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

Look at it closely. It is three wires (Radio Shack rotor wire) joined at
opposite ends to make one long wire. The 70 foot model actually has a "run"
of 210 feet - 105 feet on each side. Funny, Radio Shack rotor wire comes in
70 foot rolls.

Attach ladder line to a middle insulator. Add up the cost and you'll save a
lot by doing it yourself. I have seen a Cobra and it looks very well made
but I have never used one.

The principal is called linear loading, and it is a very effective way of
getting maximum benefit out of short antennas. It is far more efficient
than coils. ON4UN's book talks a lot about it but I couldn't find much
discussion on the effect of having the wires so close together. He does
advise, on page 8-13 of the Third Edition, "make sure the separation between
the element and the folded linear-loading device is large enough, and that
you use high-quality insulators to prevent arc-over and insulator damage."
He doesn't define "enough" and the phrasing of the sentence makes it sound
like the only concern with having them too close is arc-over. On the same
page, he warns modeling is "very tricky." From the discussion that follows
on this thread, I would say that is an understatement.
--
Radio K4ia
Craig "Buck"
Fredericksburg, VA USA
FISTS 6702 cc 788 Diamond 64


"Bert Craig" wrote in message
. net...
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