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

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.

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

I guess we scared him away Richard.

73,

Frank


"Richard Clark" wrote in message
...
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

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

On Thu, 25 Nov 2004 15:46:40 GMT, "Frank"
wrote:
I guess we scared him away Richard.

Hi Frank,

Well on to other speculations....

Sometime later this morning, I will release a study in transmission
lines that your discussion aided me with. It won't be so much about
the lines themselves, but about a compendium of topics revolving
around the Zc of the source, and what is called Mismatch Uncertainty.

I've done the grunt work, it just needs some introduction. However,
as is typical with this topic, the thread will no doubt run to half a
dozen comments, if that much. :-)

73's
Richard Clark, KB7QHC

Glad something I did helped Richard. Will look forward to your posting. I
enjoy reading these huge threads, though don't always feel I can contribute
much.

73,

Frank

"Richard Clark" wrote in message
...
On Thu, 25 Nov 2004 15:46:40 GMT, "Frank"
wrote:
I guess we scared him away Richard.

Hi Frank,

Well on to other speculations....

Sometime later this morning, I will release a study in transmission
lines that your discussion aided me with. It won't be so much about
the lines themselves, but about a compendium of topics revolving
around the Zc of the source, and what is called Mismatch Uncertainty.

I've done the grunt work, it just needs some introduction. However,
as is typical with this topic, the thread will no doubt run to half a
dozen comments, if that much. :-)

73's
Richard Clark, KB7QHC