Boy, you aren't going to let this be easy are you? What I meant by the masts
are no substitute for tower sections comment was related to using a mast to
get your 5 element 20m monobander up another 10 ft instead of buying the
correct number of tower sections.
There is absolutely nothing wrong with using a mast to get multiple antennas
on a single rotor but you have to size the mast properly to take the load and
size the tower properly to take the mast and its load. For example, that
tower with the 6ft loading spec is figured on the antenna being located at
the top of tower. When you add a mast, say another 8 ft to the tower, guess
what? The tower now is seeing the load as if it were 8 ft taller and it can
no longer handle 6 sq ft, but something less.
Also, beware loading specs from the tower manufacurer. I have recently seen
specs of say 15 sq ft that was based on discounting the antenna load by a
factor of .66. The .66 figure is applied because the antenna components are
round instead of flat. That is ok bur guess what? The antenna manufacturers
have already done that in their specs!. You can't take the .66 factor twice
or you will end up with a tower with 66% of the expected capacity.
The answer that you are looking for is not readily available. Calculations
need to be done and as I mentioned before, the torq loading data is not
available from the tower companies. Mast bending moments and twisting
calculations are no problem as the properties of the cylindrical steel masts
are well known. Properties of tower sections are not well known.
With the lack of needed data what are we to do? Well, this reflector is a
good place to hear about what did/did not work well.
Maybe someone out there would like to take on a project of testing some tower
sections to failure from torq loads. I would be happy to supply the sections
for a small nominal charge!
&3 de Gerald, K5GW, Owner & General Manager, Texas Towers
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