NEC4 & MF antenna modelling with earth
 

Hi all,
Is there anybody with some experience in MF antenna modelling with
ground effect included (surface wave/space wave) etc. ?
I have the need to find out if the NEC4 is able to modellize this type
of antenna in a general environment and with which performance.
Otherwise, I will have to find out alternative ways (FEM/...).

Thanks in advance for any suggestion


Best regards








Dr. Maurizio Lori
Antenna System Design
via P.Anfossi,15 00124 Rome Italy
Tel. + Fax: +39 06 50915496 - Cell. +39 328 1555179

On 28 Jan 2004 03:39:57 -0800, (Maurizio) wrote:

Hi all,
Is there anybody with some experience in MF antenna modelling with
ground effect included (surface wave/space wave) etc. ?
I have the need to find out if the NEC4 is able to modellize this type
of antenna in a general environment and with which performance.
Otherwise, I will have to find out alternative ways (FEM/...).

Thanks in advance for any suggestion


Best regards


Hi Maurizio,

Antenna modelers (NEC engines) render far, far field results. The
effect of ground is considered only for
1. its impact on feedpoint Z;
2. equivalent launch angle (from the perspective of very far, or DX,
receivers).

What you describe as your need sounds more like the province of
propagation modelers. There are some add-on packages that incorporate
with antenna modelers to allow for a variety of terrains; but even
those, I think, still relate to the very far, DX, receivers.

Even the propagation modelers (like VOACAP and VOAAREA) are tailored
for HF and the far, far field. Our references from the FCC for MF
broadcast stations stands as a resource. Those references go back
many years and little has advanced the science as it has served the
industry very well.

73's
Richard Clark, KB7QHC

Richard Clark wrote in message . ..
On 28 Jan 2004 03:39:57 -0800, (Maurizio) wrote:

Hi all,
Is there anybody with some experience in MF antenna modelling with
ground effect included (surface wave/space wave) etc. ?
I have the need to find out if the NEC4 is able to modellize this type
of antenna in a general environment and with which performance.
Otherwise, I will have to find out alternative ways (FEM/...).

Thanks in advance for any suggestion


Best regards


Hi Maurizio,

Antenna modelers (NEC engines) render far, far field results. The
effect of ground is considered only for
1. its impact on feedpoint Z;
2. equivalent launch angle (from the perspective of very far, or DX,
receivers).

What you describe as your need sounds more like the province of
propagation modelers. There are some add-on packages that incorporate
with antenna modelers to allow for a variety of terrains; but even
those, I think, still relate to the very far, DX, receivers.

Even the propagation modelers (like VOACAP and VOAAREA) are tailored
for HF and the far, far field. Our references from the FCC for MF
broadcast stations stands as a resource. Those references go back
many years and little has advanced the science as it has served the
industry very well.

73's
Richard Clark, KB7QHC

Hi Richard,
I know that some studies have been done in the past using NEC4 to find
the ground wave/space wave fields, but my primary interest is in the
near zone modelling of such MF antennas, in particular in presence of
non-flat ground.
The scope is to make a tool to evaluate the radiated fields in
inhabited regions with limits in the maximum field amplitude.
A tool dedicated to this task would be very useful for the site
evaluation.

I have some idea on based on recent studies, but maybe (as often
happen) that others have already done something interesting.

Are these FCC resources dealing with such issues ?
And if so are they freely available?

Thanks in advance


Maurizio

On 29 Jan 2004 05:45:02 -0800, (Maurizio) wrote:

Hi Richard,
I know that some studies have been done in the past using NEC4 to find
the ground wave/space wave fields, but my primary interest is in the
near zone modelling of such MF antennas, in particular in presence of
non-flat ground.
The scope is to make a tool to evaluate the radiated fields in
inhabited regions with limits in the maximum field amplitude.
A tool dedicated to this task would be very useful for the site
evaluation.

I have some idea on based on recent studies, but maybe (as often
happen) that others have already done something interesting.

Are these FCC resources dealing with such issues ?
And if so are they freely available?

Thanks in advance


Maurizio

Hi Maurizio,

Very few here have experience with the expensive NEC4, but I have
heard of such capabilities. Sounds like you want to investigate near
fields against complex terrain - that is going to take a lot of
modeling and a lot of CPU horsepower.

I am sure we would all be interested in the product of your effort.

The FCC does not like complexity, and as such you find a very common
and ordinary sameness to nearly every installation; unless they happen
to have a downtown location elevated above the street (there is one
like that in Spokane WA). Even there, the station conformed to the
slavish 120 radials strung out to neighboring buildings.

73's
Richard Clark, KB7QHC

Hi Richard,
my feeling is that in the past some studies have tried to find out the
structure and relative strenght of the ground/space wave fields.
Experience has then given rules to evaluate the losses due to the high
reactive fields near the antenna with the lossy ground (i have seen a
paper using a 6 dB factor to take into account antenna mismatch and
such nearby losses).
It maybe that this is almost all that has been done.
Do you agree?


Maurizio

(Maurizio) wrote in message . com...
Hi all,
Is there anybody with some experience in MF antenna modelling with
ground effect included (surface wave/space wave) etc. ?
I have the need to find out if the NEC4 is able to modellize this type
of antenna in a general environment and with which performance.
Otherwise, I will have to find out alternative ways (FEM/...).

I see you come from Italy, and (for as far as I know) the Nec4 core is
not (yet) available for none US citizens due to export restrictions.

You could consider using the Nec2 core. However not directly capable
of modelling burried radial wires you can simulate them by using
radial wires located some .001-.005 wl above ground (See www.cebik.com
for detailed info about this subject).

When modeling structures on MF (or LF) however you should also be
carefull not to violate segment length restrictions due to the
relatively low frequency compared to structure size.

You could take a look at the 4nec2 freeware
(www.qsl.net/wb6tpu/swindex.html) to see if the nec2 core is usefull.
4nec2 also has provisions for generating and visualizing far-field
patterns in which the surface wave is included.

Greetings,
Arie Voors.

wrote in message . com...
(Maurizio) wrote in message . com...
Hi all,
Is there anybody with some experience in MF antenna modelling with
ground effect included (surface wave/space wave) etc. ?
I have the need to find out if the NEC4 is able to modellize this type
of antenna in a general environment and with which performance.
Otherwise, I will have to find out alternative ways (FEM/...).

I see you come from Italy, and (for as far as I know) the Nec4 core is
not (yet) available for none US citizens due to export restrictions.

You could consider using the Nec2 core. However not directly capable
of modelling burried radial wires you can simulate them by using
radial wires located some .001-.005 wl above ground (See www.cebik.com
for detailed info about this subject).

When modeling structures on MF (or LF) however you should also be
carefull not to violate segment length restrictions due to the
relatively low frequency compared to structure size.

You could take a look at the 4nec2 freeware
(www.qsl.net/wb6tpu/swindex.html) to see if the nec2 core is usefull.
4nec2 also has provisions for generating and visualizing far-field
patterns in which the surface wave is included.

Greetings,
Arie Voors.


Thanks a lot Arie for the links which as I have rapidly seen contain
many interesting references.
My experience is mainly in microwave (and up) frequency antennas, it
is the first time I look at these low frequency problems like ground
modelling and I see that this is the right newsgroup for this field.
If I will find some way to manage this problem better than how it is
currently done, I will keep you informed.
I would need to make a comparison between simulation results from MOM
and FEM programs of a common broadcast antenna to have a better view
of the situation.
Maybe that I can arrange such simulation comparison.

Thanks again

On 30 Jan 2004 02:16:09 -0800, (Maurizio) wrote:

Hi Richard,
my feeling is that in the past some studies have tried to find out the
structure and relative strenght of the ground/space wave fields.
Experience has then given rules to evaluate the losses due to the high
reactive fields near the antenna with the lossy ground (i have seen a
paper using a 6 dB factor to take into account antenna mismatch and
such nearby losses).
It maybe that this is almost all that has been done.
Do you agree?


Maurizio

Hi Maurizio,

I worked on this a couple of years ago:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm

It deals not so much with the variation of ground proximity with a
standard antenna, instead it works against the standard ground with a
variety of antennas.

Due to the intricacy of geometry afforded by a fractal form, this:
http://home.comcast.net/~kb7qhc/ante...atic/index.htm
is the most interesting. Unfortunately, the legacy of academic
fractal research (sic) has offered no more interest than the morbid
study of Down's Syndrome among the Armadillo population.

73's
Richard Clark, KB7QHC

Richard Clark wrote in message . ..
On 30 Jan 2004 02:16:09 -0800, (Maurizio) wrote:

Hi Richard,
my feeling is that in the past some studies have tried to find out the
structure and relative strenght of the ground/space wave fields.
Experience has then given rules to evaluate the losses due to the high
reactive fields near the antenna with the lossy ground (i have seen a
paper using a 6 dB factor to take into account antenna mismatch and
such nearby losses).
It maybe that this is almost all that has been done.
Do you agree?


Maurizio

Hi Maurizio,

I worked on this a couple of years ago:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm

It deals not so much with the variation of ground proximity with a
standard antenna, instead it works against the standard ground with a
variety of antennas.

Due to the intricacy of geometry afforded by a fractal form, this:
http://home.comcast.net/~kb7qhc/ante...atic/index.htm
is the most interesting. Unfortunately, the legacy of academic
fractal research (sic) has offered no more interest than the morbid
study of Down's Syndrome among the Armadillo population.

73's
Richard Clark, KB7QHC

Hi Richard,
It seems that you have tried to shorten somewhat these cumbersome
antennas with the fractal approach.
I have seen recently an interesting paper on antennas & propagation
proceedings/magazine that was comparing the performance of fractal and
non-fractal designs.
Regarding the graphs you show in the web pages, if I have well
understood, you compare the E/H local fields (amplitudes) with the
free space impedence.
It is an alternative way to look at the near reactive fields.
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

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

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