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.

You are correct Richard -- It appears my segmentation is seriously "over the
top". The segmentation was driven by not wanting a segment to be longer
than the spacing. Some texts on NEC do recomend such constraints. So far I
have gone to 16" segments, with 1" spacing. For the transmission line model
it shows even better convergence. #24 AWG spaced by 1" has a characterestic
impedance of 550 Ohm. 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.

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?

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?
Unfortunately NEC cannot model insulated wire, so not sure how that would
effect the model. Is the VF very significant?


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.

That's what I figured.

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.

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.


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.

It sure is!

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

Now I have gotten out of my segmentation fixation I may be able to run some
more realistic models.

73,

Frank