"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

"Roy Lewallen" wrote
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?
___________________

BL&E made their surface-wave measurements 3/10 of a mile from their 3
MHz monopole transmit site.

MW ground loss for the surface wave across a path that short is low,
regardless of ground conductivity. This may be seen in the scan
linked below, which was taken from Terman's Radio Engineers Handbook,
1st Edition, page 681.

The scan doesn't show distances less than 1 mile, and the curves are
based on higher ground conductivity than BL&E had to work with -- but
an extrapolation of those curves to the BL&E conditions should
convince most reasonable readers of the conclusion in my paragraph
above.

The BL&E paper published in the Proceedings of the IRE states (page
771) "For each antenna height, 0.2 watt of power was fed into this
antenna, and the field intensity was measured at 0.3 of a mile. This
figure was then converted to a basis of a power of 1000 watts and a
distance of one mile."

So BL&E did not normalize their readings to account for ground loss
either at 3/10ths of a mile or one mile, but apparently they did
assume that the effect of the ground loss was the same at those two
distances. That error would not be large, however.

What do you think would have happened to the signal strength
if the mile of intervening ground had been replaced by a perfect
ground?

They would have measured 194.5 mV/m, referenced to 1 kW of radiated
power. As it was, they reported about 191 mV/m (max).

I'll look into the correspondence between EZNEC and FCC
predictions.

Thanks. That will be interesting.

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

RF

RC: Their paper reports by formula that I should see 194.5 mV/m. ...

RC: No? No what?

RF: 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.

RC: 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).

If you are happy with the results of your modeling, then well and good
for you.

But the near-field value calculated by EZNEC and as shown in my URL is
far short of the result of the BL&E study, and also of the FCC's
propagation curve value for those conditions.

So far neither you nor Roy has suggested that the near-field analysis
I posted was based on an incorrect model, and there was enough
information about it in my clip to determine that.

BTW, a distance of 1 km from a 1 MHz, 1/4-wave monopole is no longer
in its near field, the boundary of which in this case is less than 150
feet from the monopole.

RF

On Thu, 27 Nov 2008 05:34:56 -0800 (PST), Richard Fry
wrote:

So far neither you nor Roy has suggested that the near-field analysis
I posted was based on an incorrect model, and there was enough
information about it in my clip to determine that.

Ah, we are working from a script. I am suggesting that your model is
incorrect. You asked what to do, I would suggest fixing it. You have
been provided with the necessary references. Let us know when you
succeed, but skip reports of failure. As you say, you have already
provided enough information.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
I am suggesting that your model is incorrect.
__________

And I am suggesting that your model is incorrect.

Please show your work to the same extent that I showed mine (or more,
if you believe that to be necessary).

Otherwise all we have from you about this is undocumented.

RF

On Thu, 27 Nov 2008 11:39:07 -0800 (PST), Richard Fry
wrote:

And I am suggesting that your model is incorrect.

You have the cogent characteristics of my model, now demonstrate your
suggestion by showing its incorrect feature(s). It may even reveal
how you failed to obtain better results for your own model.

73's
Richard Clark, KB7QHC