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

Richard Clark wrote in message . ..
On 31 Jan 2004 07:42:00 -0800, (Maurizio) wrote:

However, the antenna that was simulated in the paper I was talking
about is a real antenna that has been modellized with a dedicated MOM
program and with the correct antenna geomety, and results have been
compared with measurements.
From this comparison it has been necessary the introduction of such
factor.
It seems to me that the 6 dB factor had to take into account all
losses from the transmitter to the radiated fields.
My concern is how this factor can be justified.
6 dB is a lot in terms of antenna usefull coverage distance.


Maurizio

Hi Maurizio,

I am a trained Metrologist with advanced studies in Microwaves. The
measure of power (which is intimately tied to any expression of dB) is
very difficult to achieve with great accuracy. This means that
measurements are always suspect when they purport to confound theory.

The logic of the MOM program that works at one wavelength expresses
that it will work at all wavelengths. There is no scale determinacy
whereby results in HF are corrupted in SHF. There is every potential
for human error and measuring power reveals that quicker than any
other effort.

A 6dB discrepancy is a human problem, and glaringly evident.

73's
Richard Clark, KB7QHC

Hi Richard,
I agree that the human factor can be the problem for this discrepancy,
however, it would be also very interesting to know about other
experiences with such type of measurements, just to narrow the
expected uncertainty window.
(Better if in presence of complex environments)

Maurizio

On 4 Feb 2004 00:36:07 -0800, (Maurizio) wrote:

A 6dB discrepancy is a human problem, and glaringly evident.

73's
Richard Clark, KB7QHC

Hi Richard,
I agree that the human factor can be the problem for this discrepancy,
however, it would be also very interesting to know about other
experiences with such type of measurements, just to narrow the
expected uncertainty window.
(Better if in presence of complex environments)

Maurizio

Hi Maurizio,

This is best achieved by establishing a standard with as few variables
as possible, then varying them to determine their range of error
contribution. I am sure this is already evident to you, however
experience in going through the tests can reveal how probable those
sources of error may be.

You can quickly accumulate 3dB error in simply not providing the
correct load to the power (dB) determining measurement. Temperature
too can upset readings, but often as not, the simple presence of the
observer disrupts many things. The source is all to often taken for
granted, and all too often has frequency products that add to the
power reading, but are not in the bandwidth of interest. The list
goes on....

73's
Richard Clark, KB7QHC