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#1
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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 |
#2
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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 |
#3
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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 |
#4
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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. |
#5
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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 |
#6
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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 |
#7
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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 |
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