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On Mon, 22 Nov 2004 23:08:32 GMT, "Frank"
wrote: 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. 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. 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? 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. 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. 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. 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. 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 |