Opinions please. :-)

http://www.k1jek.com/

It got pretty rave reviews on eHam.

http://www.eham.net/reviews/detail/3...9a7885c56da2cc

--
Vy 73 de Bert
WA2SI
FISTS #9384
QRP ARCI #11782

On Sun, 21 Nov 2004 15:39:31 GMT, "Bert Craig"
wrote:
Opinions please.

Hi Bert,

When I read a testimonial like:
In direct comparisons with a G5RV at the same height,
I am constantly receiving 3 S-Units better when using the Cobra.
I have to wonder about the source.

$90 for zipcord?

Well, opinion aside, I modeled this (each arm is a single wire, folded
back on itself twice) and when I tried to run a SWR sweep in EZNEC
from 2MHz to 30MHz in 0.1MHz steps, EZNEC stalled and gave up.

I suppose this may have been due to warnings of wires being too close.
As it was, I pulled the zip wires apart 1.2" (0.1'). EZNEC wanted a
minimum 6". I tried again with 1MHz steps, and the full sweep drew a
line across the top of the chart (max SWR). Hoping for more
resolution, I tried with 0.2MHz steps, and EZNEC stalled again. Third
time is charm (so the saying goes), and with 0.25MHz steps - it
stalled again.

-sigh-

73's
Richard Clark, KB7QHC

Hi Bert,

When I read a testimonial like:
In direct comparisons with a G5RV at the same height,
I am constantly receiving 3 S-Units better when using the Cobra.
I have to wonder about the source.

$90 for zipcord?

Well, opinion aside, I modeled this (each arm is a single wire, folded
back on itself twice) and when I tried to run a SWR sweep in EZNEC
from 2MHz to 30MHz in 0.1MHz steps, EZNEC stalled and gave up.

I suppose this may have been due to warnings of wires being too close.
As it was, I pulled the zip wires apart 1.2" (0.1'). EZNEC wanted a
minimum 6". I tried again with 1MHz steps, and the full sweep drew a
line across the top of the chart (max SWR). Hoping for more
resolution, I tried with 0.2MHz steps, and EZNEC stalled again. Third
time is charm (so the saying goes), and with 0.25MHz steps - it
stalled again.

-sigh-

73's
Richard Clark, KB7QHC

I agree with your comments about the testimonials. A piece of wire is a
piece of wire.

Concerning NEC modeling; the segment lengths should be equal to the wire
spacing. The wire diameter is also a factor to be considered. Anyway, to
establish the validity of the segmentation, I model a parallel wire
transmission line, and see if it is close to the expected impedance. The
antenna appeared to use something similar to 300 Ohm ribbon, so half inch
spacing is probably closer to the correct dimensions.

73,

Frank

On Sun, 21 Nov 2004 23:24:45 GMT, "Frank"
wrote:
so half inch spacing is probably closer to the correct dimensions.

Hi Frank,

Well, my wires were separated by nearly three times that, it was
suggested by a warning window that it should have been 6" and all this
is immaterial to the design being Zip cord.

OK, so we proceed with an antenna that is "like" the topic, but is
spread out to attend to the modeler. I took all wires and separated
them by those 6" and did a SWR sweep from 2 to 30MHz by 0.2MHz and
EZNEC plunged on through them all without clutching up.

Results are
12 +j550 Ohms at 2MHz -11dBi @ 10°
resonates below 80M -6dBi @ 10°
1975 -j160 Ohms at 60M -4dBi @ 10°
570 -j890 Ohms at 40M -0.7dBi @ 10°
104 -j680 Ohms at 30M 1.1dBi @ 10°
440 -j610 Ohms at 20M 5.5dBi @ 10°
210 +j1300 Ohms at 17M 7.7dBi @ 10°
420 -j520 Ohms at 15M 8.2dBi @ 10°
173 +j404 Ohms at 12M 10dBi @ 10°
330 -j360 Ohms at 10M 10dBi @ 10°

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 21 Nov 2004 23:24:45 GMT, "Frank"
wrote:
so half inch spacing is probably closer to the correct dimensions.

Hi Frank,

Well, my wires were separated by nearly three times that, it was
suggested by a warning window that it should have been 6" and all this
is immaterial to the design being Zip cord.

OK, so we proceed with an antenna that is "like" the topic, but is
spread out to attend to the modeler. I took all wires and separated
them by those 6" and did a SWR sweep from 2 to 30MHz by 0.2MHz and
EZNEC plunged on through them all without clutching up.

Results are
12 +j550 Ohms at 2MHz -11dBi @ 10°
resonates below 80M -6dBi @ 10°
1975 -j160 Ohms at 60M -4dBi @ 10°
570 -j890 Ohms at 40M -0.7dBi @ 10°
104 -j680 Ohms at 30M 1.1dBi @ 10°
440 -j610 Ohms at 20M 5.5dBi @ 10°
210 +j1300 Ohms at 17M 7.7dBi @ 10°
420 -j520 Ohms at 15M 8.2dBi @ 10°
173 +j404 Ohms at 12M 10dBi @ 10°
330 -j360 Ohms at 10M 10dBi @ 10°

73's
Richard Clark, KB7QHC

Hi Richard:

I have modeled wires separated by 0.5" as transmission lines, and gotten
good agreement with the predicted impedance. Only problem, with an antenna
of this type, is you exceed the 2000 maximum number of allowable segments.
I think I can go to 2" spacing, with 2" segmentation, and will give it a
try. I suspect I will get results very similar to yours. I also suspect --
but have yet to verify -- that these antennas will behave pretty much the
same as a conventional dipole of the same overall length. Note that a 140
ft dipole resonates at 3.34 MHz.

73,

Frank

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

"Richard Clark" wrote in message
news
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

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

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

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