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## [TowerTalk] wideband 80 meter dipoles (long-delete if irrelevant)

 To: [TowerTalk] wideband 80 meter dipoles (long-delete if irrelevant) cebik@utkux.utcc.utk.edu (L. B. Cebik) Thu, 27 Nov 1997 08:13:45 -0500 (EST)
 ```Dave Leeson brought to our attention an interesting technique for achieving wide-band operation on the lower HF bands, derived from mentions in texts and references in ARRL publications by Frank Witt, AI1H. The technique involves choosing a geometric average frequency between two frequencies of interest--then, for that frequency, cutting a length of 50-ohm coax a multiple of 0.5 wl (allowing for velocity factor), with a 0.25 wl length of 75-ohm coax (again, allowing for velocity factor) at the station end of the line. This is a bit of follow-up that seemed interesting as the numbers emerged from some modeling exercises. I thought I would pass them on. The SWR at the antenna relative to 50 ohms does not change, but line losses at the lower HF bands are not--for many purposes--sufficiently large to make a case against this or other wide-banding techniques with coaxial feed lines. The factors that produce wide-band operation (using the conventional <2:1 SWR measure for convenience) include the impedance transformation along the transmission line at frequencies above and below the dipole resonant length and the physical lengths of coax cut for that resonant frequency. Since the situation described by Dave can be modeled directly in NEC-2 or NEC-4, using the transmission line feature available on NEC, I decided to look at some SWR curves across 80 meters. My dipole was resonated at 3.75 MHz to ensure that the 2:1 SWR points fell within the band. I used the NEC mathematical models of 50-ohm, 0.765 VF transmission line for 0.5, 1.0, 1.5, and 2.0 wl, followed by a 0.25 wl section of 75-ohm, 0.66 VF cable to the feedpoint/station end. My dipole at 120' over level medium ground had an independent feedpoint Z of 76 ohms. I am reading from graphs at this point, but hope to make the data more precise later. 50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR 0.5wl 3.57 3.96 0.39 1.45 1.0 3.55 3.96 0.41 1.30 1.5 3.57 3.93 0.36 1.10 2.0 3.58 3.91 0.33 1.05 The table has several interesting features. First, for a 0.5 wl 50-ohm run, there is only one SWR minimum, roughly at the self-resonant frequency of the dipole. With an independent feed Z of 76 ohms, the SWR shows a shallow curve. Second, for lengths of 50-ohm coax of 1 wl and up, the double minima curve emerges. With the given independent dipole feed Z, bandwidth is greatest with a 1 wl run and diminishes above that. In fact, as the length of 50- ohm coax is increased, the rise in SWR is steeper at both the low and high ends of the band. However, the minimum SWR become lower with increases in 50-ohm line length. The SWR at the dipole's self-resonant frequency remains almost unchanged (1.4 to 1.5) throughout. I reran the exercise, each time lowering the dipole height by 10' in order to see what effect an increasing independent feed Z might have on the curves. I adjusted the independent dipole length as necessary for resonance and imported that length to the model with transmission lines. First the numbers: 110' up" Z=83 ohms 50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR 0.5wl 3.55 3.97 0.42 1.35 1.0 3.54 3.96 0.42 1.25 1.5 3.56 3.93 0.37 1.10 2.0 3.58 3.91 0.33 1.05 100' up" Z=89 ohms 50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR 0.5wl 3.53 3.97 0.44 1.30 1.0 3.53 3.96 0.43 1.20 1.5 3.56 3.92 0.36 1.05 2.0 3.58 3.90 0.32 1.05 90' up" Z=92 ohms 50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR 0.5wl 3.53 3.99 0.46 1.25 1.0 3.54 3.96 0.42 1.10 1.5 3.56 3.92 0.36 1.01 2.0 3.58 3.90 0.32 1.05 As the independent feedpoint impedance of the dipole increases (within the boundaries of the test runs), the performance of the 0.5 wl 50-ohm coax run improves. The curves over all the tests for this length of line are largely congruent, and the improved performance with increasing feed Z occurs because the impedance presented to the 0.25 wl 75-ohm matching section grows closer to the value needed for a 50-ohm impedance at the transmitter end. Although not especially extreme, the slope of the SWR curves for the two longest runs of 50-ohm line grow steeper with increasing independent dipole feed Z. Band edge values are about 5:1 for 2 wl runs and 4:1 for 1.5 wl runs. By contrast, with a 1.0 wl run, the band-edge SWRs are close to 3:1, while with a 0.5 wl run, the band-edge values are about 2.7:1 for the worst case and 2.5:1 for the best case (at the lower end, with lower values at the upper end of the band). These values do not account for dissipative line losses that ordinarily show up at the shack end of the line as slightly lower SWR readings. So, what is the use of all this modeling? If all one needs are two low-SWR points within the band, then any of the 50-ohm lengths might be in order. However, if one is seeking the maximum possible coverage of 80-75, then one might consider restricting the length of initial 50-ohm coax run to 0.5 wl or at most 1.0 wl. One can insert the 75-ohm matching section at this point and use 50-ohm coax the rest of the way to the shack. Since the impedance values fluctuate across the band by as much above 50 ohms as below it, some further impedance transformation will occur, but it will be in virtually all cases less radical at the band edges than would be the case of using the longer initial 50-ohm runs indicated in the charts. The numbers and suggestions are limited, of course, by the limitations of the models and modeling program. They may require field adjustment in accord with the circumstances of any given installation. However, I hope they are useful to those thinking about using the feed system Dave Leeson has brought to our attention. -73- LB, W4RNL L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215 1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335 Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509 37938-4443 USA / \ \ \ \ || cebik@utk.edu URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search ```
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