To continue on this business of C31XR stacking, I would like to thank Natan
Huffman for forwarding some literature, wherein lies the genesis of the 37-40
foot spacing. I have determined much smaller optimums and will still hold issue
with K7LXC's comments RE the 37-40' optimum. Why?
Because the figures change as you go from tall towers to shorter ones. As the
10 meter section is the real long boom, let's take 28.6 for computation. Or
another way to put it, there are large tower rules and small tower rules for
If you model a 2 stack of C31's in free space, you get: (stack distance)
gain in dbi.
(26) 12.42, (28) 12.65, (30) 12.84, (32) 12.97, (34) 13.04, (36) 13.08, (38)
13.10, (40) 13.10
Note first of all that there is not quite 0.7 dB difference between 26 & 40 foot
stacking distances, and only 0.13 between 32 and 40. This is a very broad and
gentle curve, and with the max up around 38-40 feet. This implies that at large
heights, the best stacking distance is 38-40 feet. But this inference does not
hold up as the stack approaches the ground and interaction with reflected energy
can vary phase in the elements.
The reason why I was getting different stacking distances is because I was
modeling on smaller towers that I had a chance of putting up. To make things a
little easier to understand and visualize, I will hold the top antenna constant
(e.g. top of a tower) and vary the height of the lower antenna in the stack.
To give a reference, a two stack at 140/100 has 18.67 dbi gain, the best I was
able to get in a half hour's fiddling for a top score.
With top antenna at 100', we get: (height of bottom C31) gain in dbi
(75) 17.82, (70) 18.24, (68) 18.33, (65) 18.34, (60) 18.18, (55) 17.94 Note
that if this is a 3 guy set tower that happily the max gain point for the lower
antenna is obtained right above the second set of guys. You can see that the max
is just starting to move away from the 38-40 max of free space. And it's only
0.35 dB less that the 40 foot stack way up there, and pretty close to identical
if you account for the extra feedline to get up there.
With top antenna at 90', we get: (70) 17.77, (65) 17.75, (60) 18.10, (57)
18.16, (55) 18.16, (50) 17.99
With top antenna at 80', we get: (60) 17.67, (55) 17.70, (50) 18.02, (48)
18.04, (45) 17.95, (40) 17.63
With top antenna at 70', we get: (50) 17.51, (45) 17.36, (40) 17.62, (38)
17.65, (35) 17.59, (30) 17.21
Note that at 70' height the gain at 40' spacing is almost a half dB down from
the optimum at 32', a clear departure from the free space figures.
I have not addressed at all the details of front to back, large vertical and
high angle lobes which roughly are at their best with distances, with two stacks
under 100' anyway, around 30'.
Setting out an absolute rule for 37-40' feet stacking gives an appearance that a
two stack on a shorter tower will not work well, when in fact 100/67 will do
VERY nicely, using the same rules that ask for 140/100.
Hope this satisfactorily explains my earlier reaction.
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Apex, NC, USA
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