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
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