereas the near field calculations clearly show the
ground wave effect.

The actual mV/m values in this example are for a radius (distance from
the coordinate system origin) of 1000 meters and a power level of 1000
watts. The full screen image shows the green dot marker on the near
field trace with a value of about 148 mV/m at 5° elevation. The
plot-only image shows the marker moved to the far field trace,
calculated (without ground wave included) to be about 90 mV/m at the
same elevation. At elevations below about 30° the normal far field
results are clearly inaccurate. Above about 30° elevation the near
field and far field results are almost identical.

You can also plot the field strength for an azimuth slice, 360° around
at a given observation height above the ground.

www.qsl.net/ac6la/adhoc/ambcast.gif shows an E-field strength pattern
(using Near Field calcs) at a radius of 1000 meters and a height above
ground of 2 meters. In this case the antenna model is for a
real-world AM broadcast radio station in California and includes four
towers, guy wires, and a buried ground screen. (This calculation was
done using NEC-4 but you could also use NEC-2 by raising the entire
structure slightly above ground.) I believe this type of plot is
required as part of the FCC licensing application. I am not a
professional engineer (far from it!) but I worked closely with someone
who is in order to implement this plotting function in MultiNEC.

The MultiNEC program is not freeware but the price is very modest.
For more information and a demo download please see
www.qsl.net/ac6la/.

Dan