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Old November 22nd 04, 11:08 PM
Frank
 
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"Richard Clark" wrote in message
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On Mon, 22 Nov 2004 01:52:00 GMT, "Frank"
wrote:
Note that a 140 ft dipole resonates at 3.34 MHz.


Hi Frank,

I also note that the "Cobra" doubles the number of resonances across
the 2 to 30 MHz spectrum with the resonances coming in adjacent pairs.
Our sometime correspondent, Dr. Slick, thinks he invented this
characteristic with his mystery, garden variety dipole; but it is a
simple matter of putting a bend in the wire.

This one manipulation was also the foundation of the "fractal"
antenna, but "scientists" gushed on with baroque explanations and
hyperbolic claims to disguise a rather mundane oddity.

73's
Richard Clark, KB7QHC


Hi Richard:

Here, below, are the data from a 140 ft "Cobra" type antenna compared with a
140 ft dipole. I modeled #24 AWG, spaced 1", with 1" segments. My
previous comment about 2000 segments maximum was incorrect, and NEC can go
to 10,000 segments. As it is, the model contained over 5,000 segments, and
took almost 2 hours to run. The NEC output file contained about 250,000
lines of data in a 14.5 MB text file. While the 1" segmentation is well
below the minimum lambda/1000 recommended by the NEC manual, a transmission
line with similar dimensions gave results very close to the expected value
of 550 Ohms. These results could therefore be considered as a reasonable
validation of the antenna model. Not sure why EZNEC cannot deal with such a
model.

From an operational point of view there seems to be very little difference
between a 140 ft "Cobra" antenna, and a 140 ft dipole. On some frequencies
it is noted that the Cobra exhibits a lower efficiency.

The data are noticeably different from your results (note that at 2 MHz both
our results show an imaginary part of 0), but probably due to the large
difference in dimensions. I was not able to verify the number of resonances
due to the very long run times. If I had attempted 0.2 MHz steps the
program would have run for 8 to 10 hours and generated over a million lines
of output data. I think what I need is 64 bit dual processor system to
tackle such a problem.

Most of the claims I have read on e-ham.net can be considered as
unverifiable nonsense. I certainly agree with your comments on the design.
As for "fractal" antennas, I do not really know anything about them, so
cannot comment.

One thing that does come out of this modeling is that an antenna does not
have to be very much longer than a 1/4 wave long to provide reasonable input
impedances.

Incidentally my model was run in free space, so have no data on take-off
angle. I doubt it would be very different.

73,

Frank

Cobra
140 ft Dipole

Frequency Real(Z) Imag(Z) Efficiency Real(Z)
Imag(Z) Efficiency
(MHz) (Ohms) (Ohms) (%) (Ohms)
(Ohms) ( %)

2 71.216 201.878 36
18 -737.554 98

4 140.456 810.61 66 125.32
272.626 99

5 511.886 -23.295 92 359.372
872.604 99.6

7 1889.15 -1965.46 98
2670.88 -2355.48 99.6

10 137.527 -758.652 46
90.313 -165.43 99

14 1465.88 -1284.19 97
2158.99 -1485.13 99.6

18 233.57 679.892 65 180.16
224.359 99.2

21 1272.82 -1009.83 97
1926.04 -1113.17 99.5

24 217.887 -864.421 36
120.373 -159.68 98.9

28 1155.79 -852.805 97
1783.27 -886.08 99.5


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Old November 23rd 04, 01:00 AM
Richard Clark
 
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On Mon, 22 Nov 2004 23:08:32 GMT, "Frank"
wrote:

Hi Richard:

Here, below, are the data from a 140 ft "Cobra" type antenna compared with a
140 ft dipole. I modeled #24 AWG, spaced 1", with 1" segments. My
previous comment about 2000 segments maximum was incorrect, and NEC can go
to 10,000 segments. As it is, the model contained over 5,000 segments, and
took almost 2 hours to run. The NEC output file contained about 250,000
lines of data in a 14.5 MB text file. While the 1" segmentation is well
below the minimum lambda/1000 recommended by the NEC manual, a transmission
line with similar dimensions gave results very close to the expected value
of 550 Ohms. These results could therefore be considered as a reasonable
validation of the antenna model. Not sure why EZNEC cannot deal with such a
model.


Hi Frank,

Well, your segmentation at one inch intervals over 420 feet of wire
seems rather over-the-top, but I suppose you arrived at that through
your transmission line modeling. Perhaps I should attempt that myself
in my own TL models.

EZNEC, itself, offers segmentation and geometry tests and where there
are issues it will remedy them itself. For this turn in the road I am
simply the loose nut behind the steering wheel. The first pass I
offered here (that brought EZNEC to its knees) consisted of only 263
segments. I then spread the wires out to accommodate those issues
EZNEC reported but would not correct of its own volition.

Were your wires as close as conventional 3-wire zip cord?

From an operational point of view there seems to be very little difference
between a 140 ft "Cobra" antenna, and a 140 ft dipole. On some frequencies
it is noted that the Cobra exhibits a lower efficiency.


No doubt from one proximal wire bucking the other.

The data are noticeably different from your results (note that at 2 MHz both
our results show an imaginary part of 0), but probably due to the large
difference in dimensions.


I would offer more likely due to nearby embrace of earth.

I was not able to verify the number of resonances
due to the very long run times. If I had attempted 0.2 MHz steps the
program would have run for 8 to 10 hours and generated over a million lines
of output data. I think what I need is 64 bit dual processor system to
tackle such a problem.

Most of the claims I have read on e-ham.net can be considered as
unverifiable nonsense. I certainly agree with your comments on the design.
As for "fractal" antennas, I do not really know anything about them, so
cannot comment.


Consult:
http://www.qsl.net/kb7qhc/antenna/fr...och/review.htm
for one small sample from many hundred pages of results.

One thing that does come out of this modeling is that an antenna does not
have to be very much longer than a 1/4 wave long to provide reasonable input
impedances.

Incidentally my model was run in free space, so have no data on take-off
angle. I doubt it would be very different.

73,

Frank

Cobra 140 ft Dipole

Frequency Real(Z) Imag(Z) Efficiency Real(Z) Imag(Z) Efficiency
(MHz) (Ohms) (Ohms) (%) (Ohms) (Ohms) ( %)

2 71.216 201.878 36 18 -737.554 98

mine 12 +j550 Ohms
14 1465.88 -1284.19 97 2158.99 -1485.13 99.6

mine 440 -j610
21 1272.82 -1009.83 97 1926.04 -1113.17 99.5

mine 420 -j520

It appears that your reader, Outlook Express (a piece of software crap
by the way) is line length limited. Comparisons were difficult at
best so I confined them to those above. I was going to do a
side-by-side comparison with the same 140' dipole, but it was
unremarkable when earth dominates the drive point Z.

Another explanation for variations may be simply found in the high
degree of Z variation across frequency, we do agree on the imaginary
sign and the values do track in the higher bands.

73's
Richard Clark, KB7QHC
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Old November 23rd 04, 05:14 AM
Frank
 
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Hi Frank,

Well, your segmentation at one inch intervals over 420 feet of wire
seems rather over-the-top, but I suppose you arrived at that through
your transmission line modeling. Perhaps I should attempt that myself
in my own TL models.


You are correct Richard -- It appears my segmentation is seriously "over the
top". The segmentation was driven by not wanting a segment to be longer
than the spacing. Some texts on NEC do recomend such constraints. So far I
have gone to 16" segments, with 1" spacing. For the transmission line model
it shows even better convergence. #24 AWG spaced by 1" has a characterestic
impedance of 550 Ohm. My transmission line model is 100 ft long with 75
segments in each side. It would be interesting to know if EZNEC would
produce the same result.

EZNEC, itself, offers segmentation and geometry tests and where there
are issues it will remedy them itself. For this turn in the road I am
simply the loose nut behind the steering wheel. The first pass I
offered here (that brought EZNEC to its knees) consisted of only 263
segments. I then spread the wires out to accommodate those issues
EZNEC reported but would not correct of its own volition.


Were your wires as close as conventional 3-wire zip cord?


No, as explained above they are #24 spaced by 1". Possibly I missed
something there. Is the antenna made from simple 3-wire zip cord?
Unfortunately NEC cannot model insulated wire, so not sure how that would
effect the model. Is the VF very significant?


From an operational point of view there seems to be very little difference
between a 140 ft "Cobra" antenna, and a 140 ft dipole. On some
frequencies
it is noted that the Cobra exhibits a lower efficiency.


No doubt from one proximal wire bucking the other.


That's what I figured.

The data are noticeably different from your results (note that at 2 MHz
both
our results show an imaginary part of 0), but probably due to the large
difference in dimensions.


I would offer more likely due to nearby embrace of earth.


Very likely. How high did you place your model? Did you use the
Sommerfeld/Norton grouind model? Probably not really a factor at 30 ft, or
more, above ground.


Consult:
http://www.qsl.net/kb7qhc/antenna/fr...och/review.htm
for one small sample from many hundred pages of results.

One thing that does come out of this modeling is that an antenna does not
have to be very much longer than a 1/4 wave long to provide reasonable
input
impedances.

Incidentally my model was run in free space, so have no data on take-off
angle. I doubt it would be very different.

73,

Frank

Cobra
140 ft Dipole

Frequency Real(Z) Imag(Z) Efficiency Real(Z) Imag(Z)
Efficiency
(MHz) (Ohms) (Ohms) (%) (Ohms) (Ohms)
( %)

2 71.216 201.878 36
18 -737.554 98

mine 12 +j550 Ohms
14 1465.88 -1284.19 97
2158.99 -1485.13 99.6

mine 440 -j610
21 1272.82 -1009.83 97
1926.04 -1113.17 99.5

mine 420 -j520

It appears that your reader, Outlook Express (a piece of software crap
by the way) is line length limited.


It sure is!

Comparisons were difficult at
best so I confined them to those above. I was going to do a
side-by-side comparison with the same 140' dipole, but it was
unremarkable when earth dominates the drive point Z.

Another explanation for variations may be simply found in the high
degree of Z variation across frequency, we do agree on the imaginary
sign and the values do track in the higher bands.

73's
Richard Clark, KB7QHC


Now I have gotten out of my segmentation fixation I may be able to run some
more realistic models.

73,

Frank


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Old November 23rd 04, 06:47 AM
Richard Clark
 
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On Tue, 23 Nov 2004 05:14:14 GMT, "Frank"
wrote:

My transmission line model is 100 ft long with 75
segments in each side. It would be interesting to know if EZNEC would
produce the same result.


Hi Frank,

I will try to fill that request. If not soon, eventually.

Were your wires as close as conventional 3-wire zip cord?


No, as explained above they are #24 spaced by 1". Possibly I missed
something there. Is the antenna made from simple 3-wire zip cord?


Yes, by inference from close-up photography at the web site.

Unfortunately NEC cannot model insulated wire, so not sure how that would
effect the model. Is the VF very significant?


EZNEC can accommodate insulation, but I rarely fine tune to that
degree when I am looking at the panorama of SWR sweeps. Really, this
will only nudge things in the single digit percentage range and such
things are swamped from one Ham's situation to another's.

I would offer more likely due to nearby embrace of earth.


Very likely. How high did you place your model? Did you use the
Sommerfeld/Norton grouind model? Probably not really a factor at 30 ft, or
more, above ground.


I placed it at the reported height, 35 feet, that raised the signal
levels 2 S-Units over a similarly mounted G5RV. ;-) I choose one of
two ground models when I work on these things: Perfect, or average,
high accuracy lossy. That height seems to be a commonly reported
value with those who put up wire antennas and have no tower to work
from, so it works for me as an "Everyman's" best.

73's
Richard Clark, KB7QHC
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Old November 23rd 04, 08:04 AM
Richard Clark
 
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On Tue, 23 Nov 2004 06:47:32 GMT, Richard Clark
wrote:

On Tue, 23 Nov 2004 05:14:14 GMT, "Frank"
wrote:

My transmission line model is 100 ft long with 75
segments in each side. It would be interesting to know if EZNEC would
produce the same result.


Hi Frank,

I will try to fill that request. If not soon, eventually.


Hi Frank,

I gave this several passes with various segmentations.

The best, flattest response appeared at 400 (total) segments over a
10M length of line of #24 separated by 1". This resulted in a
characteristic 575 Ohm line with a 1.045:1 SWR ripple over the 2 MHz
to 30 MHz band.

When I doubled and then tripled the segments, the low end got a little
whacked out (getting worse as the segments went up).

I was working from many segments down, so I did not lower the count to
your longer segments - maybe tomorrow, I was paying attention to your
(and EZNEC's) advice about segment length equaling line separation.

73's
Richard Clark, KB7QHC


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Old November 24th 04, 03:49 AM
Frank
 
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Hi Frank,

I gave this several passes with various segmentations.

The best, flattest response appeared at 400 (total) segments over a
10M length of line of #24 separated by 1". This resulted in a
characteristic 575 Ohm line with a 1.045:1 SWR ripple over the 2 MHz
to 30 MHz band.

When I doubled and then tripled the segments, the low end got a little
whacked out (getting worse as the segments went up).

I was working from many segments down, so I did not lower the count to
your longer segments - maybe tomorrow, I was paying attention to your
(and EZNEC's) advice about segment length equaling line separation.

73's
Richard Clark, KB7QHC


Hi Richard:

Found some three conductor zipcord and measured its dimensions: wire,
stranded, #14 AWG, center to center spacing 3.5mm (0.138"). Experimented
with a 100ft NEC transmission line model, with various segmentations from 4"
to 16". Characteristic impedance agreed with :- 276*log(2*S/d) at 171
Ohms, where S is the center to center wire spacing, and d is the wire
diameter. From your comments it seems EZNEC is in close agreement with my
NEC program.

Applied the above dimension to a 140 ft "Cobra" type antenna at 35 ft above
an average ground of: sigma 5 mS/m, and Er = 13. The antenna was segmented
at 6". Hope the formatting of the following results is not totally
destroyed by Microsoft's Outlook Express.

Cobra Antenna:

Freq Re Im S Eff.
(MHz) (450) (%)
2 29.8 93.8 15.7 66.3
3.8 109 637 12.6 83.4
5 251 58 1.8 95.4
7.2 999 -1381 6.8 99.0
10 516 1850 16.7 68
14.2 1287 -989 4.1 98.9
18 315 861 7.2 81.3
21.2 1096 -915 3.3 98.6
25 301 672 5.3 82.7
28.6 669 -712 3.6 98.2


140 ft Dipole compared

Freq Re Im S Eff.
(MHz) (450) (%)
2 8.8 -856 235 66.3
3.8 79 238 7.3 83.3
5 361 1127 9.8 95.4
7.2 1959 -2379 13 99
10 111 -195 4.9 68
14.2 2581 -2020 9.3 98.9
18 181 251 3.4 81.3
21.2 2225 -1871 8.5 98.6
25 188 233 3.1 82.7
28.6 1252 -1577 7.4 98.2

If the NEC models are correct there does not seem to be a lot of difference
between the "Cobra" and a 140 ft dipole. Interesting to note that the
Imaginary part of Zin, at 2 MHz is still 0. With 100 ft of 450 Ohm line on
2 MHz the loss is only 0.12 dB compared with 1.5 dB on the regular dipole.


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Old November 24th 04, 05:00 AM
Richard Clark
 
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On Wed, 24 Nov 2004 03:49:20 GMT, "Frank"
wrote:
If the NEC models are correct there does not seem to be a lot of difference
between the "Cobra" and a 140 ft dipole.


Hi Frank,

So it would seem.

Hi Bert,

You asked for comments, but you've been quite silent yourself. Would
you think that the extra wire was worth the $90? Would you think you
could trust testimonials, especially those claiming 2 S-Units gain
over a G5RV?

73's
Richard Clark, KB7QHC
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