"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