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Stacks and SWR

 To: Stacks and SWR cebik@utkux.utcc.utk.edu (L. B. Cebik) Wed, 12 Feb 1997 08:27:38 -0500 (EST)
 ```Some beam stackers have experienced a rise in SWR if the beams are pointed in opposite directions. Others have not. The degree of SWR rise was not specified. To shed some light on this question, I ran a pair of identical beams through NEC-4. The individual beams, cut to be resonant at 14.175 MHz had the following free space properties: TO angle Gain (dBi) F-B ratio (dB) Beamwidth Feed Z (degrees) (degrees) (R+/jX) --- 8.1 26.6 62 25.5 - 0.1 Here are the numbers for the beam alone at heights of 1 wl (70') and at 1.5 wl (105') 70' 13 13.4 24.3 62 25.0 - 0.0 105' 9 13.7 24.7 62 25.2 - 0.0 Let's stack the beams and feed each separately to see if interaction changes anything. Remember, the beams are stacked just about 1/2 wl apart. Feed lower only: 14 13.3 21.5 62 25.7 - 1.0 Feed upper only: 9 13.6 21.7 60 26.0 - 1.1 Feed both: 10 15.8 20.8 62 Lower: 24.6 + 1.3 Upper: 24.7 + 1.2 Notice that at 1/2 wl spacing, where the lower beam is 1 wl up, the only possibly significant change is a decrease in F-B ratio in the 3-4 dB range. Now let's point the beams in opposite directions and see what a dual feed produces: 9 11.3 1.2 60 Lower: 23.7 - 5.2 Upper: 23.8 - 5.4 Depending upon the sensitivity of the feed system to changes under 10% in Z and depending upon the basic match between antenna feed, antenna match and lines, and in which direction any small mismatch might go, one may well see a small rise in SWR. Although 5+ ohms of reactance may not be considered much, at an R of 25 ohms, it is 20% of the R value. Although this does not change R significantly, it changes SWR more significantly. (See some older ARRL books for calculating SWR directly from Zo and R +/- jX.) Some prefer to stack beams at a 5/8 wl spacing to maximize gain. So I raised the upper beam 9' to 114' to check the results. Easier on NEC than on a tower. Feed lower only: 12 14.2 18.8 64 24.4 + 0.1 Feed upper only: 9 14.6 19.5 64 24.9 + 0.4 Feed both: 10 16.2 17.4 62 Lower: 24.0 + 1.3 Upper: 24.5 + 1.7 Notice the slight parasitical improvement in individual beam gain, as well as the higher stacked gain, relative to 1/2 wl spacing. However, notice also the continued degradation in F-B performance. You pays your money and you takes your choice. Now let's point the beams in opposite directions and see what a dual feed produces: 9 11.9 0.9 62 Lower: 21.2 - 1.7 Upper: 22.0 - 1.5 Although the reactance has not climbed very much, the feedpoint resistance is down about 12% or so. Again, depending upon the sensitivity of the matching system to changes of this order and in which direction any inaccuracies in match are directed, a rise in SWR can be expected in many cases. If a perfect 1:1 is initially established, a 1.2-1.3:1 may result--noticeable, but not in any sense fatal. However, I have idealized the case with a very nice 25-ohm natural feedpoint Z so that accurately cut feedlines can do all the matching. Commercial beams have their own systems to place 50 ohms at the coax connection, and we construct phase lines from there. Sensitivity to 10-15% mismatches is thus not predictable by NEC alone. However, if one does not see that slight rise, one has to wonder if it is function of losses somewhere in the system. All models done in NEC-4 over S-N .005/13 ground, with Al elements. If this is in any way useful data, I'll pass along some modeling data on misphased stacks. If not, I'll file it. -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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