[TowerTalk] Rohn 25 & Mast lingth
grants2 at pacbell.net
Mon Apr 17 17:26:46 EDT 2017
I had 3x 25G sections on a house bracket and about 14' of 2" x 0.375"
wall 6061T6 exposed mast to a TH7DX, which probably is in the large size
category for tribanders. Survived several hurricanes in E. Ma.
Probably 80kts at my house. Although it was plenty strong enough, I
would not use Al mast again, it is too bendy. I could tilt the tower
over and went with Al to save weight against the raising tackle.
There is no structural advantage to more mast inside the tower beyond 3
ft or so. More doesn't matter either way.
How you reinforce the wall the bracket attaches to is important. Besides
internal cross bracing of 2x8's on the studs, I structurally nailed a
4x8 sheet of 3/4 ply down as the attic floor. That diagonally tied from
floor to wall bracket with a 6' long steel angle. End walls on roofs
have just a few nails top and bottom in the studs. Structurally, a
floor membrane should take the tower loads. This is a bit harder if the
room inside the bracket is finished space and your decorating police
have a vote.
You don't want to cut an alloy mast at home except with an abrasive saw
or a cobalt tipped bandsaw blade and plenty of coolant.
On 4/17/2017 13:54 PM, Clifton Keely via TowerTalk wrote:
> I am in the process of erecting a Rohn 25G to 30 feet with a house bracket. My question is how long should the mast be within the tower? I have a thrust bearing top plate and a good rotor for the load, but should the mast extend 2 feet below the top plate or 6 or 8 or 12 feet. Would there be a real advantage to letting the mast go below the next tower joint down (10 ft). DX sells a high budget mast they they won’t cut, it is 22 feet long. I don’t think I want to have a mid sized triband 10 or 12 feet above the top plate/thrust bearing. My plan is to place the antenna at about 5 feet above the top plate/bearing Speak to me someone who knows. Please. All advice greatly appreciated. Clif AA6FE.
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