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
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