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

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

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