Ref: F.T. Brady message about long masts:
A couple of thoughts about the long mast idea:
1) The tower will still have a large wind load thrust upon it as the tower
sections themselves represent a major part of the wind load of such
strutures. Even small towers like Rohn 25G have a wind load of about 7lbs per
lineal foot with 70 mph winds. A 60 ft tower in this case will have about 420
lb wind load (60 X 7) which is roughly the equivalent of 21 square ft of
surface area (420/20).
2) The amount of twist in the mast will surprise you. For example, with a 60
ft mast 2" in diameter with a .25" wall, made of steel supporting a load
creating a twisting moment of say 5000 ft/lb (like a 200 pound antenna stack
X a 25 foot length) will twist more than 16 degrees. Aluminum of same
dimensions is even worse with over 50 degrees twist!
One of my local friends and good customer, K5YNZ has a similar set up on an
85ft self supporting tower with a big 4 element quad on top. Because of the
twisting problem he is never really sure where the antenna is pointed and is
making plans to convert the system back to a conventional length mast set up.
Keep up the good ideas! It is making us all think! 73 de Gerald Williamson,
K5GW, Owner/General Manager, Texas Towers
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