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