"Roy Lewallen" wrote
Of course the standard far field analysis doesn't accurately depict the
field close to the antenna -- it's a plot of the field at points very
distant from the antenna, as clearly explained in the manual. NEC allows
you to include the surface wave if you want, and it accurately shows the
total field including the surface wave at a distance of your choice.
_________

Not the versions of NEC used by most amateurs.

Those versions show zero or very low gain in/near the horizontal plane
for a
vertical monopole over real earth. This leads to the common (mis)
belief
that those are the gains of the radiation pattern _originally
generated_
by the monopole.

But that belief is untrue.

RF

On Wed, 26 Nov 2008 03:39:41 -0800 (PST), Richard Fry
wrote:

"Roy Lewallen" wrote
Of course the standard far field analysis doesn't accurately depict the
field close to the antenna -- it's a plot of the field at points very
distant from the antenna, as clearly explained in the manual. NEC allows
you to include the surface wave if you want, and it accurately shows the
total field including the surface wave at a distance of your choice.
_________

Not the versions of NEC used by most amateurs.

Those versions show zero or very low gain in/near the horizontal plane
for a
vertical monopole over real earth. This leads to the common (mis)
belief
that those are the gains of the radiation pattern _originally
generated_
by the monopole.

But that belief is untrue.

RF

Roy and others have answered this one in the past too. You employ the
near field table to observe the ground wave. It works approximately
well, even out to the edge of the implicit flat universe. If you
object to flat universes, you are no longer in the realm of ground
wave. If anything, modelers give MORE response in comparison to the
BL&T data.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

Roy and others have answered this one in the past too. You employ the
near field table to observe the ground wave. It works approximately
well, even out to the edge of the implicit flat universe. If you
object to flat universes, you are no longer in the realm of ground
wave. If anything, modelers give MORE response in comparison to the
BL&T data.

With EZNEC, you have to use the near field analysis to include the
ground wave; direct ground wave analysis isn't included in EZNEC because
it's of very limited use to most amateurs. In fact, it's really of
interest only to AM broadcasters and a very few other very small and
specialized users, who just about universally use EZNEC Pro which does
include direct ground wave analysis.

But anybody who's interested in direct ground wave analysis can also use
one of the other modeling programs which includes it, or use NEC-2
itself, available free from http://www.si-list.net/swindex.html. The
manual is available there also. NEC-2 produces field strength data
including the ground wave if requested with the proper entry on the RP
"card".

Roy Lewallen, W7EL

"Roy Lewallen" wrote
With EZNEC, you have to use the near field analysis to include
the ground wave; direct ground wave analysis isn't included in
EZNEC because (etc).
_________

After the comments of Richard Clark and you, Roy, I attempted to use
EZNEC to determine the ground wave (see link below).

The near-field analysis of EZNEC for radiation in the horizontal plane
at a point 1 km from a 1/4-wave monopole having two ohms in series
with a Mininec r-f ground, while radiating 1 kW over an earth
conductivity of 8 mS/m is shown as 72 mV/m.

The same setup when analyzed using the FCC's radiation efficiency for
this monopole height, and their propagation charts for these
conditions shows about 295 mV/m as the result, which value is
supported by the measured performance of real-world AM broadcast
stations, and is also a value in a range that could be expected from
the BL&E data.

Hopefully you or Richard Clark can tell me the reason(s) for this
difference, which could easily be my own setup of the NEC model.

Roy, would you mind posting the ground wave value EZNEC Pro reports
for these conditions?

http://i62.photobucket.com/albums/h8...FldExample.gif

RF

On Wed, 26 Nov 2008 11:41:04 -0800 (PST), Richard Fry
wrote:

Hopefully you or Richard Clark can tell me the reason(s) for this
difference, which could easily be my own setup of the NEC model.

I modeled their structures as they built them explicitly (they had
many variations), at the frequency they used, took readings at the
distance they reported. For your 1 kilometer distance (not one they
used), I get 303 mV/m at 3 MHz for their 70 foot radiator over a field
of 113 x 135 foot radials with an average ground conductivity. When I
use their distance of a mile, I get 188 mV/m, all else identical.
Their paper reports by formula that I should see 194.5 mV/m.

It would appear that with the average of the two distances, my model
accords quite closely to BL&E. That average would suggest results are
within an unreasonable accuracy given my experience with making RF
power determinations at the bench. However, my model is repeatable,
the paper is chiseled into the granite of history and your original
complaint seems to be moot.

73's
Richard Clark, KB7QHC

Earlier posts in this thread:

From: "Richard
Date: Wednesday, November 26, 2008 12:00 PM

Roy and others have answered this one in the past too. You employ the
near field table to observe the ground wave. It works approximately
well, even out to the edge of the implicit flat universe. If you
object to flat universes, you are no longer in the realm of ground
wave. If anything, modelers give MORE response in comparison to the
BL&T data.

Then I posted my result of using the near-field analysis of EZNEC
showing a value much LESS than the "modeler" value for those
conditions when using the BL&E data and the FCC curves.

Following that is posted:

From: "Richard Clark"
Date: Wednesday, November 26, 2008 3:32 PM

I modeled their structures as they built them explicitly (they had
many variations), at the frequency they used, took readings at the
distance they reported. For your 1 kilometer distance (not one they
used), I get 303 mV/m at 3 MHz for their 70 foot radiator over a field
of 113 x 135 foot radials with an average ground conductivity. When I
use their distance of a mile, I get 188 mV/m, all else identical.
Their paper reports by formula that I should see 194.5 mV/m. ...

No, the BL&E paper (accurately) stated that 194.5 mV/m is the
theoretical maximum field possible at 1 mile for 1 kW radiated by a
perfect 1/4-wave monopole over a perfect ground plane. The peak
values they measured came very close, but never quite achieved that
value.

It would appear that with the average of the two distances, my model
accords quite closely to BL&E.

Mr. Clark - kindly note that in your first quote above you say that,
if anything, "modelers" show MORE response than BL&E Then when
pressed a bit you say that your model "accords quite closely" with
BL&E.

Yet the results of my EZNEC near-field model showed considerably LESS
ground wave field at 1 km than either the FCC approach or the BL&E
data.

Clarifications, please?

RF

Richard Fry wrote:
. . .
No, the BL&E paper (accurately) stated that 194.5 mV/m is the
theoretical maximum field possible at 1 mile for 1 kW radiated by a
perfect 1/4-wave monopole over a perfect ground plane. The peak
values they measured came very close, but never quite achieved that
value.

Can you explain why they very nearly accomplished this perfect ground
value even though the ground wave signal had to propagate one mile over
ground of finite conductivity? What do you think would have happened to
the signal strength if the mile of intervening ground had been replaced
by a perfect ground?

It would appear that with the average of the two distances, my model
accords quite closely to BL&E.

Mr. Clark - kindly note that in your first quote above you say that,
if anything, "modelers" show MORE response than BL&E Then when
pressed a bit you say that your model "accords quite closely" with
BL&E.

Yet the results of my EZNEC near-field model showed considerably LESS
ground wave field at 1 km than either the FCC approach or the BL&E
data.

If you can answer the questions I asked above, you should understand why
EZNEC doesn't predict the same value as the obviously (to me) normalized
BL&E values. I'll look into the correspondence between EZNEC and FCC
predictions.

Roy Lewallen, W7EL

On Wed, 26 Nov 2008 14:49:06 -0800 (PST), Richard Fry
wrote:

When I
use their distance of a mile, I get 188 mV/m, all else identical.
Their paper reports by formula that I should see 194.5 mV/m. ...

No, the BL&E paper (accurately) stated that 194.5 mV/m is the
theoretical maximum field possible at 1 mile for 1 kW radiated by a
perfect 1/4-wave monopole over a perfect ground plane. The peak
values they measured came very close, but never quite achieved that
value.

No? No what? Is your rejection rhetorical? a dramatic conceit? Is
there some cognitive gap between "by formula" and "theoretical" you
are trying to mine? To what purpose?

Are you demanding an exact accounting between measured vs. modeled? If
so, my model comes within 2mV/m of their graphed data (which, in its
own right, does not mean they actually measured that particular
cardinal point but as it encompasses their explicitly stated variables
is tantalizingly close enough). Expectations of accuracy performed in
the field for a continuum of points (verging on 1%) for a fabricated
argument of more/less is seeking advantage where there is no salvation
to be found.

It would appear that with the average of the two distances, my model
accords quite closely to BL&E.

Mr. Clark - kindly note that in your first quote above you say that,
if anything, "modelers" show MORE response than BL&E Then when
pressed a bit you say that your model "accords quite closely" with
BL&E.

There is more than one model involved as described by BL&E. I
explicitly selected from one of several available - all of which I
have modeled. The model I describe conforms to far more of their
variables available than those expressed by you. It also exhibited
more response than your 1kM touchstone. Is this touchstone derived
from BL&E or some other source unknown to all here, but you? It seems
when I followed your offering, you want to challenge its authority.

Those two data points I offer exhibit variations of barely a quarter
dB about the touchstones you supply (one available from BL&E), and
which you fall considerably short of in your own effort. Their
average around these touchstones average is an amazingly small
difference. The difference between the model I selected, and the one
they report (one in the same) is on order of 0.1dB. If this does not
constitute an accord, then I would suggest you have more water to
carry than myself to turn modeling results into congruency. I am not
particularly motivated to improve things when my experience suggests
that it is a fool's mission given it implies accuracies that were
beyond what was achievable in that cold winter field, 70 odd years
ago.

Yet the results of my EZNEC near-field model showed considerably LESS
ground wave field at 1 km than either the FCC approach or the BL&E
data.

Clarifications, please?

You don't provide enough detail of your model to be able to point to
anything in error, but by the multitude of your statements, it doesn't
sound like you have spent enough time in the practice of modeling. The
rest of my discussion below hardly reveals anything beyond the obvious
- for one versed in the craft.

My models were arrived at through the simple, but tedious craft of
close reading and conforming to expressed facts in the literature.
Some art was involved in the selection from a choice of grounds, for
which such choice drives a wide variation of results. Does this sound
familiar? Even there, calling it art denies the information supplied
by photographs revealing a very commonplace description: Pastoral. My
choice of ground characteristics, if anything, hardly exhibits a
radical departure. In fact I choose no other ground than average for
the vast majority of my modeling. Within the confines of the
abilities of the model to support buried wire, that was performed by
suggestions offered in the help manual (clarity is achieved in reading
that too and is generally obtained in the course of considerable
exposure to the toolset). Here, the radials hovered less than half an
inch above ground instead of buried six inches beneath. Perhaps this
explains the remaining 0.1dB variation, but I doubt it. To infer such
tight coupling between model and measure is a fantasy only Art would
embrace to prove we can't trust established theory.

73's
Richard Clark, KB7QHC

Richard Fry wrote:
"Roy Lewallen" wrote
With EZNEC, you have to use the near field analysis to include
the ground wave; direct ground wave analysis isn't included in
EZNEC because (etc).
_________

After the comments of Richard Clark and you, Roy, I attempted to use
EZNEC to determine the ground wave (see link below).

The near-field analysis of EZNEC for radiation in the horizontal plane
at a point 1 km from a 1/4-wave monopole having two ohms in series
with a Mininec r-f ground, while radiating 1 kW over an earth
conductivity of 8 mS/m is shown as 72 mV/m.

The same setup when analyzed using the FCC's radiation efficiency for
this monopole height, and their propagation charts for these
conditions shows about 295 mV/m as the result, which value is
supported by the measured performance of real-world AM broadcast
stations, and is also a value in a range that could be expected from
the BL&E data.

Hopefully you or Richard Clark can tell me the reason(s) for this
difference, which could easily be my own setup of the NEC model.

Roy, would you mind posting the ground wave value EZNEC Pro reports
for these conditions?

http://i62.photobucket.com/albums/h8...FldExample.gif

RF

My model has 120 0.5 wavelength radials buried 1.2 feet deep (the
unusual depth due to rescaling another model). Ground conductivity 8
mS/m, dielectric constant 13. The antenna is 0.25 wavelength high. The
whole structure is made from #12 wire to eliminate any problems due to
dissimilar diameters. Field strength is Ez at 1000 meters with 1000
watts applied.

Using the NEC-4D calculating engine, EZNEC Pro/4 shows (NF = near field
analysis, GW = far field analysis with ground wave):

Z = 40.08 + j27.91
GW = 297.7 mV/m
NF = 297.7 mV/m

Same, but with 0.25 wavelength radials:

Z = 39.56 + j26.55
GW = 292.7 mV/m
NF = 292.7 mV/m

Note that the feedpoint R and field strength don't exactly correlate if
you make the assumption that the resistance difference is due to loss.
This would be due to a slightly different current distribution on the
radiator due to interaction with the different ground fields. Other
experiments have shown that the impedance will also vary some with
radial burial depth.

Following are the results using the NEC-2D engine with a 0.25 wavelength
vertical and 120 0.5 wavelength radials one foot above the ground, all
other conditions otherwise the same. This analysis can be run with
EZNEC+, but only the NF results will be available:

Z = 66.83 + j1.894
GW = 230.0
NF = 229.7

As above, but 0.25 wavelength radials:

Z = 32.42 + j18.87
GW = 311.4
NF = 311.4

Elevated radials, even when elevated only this amount, show distinct
resonance effects, and making them longer than about 0.25 wavelength
often results in reduced efficiency which I think is due to movement of
the radial current maxima away from the center. The above results
illustrate these phenomena. While slightly elevated radials can be used
to approximate buried ones, as you can see the substitution isn't perfect.

The same 0.25 wavelength vertical over perfectly conducting (or MININEC)
ground showed a Z of 37.95 + j21.49 ohms. However, the resistances of
the various examples above aren't just this resistance plus loss
resistance, since the current distribution isn't quite the same when
radials are present.

The results you got weren't valid due to use of MININEC ground with near
field analysis, as I explained in another posting. As you can see, you
can get reasonably good results using EZNEC+ and near field analysis,
although the vast majority of people this intensely interested in the
mechanisms of AM broadcasting aren't hobbyists but rather professional
engineers who are using EZNEC Pro/4.

Roy Lewallen, W7EL

On Nov 29, 1:10*pm, Roy Lewallen wrote:

As you can see, you can get reasonably good results using
EZNEC+ and near field analysis, although the vast majority
of people this intensely interested in the mechanisms of AM
broadcasting aren't hobbyists but rather professional
engineers who are using EZNEC Pro/4.
________

Thanks very much for your numbers and comments, Roy.

I would never have thought to try to use EZNEC near-field analysis to
compute the groundwave if I hadn't read the suggestion to do so in
this thread. That was my first, and will be my last attempt at that.

When I need to calculate the MW ground wave for a particular distance,
monopole height, frequency and ground conductivity I use the FCC
method of first determining the inverse distance field of the radiator
at 1 km for 1 kW of radiated power, and then using that value in a
program I have with the FCC's MW propagation curves in digitized form.

My point when starting this thread was to show that the elevation
pattern radiation actually launched by vertical monopoles on any
frequency does not have a zero/very low relative amplitude at/near the
horizontal plane, which from what I read on these NGs seems to be a
popular belief.

RF