Unfortunate I have only a NEC 2 based program (Nittany Scientific's NEC-Win
Pro), but with the Sommerfeld/Norton ground model you can approach the
ground to within 1/1000 of a wavelength closely approximating the results of
buried wires, and in very close agreement with your "RADIALS2" program. NEC
programs do require a value for ground permittivity. NEC 4 based software
is more expensive, being in the $800.00 range, plus a $500.00 license from
the Lawrence Livermore Laboratory. In any case the data are easily compared
with your program results.

I have measured my ground conductivity, but not yet attempted to measure the
permittivity.

If anybody wants to get serious with antenna modeling I recommend Ansoft's
HFSS (Often known as "Highly Frustrating Structure Simulator"). It costs a
mere $30,000, with a $10,000 per year support payment.

Regards,

Frank


"Reg Edwards" wrote in message
...
Frank wrote -
I have also used your software for modeling verticals, and it is
in very close to the results produced by NEC. The one problem with NEC 2
(Though not with NEC 4) is that it cannot model buried radials, but can
get
very close to the ground.

================================

The only program I am reasonably familiar with is the several years old
free
EZNEC. I don't know whether it has been updated or not and I make very
little use of it. Come to think of it, I don't make much use of my own
programs either.

Regarding shallow buried radials in conjunction with a vertical, have you
tried my recent program RADIALS2 ?

It is intended to demonstrate performance of the radials themselves in a
given ground rather than antenna performance. Which I suspect is the
reverse
of NEC-4.

As you probably know, the effects of above-ground radials change very
rapidly as they get within a few inches of the ground surface. But once in
the ground they tend to remain static.

RADIALS2 uses an entirely different, unconventional form of performance
analysis. If other programs don't take soil permittivity into account at
HF,
predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a
form suitable for a direct comparison with my simple program?

But in view of the large uncertainties involving ground conditions,
accuracy
is not worth making much of a song and dance about.
----
Reg, G4FGQ

Reg Edwards wrote:

The only program I am reasonably familiar with is the several years old free
EZNEC. I don't know whether it has been updated or not and I make very
little use of it. Come to think of it, I don't make much use of my own
programs either.

Regarding shallow buried radials in conjunction with a vertical, have you
tried my recent program RADIALS2 ?

It is intended to demonstrate performance of the radials themselves in a
given ground rather than antenna performance. Which I suspect is the reverse
of NEC-4.

As you probably know, the effects of above-ground radials change very
rapidly as they get within a few inches of the ground surface. But once in
the ground they tend to remain static.

RADIALS2 uses an entirely different, unconventional form of performance
analysis. If other programs don't take soil permittivity into account at HF,
predictions must lose accuracy. Are the inputs and outputs of NEC-4 in a
form suitable for a direct comparison with my simple program?

Yes. I made a few comparisons long ago, shortly after you introduced
your program, and found major disagreement. NEC-4 approximately agrees
with the measurements made long ago by Brown, Lewis, and Epstein (whom I
know you've never heard of), once you make reasonable assumptions of
ground conductivity and dielectric constant. Your program gives very
different answers. At the time, I concluded that there's considerable
coupling between radials, which your program doesn't seem to account
for. Interested readers should look in the google archives for postings
in this group on the thread "Ground Radials" in July 1998 and
"Evaluation of G4FGQ Freeware Antenna Software" in September 1998.


But in view of the large uncertainties involving ground conditions, accuracy
is not worth making much of a song and dance about.

True, but in the past, you've used the results from your program to
reach conclusions about radial systems that I didn't, and don't, believe
to be valid. (See the threads mentioned above.) I don't think it's wise
to draw conclusions from a program that gives results which are
demonstrably very different from the only measurements regarded to be
reasonably well made.

Roy Lewallen, W7EL

"Frank" wrote -
I cannot claim to have done anything requiring much thought. I
just considered, since so much discussion is centered on current
distribution, that some might be interested in the posted curves. So far
yours is the only response.

==================================

That's because your graph embarasses that half of the contestants who insist
the coil's input and output currents are identical and so the less said
about it the better.

And it gratifies the exhausted sentiments of the other half who are
unwilling to grant you the credit for providing the convincing evidence
after all the hard work they have put into stating the bleeding obvious. And
still are.

(After John Cleese who joked about the value of the distinction of a
first-class university honours degree in stating the bleeding obvious.)

Neither was there was any response to my brief statement of 4 Facts except
yours. And for similar reasons. I did not expect any response. But after
all, any unsupported statement of mine (no Terman, no Kirchoff, no Kraus, no
Thevenin, no Balony) can hardly be construed as evidence of anything.

smileysmiley

----
Reg, G4FGQ

Frank wrote:
So far yours is the only response.

Hi Frank, your results look reasonable and thanks for your input.

I was also puzzled by
the slight increase in current just under the loading coil, but suspect it
was caused by coupling between the lower conductor and the base of the coil.

A number of us have reported the same thing. In my EZNEC octcoil.ez simulation,
(available from my web page below) the net current decreased by 2% from the feedpoint
to the coil and then in the first part of the bottom of the coil, it increased by 5%.
(Whoops, I almost said it "dropped" by 2% and "rose" by 5% which would have elicited
endless nitpicking. :-)

There is, no doubt, an impedance discontinuity at each end of the coil. The
net current standing-wave values at each end of the coil that differ from just
a straight wire are probably due to interference effects among the various
forward and reflected wave components. A conceptual idea of what is happening
might be the following solvable example (S-parameter analysis).

a1-- | --a2
--b1 | b2--

-----Z1-----+-----Z2-----+-----Z3-----open

bottom coil top
wire wire

The Z1 to Z2 impedance discontinuity will cause reflections in both directions.

b1 = s11*a1 + s12*a2 b2 = s21*a1 + s22*a2

The Z2 to Z3 impedance discontinuity will also cause reflections in both directions.

same equations as above with different parameters

The net current at the bottom and top of the coil cannot be expected to be the
same as in a straight piece of wire with no step functions in the impedance.
--
73, Cecil http://www.qsl.net/w5dxp


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On Sun, 07 Nov 2004 04:31:00 GMT, "Frank"
wrote:


Hi Frank,

|Thanks for your comments Reg. The fact is my graph is produced in Excel
|from data provided by a NEC 2 output file generated by few trivial lines of
|NEC code. I cannot claim to have done anything requiring much thought. I
|just considered, since so much discussion is centered on current
|distribution, that some might be interested in the posted curves. So far
|yours is the only response. I will probably delete the page in a day or so.
|The loading inductor is 2.5" diameter, 6" long, with turns spaced at 0.5".
|The NEC code is listed on the site, so anybody can copy to verify the
|validity of my results -- or the validity of the code.

I haven't run your code, but I did something similar, and announced
the results here, almost a year ago. Perhaps that explains the lack of
response.

http://www.qsl.net/n7ws/Loaded%20antennas.htm

or in downloadable form:

http://www.qsl.net/n7ws/Loaded_Antennas.pdf

the antenna files are he

http://www.qsl.net/n7ws/AntennaModels.zip

Regards,

Wes

Wes Stewart wrote:
I haven't run your code, but I did something similar, and announced
the results here, almost a year ago. Perhaps that explains the lack of
response.

http://www.qsl.net/n7ws/Loaded%20antennas.htm

Yet many keep insisting that the net currents at each end of a
loading coil are the same magnitude.
--
73, Cecil http://www.qsl.net/w5dxp


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"Wes Stewart" wrote in message
...
On Sun, 07 Nov 2004 04:31:00 GMT, "Frank"
wrote:


Hi Frank,

|Thanks for your comments Reg. The fact is my graph is produced in Excel
......
|...........validity of my results -- or the validity of the code.

I haven't run your code, but I did something similar, and announced
the results here, almost a year ago. Perhaps that explains the lack of
response.

http://www.qsl.net/n7ws/Loaded%20antennas.htm

or in downloadable form:

http://www.qsl.net/n7ws/Loaded_Antennas.pdf

the antenna files are he

http://www.qsl.net/n7ws/AntennaModels.zip

Regards,

Wes

Thanks for the information Wes. I have only recently noticed the loading
coil discussions going on, so guess I missed a lot of the earlier postings.
Your curves are very similar to mine, although I plotted only those currents
within the loading coil, that are spaced by the winding separation (Which
happens to be the same as the overall segmentation). The far field is
proportional to the portion of the current (within the helix) in the "z"
direction, and assume that the x and y components are cancelled out. Such
plotting seems to be supported by computing the integral of I(z)dz -- while
realizing the limitation of such expressions.

I just tried to run your EZNEC files, but cannot since I only have the demo
program. Will have to figure out a way to extract the NEC code.

The qsl.net site is incredibly slow, I had to make a couple of attempts to
download your files.

Regards,

Frank

Reg Edwards wrote:
The input and output currents of a loading coil of finite length are always
different from each other.

There's hardly any "input and output currents" for a loading coil in
a standing wave antenna. That concept is why W8JI is in trouble with
his explanations. What is actually being measured is the magnitude
of the standing current wave.

There is forward current flowing into the bottom of the coil and out
the top. There is reflected current flowing into the top of the coil
and out the bottom. The net current is a standing current wave. If we,
as Kraus suggests, assume that the forward current equals the reflected
current (relatively small error in doing so) then there is zero net current
flowing in and out of the coil. The standing wave current is, well, just
standing there and is not "going" anywhere.

The gross error that a lot of people are making is that standing
wave current flows. If the forward and reflected currents are equal,
as Kraus assumes for purpose of discussion, then there is zero net
current flow through the coil. Yet, net current is what everyone
is measuring. What they are actually measuring is the value of
the standing wave current at each end of the coil and it is not
flowing. It is only an artifact of the superposition of the two
waves that are flowing.

The magnitude of the reflected current can be estimated from the
feedpoint impedance. The lower the feedpoint impedance, the closer
in magnitude is the reflected current to the forward current. For
a center-loaded mobile antenna, the reflected current through the
coil appears to be well within 5% of the value of the forward
current.
--
73, Cecil http://www.qsl.net/w5dxp


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W5DXP wrote:
There is forward current flowing into the bottom of the coil and out
the top. There is reflected current flowing into the top of the coil
and out the bottom. The net current is a standing current wave.

In view of the above, for practical putposes, trying to get maximum performance
out of the loaded radiator, it should be beneficial to have the same diameter
of whip above the coil, rather than tapering whip?
One might deduct that if the current is diminishing towards the top, that the
diameter of the radiator (RF resistance) could be tapered also. But since the
RF current has to flow to the tip and then reflect and go back and interfere
with itself, we should make it uniform, where possible.
We are probably talking about fraction of a peanut, but for the purists and
sake of argument.

Yuri, K3BU.us

Yuri Blanarovich wrote:

W5DXP wrote:
There is forward current flowing into the bottom of the coil and out
the top. There is reflected current flowing into the top of the coil
and out the bottom. The net current is a standing current wave.

In view of the above, for practical putposes, trying to get maximum performance
out of the loaded radiator, it should be beneficial to have the same diameter
of whip above the coil, rather than tapering whip?

That doesn't help much. What helps is a humongous top hat which does
help to equalize the current at the bottom of the coil and the current
at the top of the coil.

One might deduct that if the current is diminishing towards the top, that the
diameter of the radiator (RF resistance) could be tapered also. But since the
RF current has to flow to the tip and then reflect and go back and interfere
with itself, we should make it uniform, where possible.
We are probably talking about fraction of a peanut, but for the purists and
sake of argument.

Nope, not a fraction of a peanut at all - more like 100% more radiated
power because of that humongous top hat. I'm considering a humongous top
hat for my pickup that runs the length of the truck and is mounted about
a foot above the cab - maybe 50 square feet in all. That would minimize
the size of the loading coil and ensure maximum current in the bottom
section. The maximum height above ground would be about seven feet, a
definite advantage for a mobile antenna around here.
--
73, Cecil http://www.qsl.net/w5dxp


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