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#14
April 11th 04, 01:59 AM
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In article , Richard Clark
writes: On 10 Apr 2004 18:25:04 GMT, PAMNO (N2EY) wrote: All of this supports both your experiences and those of others. If the exact characteristics of tubular Twin Lead are important to the design, it's no wonder that the reported results cannot be obtained. One more point. The SWR graphs shown for the 5 band version show quite narrow SWR bandwidths. If anyone is interested I can report them in a future post. Hi Jim, I stand corrected on the tubular twin lead (must've died in the mid-60s then). No biggie. I did not expect to find it in the 1953 catalog, but there it was, and not a new item either. Probably unobtanium now. It offers nothing substantially different to the physics of design however, just a different velocity factor which is scalable to suit any implementation. Lattin seemed to think it was important. Do you have the 1960 QST article? However, my data to this point confounds the "theory" of it all by exhibiting stubs that do NOT resonate at their computed length (complete with correction for ANY velocity factor commonly observed). More data is always appreciated. Well, here's the info from the 1960 QST article. Dipole dimensions: Innermost section (not a stub; shorted at both ends): 8' Next section (stub; open at inner end): 6' 11" [resonant at 10 meters?] Next section (stub, open at inner end): 13' 10" [resonant at 20 meters?] Outermost section (stub, open at inner end): 27' 5" [resonant at 40 meters?] Entire antenna 57' 2" per side (plus connections) SWR, 50 ohm coax, coax length not given (all numbers guesstimated from graph): 80 meters: Minimum SWR: 3725 kHz - 1.1:1 2:1 SWR points: 3625 & 3800 kHz (175 kHz) SWR at 3500: 5:1 SWR at 4000: 4.5:1 40 meters: Minimum SWR: 7225 kHz - 1.6:1 2:1 SWR points: 7150 & 7275 kHz (125 kHz) SWR at 7000: Off end of scale (5:1 at 7075 kHz) SWR at 7300: 3:1 20 meters: Minimum SWR: 14250 kHz - 1.3:1 2:1 SWR points: 14100 & 14375 kHz (275 kHz) SWR at 14000: 3.5:1 SWR at 14350: 1.8:1 10 meters: Minimum SWR: 28600 - 1.6:1 2:1 SWR points: 28500 & 28750 kHz (250 kHz) SWR at 28900: 3:1 Chart for 10 meters covers 28500 to 28900 only. No chart for 15 meters but text says it will work there on 3/2 wavelength resonance of 40 meter section and SWR of not less than 3:1. Note how tight the 2:1 SWR points are, even on the higher bands. -- Also described is an 80/40 dipole. Inner section is 28' and shorted both ends, outer stub section is 27' 5". Interesting feature of this one is that in order to achieve 40 meter resonance there are pieces of wire 2' 6" long hung from the junction of the two sections. The text and diagram say the outer section resonates on 40 but the inner section has to be kept at 28 feet or the 80 meter resonance will be too low. -- Seems to me that the classic W3DZZ trap dipole would be a lot less aggravation to model and get working, plus more flexible in choice of minimum SWR points. Particularly if additional "resonance wires" were added if needed. Certainly easier to make mechanically strong traps than stubs, and adjusting single wire lengths with Burndys is a lot easier than fooling with tubular Twin-Lead. 73 de Jim, N2EY |
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