[TowerTalk] 24' quad stacking (long)
L. B. Cebik
cebik@utkux.utcc.utk.edu
Sat, 5 Aug 2000 11:40:40 -0400 (EDT)
Because there appears to be interest in the stacking potential of quads,
especially 2-element, 5-band quads, I finally decided to stack a pair of
models and see what they would do. I used Gerhard's 24' spacing between
array centers. I first ran the single array in free space, followed by the
pair in free space, for a relative gain and general property check. I then
placed the lower quad 50' up and the higher 24' above that to see if ground
would create any undesirable effects.
The quads used in this initial run are models in NEC-4 of the KC6T quad in
April, 1992 QST (p.52), one of the finest planar quad designs I have found
with a constant 8' spacing. It uses loading capacitance (modeled as a
value of C and not as a value of -jX) in the reflector to set the operating
frequency and my models are self-resonant without a matching network. The
models have been set for the most desirable combination of gain, F-B, and
impedance at mid-band to reveal the rate of change of these parameters both
above and below the design frequency.
If the length of the entry is forgivable and there is any interest, I can
make the same runs on one of the spider designs in my collection to see if
there is any significant difference.
The data consists of gain in dBi, TO angle (where relevant), F-B,
beamwidth, feed Z(s) and 75-Ohm SWR (for which the original array had been
set). These tables are produced in Courier and converted to straight ASCII
with hard spaces, not tabs. Hence, reading them may require a change to a
monospaced font for the numbers to line up.
KC6T Quad in Free Space
Fq Gain F-B B/W Feed Z SWR-75
14.0 7.6 8.3 69 34.7-45.6 3.10
14.175 7.2 24.0 73 76.6+ 1.6 1.03
14.35 6.4 11.6 76 112.2+ 9.4 1.52
18.118 7.3 31.7 74 69.5+ 1.7 1.08
21.0 7.7 12.6 72 47.3-30.0 1.96
21.225 7.3 34.4 75 69.5+ 1.7 1.08
21.45 6.7 14.2 77 89.6+19.3 1.34
24.94 7.2 30.6 76 77.0+ 0.3 1.03
28.0 7.7 14.5 75 65.9-54.2 2.15
28.5 7.5 22.8 77 75.4- 0.3 1.01
29 7.3 37.1 78 87.1+50.1 1.87
2 KC6T Quad stacked 24' apart in Free Space: Z1 (upper entry) = lower
quad; Z2 (lower entry) = upper quad. Since both quads are fed on the lower
element, some differentials in values are normal.
Fq Gain F-B B/W Feed Z 1/2 SWR-75 1/2
14.0 9.0 11.3 70 64.6-18.9 1.36
66.7-20.4 1.30
14.175 8.8 20.4 73 120.8+23.2 1.70
117.2+13.5 1.60
14.35 8.3 12.4 75 164.8+14.3 2.22
154.2+12.3 2.07
18.118 9.5 21.5 73 86.8+ 3.6 1.17
85.9+ 4.1 1.16
21.0 10.1 12.9 72 56.3-29.9 1.71
56.1-29.6 1.71
21.225 9.9 22.3 75 81.3+ 1.3 1.09
81.1+ 1.8 1.09
21.45 9.5 13.3 77 104.1+14.6 1.44
104.2+15.2 1.45
24.94 10.2 23.1 77 84.1- 5.0 1.14
84.6 -4.9 1.15
28.0 10.8 15.1 75 70.0-57.0 2.16
70.2-56.7 2.15
28.5 10.7 23.5 77 79.7- 2.2 1.07
79.9- 2.0 1.07
29 10.5 26.8 78 92.5+48.6 1.84
92.7+48.9 1.85
Stacking Gain averaged by bands:
20 17 15 12 10
1.6 2.2 2.6 3.0 3.2 dB
2 KC6T Quad stacked 24' apart in 50' and 74' above average ground: Z1
(upper entry) = lower quad; Z2 (lower entry) = upper quad. Since both
quads are fed on the lower element, some differentials in values are
normal.
Fq Gain F-B B/W Feed Z 1/2 SWR-75 1/2 TO
14.0 13.6 11.7 71 64.2-19.1 1.37 15
66.6-20.0 1.36
14.175 13.4 20.5 74 117.0+19.6 1.63
120.1+18.5 1.66
14.35 13.0 12.1 76 156.4+20.5 2.13
162.8+ 8.5 2.18
18.118 14.4 21.6 74 87.7+ 4.5 1.18
85.3+ 3.5 1.15
21.0 15.1 13.1 72 56.7-29.8 1.70
55.8-29.7 1.72
21.225 15.0 22.0 75 82.3+ 1.1 1.10
80.5+ 2.1 1.08
21.45 14.6 13.1 78 105.1+13.4 1.45
103.8+16.3 1.45
24.94 15.4 22.1 77 82.7- 5.8 1.13
86.2 -5.5 1.17
28.0 15.9 14.9 75 69.1-56.1 2.15
69.3-57.5 2.18
28.5 15.8 22.9 77 79.3- 0.9 1.06
78.7- 1.9 1.06
29 15.7 26.5 79 92.7+50.2 1.87
91.8+49.6 1.86
As is clearly evident, the particular quad design explored here does not
suffer from being placed over ground at 50' for the center of the bottom
array and at 74' for the center of the top array. The front-to-back and
impedance values hold closely to their free-space values--sufficiently so
that I could not think of recommending a design change. In addition,
stacking appears to shift the operating parameters to provide operating
bandwidth in the stack that is superior to that of the single array.
Assuming that an in-phase feedline harness can be devised, the coax run
should bring virtually all SWR values under 2:1 at the shack. For the
stacks, SWRs are all well within rig-tuner range without excessive loss.
I have been hesitant in the past to recommend stacking multi-band quads,
given the fact that a quad is already a stack in itself. However, these
figures suggest that--if one can handle the mechanics and weather--the
enterprise may prove worthy, even with relatively close spacing, as used in
these models.
As always, differences of design may yield different stacking results, and
every design should be thoroughly modeled before capital investment.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / Tel: (865) 938-6335
1434 High Mesa Drive / \/ \/\ ----/\--- http://www.cebik.com
Knoxville, Tennessee /\ \ \ \ / / || / e-mail: cebik@utk.edu
37938-4443 USA / \ \ \ \ || e-mail: w4rnl@arrl.org
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