I found my stacking studies which were made for
wide spaced a 3L20, 4L15, and 5L10 which comes
pretty close to representing the C31XR.
On 20M, stacking gain just begins to fall off below
40 ft spacing for 3L on a 24 ft boom (8.5 dBi).
On 15M, 40 ft spacing is optimum for 4L (9.5 dBi).
On 10M, stacking gain is falling off at/above 40 ft spacing
for 5L on a 26 ft boom (10.5 dBi).
Bottom line, 35 to 45 ft spacing for 3 C31XR's is a
reasonable approach. Something like 122 / 82 / 42
to 122 / 79 / 36 would work on your 130 ft Rohn 45
tower guyed at 121 / 78 / 35 ft. Note that wider spacing
will definitely result in lower stacking gain on 10M.
It would be useful to model the 3 C31XR's at these heights
to verify that they track with my wide spaced monobanders.
(A stacked pair of C31XR's at 70 and 35 to 40 ft looks attractive)
If you use the above then look at the bending moment
with your 3L40 at 131 ft. Forget about the chrome molly
mast above the top of your tower with your top guys 9 ft below.
A 2L40 on the other tower at 131 ft. would add flexibility
and be within ratings. Stacked 2L40's with less than
70 ft separation hardly seems worth the effort (1 dB gain)
but a low (60 to 90 ft) 2L40 is useful before sunset / early
evening and after sunrise.
de Tom N4KG
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