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Misphasing stacks

 To: Misphasing stacks cebik@utkux.utcc.utk.edu (L. B. Cebik) Wed, 12 Feb 1997 20:56:08 -0500 (EST)
 ```Misphasing of stacked beams: There are two questions one can ask. 1. Is something wrong when stacked beams are misphased? 2. Can I do anything useful by reversing the phase of one of a stack of beams. To see what might happen, I modeled the same 3-element Al 20 meter beam as in the last report over S-N ground .005/13 on EZNEC/4. I added transmission lines--1/4 wl section joined to form a feedpoint. Since NEC feedlines are mathematical, not physical, I simply used the sections to transform the 25 ohm resonant Z of the individual beams to 100 ohms each, in parallel for a pair of beams making 50 ohms. This provided a baseline for watching variations. The 3-3 stack used 1/2 wl separation, again at 70' and 105' (which may be translated for any upper HF band in terms of wavelengths of height and separation) Here are the results: TO angle Gain (dBi) F-B ratio (dB) Beamwidth Feed Z (degrees) (degrees) (R+/jX) In-phase: 10 15.83 20.8 60 50.6 - 2.8 As predicted in model construction, the transmission line transformers yield a very matchable condition. Out-of-phase: 50 12.34 18.9 82 (oval) 45.9 + 6.4 25 11.47 28.8 66 Reverse phasing produces two major lobes in the elevation pattern at the indicated angles, each down at least 3.5 dB from the main lobe of the in- phase model. Both lobes are quite high relative to desired dx angles. However, such a configuration, if switchable, might be useful for a domestic contest. The upper lobes on the in-phase model are down by 7 dB or more. I next tried a 3-3-3 stack at .5-1.0-1.5 wl (70'-105'-140'). I again used 1/4 wl transmission line transformers joined at a distant feedpoint, resulting in an anticipated baseline feedpoint impedance of 33.3 ohms R. Since throughout the exercise I used 41 segments per 1/2 wl to ensure convergence without having to recheck each model, the 370-segment model was the limit of my efforts. Here are the results. TO angle Gain (dBi) F-B ratio (dB) Beamwidth Feed Z (degrees) (degrees) (R+/jX) In-phase: 8 17.3 21.7 60 32.7 - 1.7 This result tallies well with expectations. Notable in the 3-beam stack is a return of some of the F-B ratio lost in the 2-beam stack. The stack has lesser lobes at 26 degrees (down 8.5 dB) and at 41 degrees (down 15 dB): these figures are given for comparison with lobes, both main and secondary, of the same stack with one of the beams out of phase with the other two. Highest out-of-phase: 17 13.5 21.3 62 33.8 + 1.2 36 13.2 29.2 68 Although the gain of each lobe is no more than that of a single beam, the elevation angles may also be useful for domestic work. The feedpoint impedance is quite usable. Middle out-of-phase: 55 13.3 14.5 88 27.7 + 8.0 This configuration may be least useful due to the very high angle of the main lobe. There are lesser lobes at 8 degrees (down 6 dB) and at 33 degrees (down 4.5 dB). The feedpoint impedance is down 18% on the resistance side, with a significant reactance, which may show an undesirable rise in SWR in a switched system, even after the anticipated 33 ohms is matched back to a 50-ohm cable. Lowest out-of-phase: 22 14.3 21.6 62 34.1 + 1.0 This configuration shows lesser lobes at 7 degrees (down 4 dB) and at 28 degrees (down 3.5 dB). The feedpoint impedance is acceptable. The configuration may be useful as an alternative dx configuration, despite the loss of gain from in-phase maximum, since--under some circumstances-- capturing the proper elevation of signal angle may be more important than raw gain. The exercise strongly suggests that a switchable phasing system may prove useful, depending upon one's operating goals and activities. Although some options may yield less than useful patterns, most of the patterns-- especially in terms of altered elevation angles of maximum radiation--have a certain utility. I suspect that the upshot to consider is this: If you are going that high with that much rotatable metal, you might as well throw in a phase-reversal switching system, just in case . . .. And that is using only 3-element Yagis as the foundation. There is no reason to believe that results for larger individual beams cannot be extrapolated with reasonable reliability--or that 5/8 wl spacing will not show similar small gains above 1/2 wl spacing. In the latter case, however, phase reversal patterns might differ, since beam interaction differs a bit. Hope this data is useful, at least in setting up reasonable expectations. -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 -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: K7LXC@contesting.com Sponsored by Akorn Access, Inc & N4VJ / K4AAA ```
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