[TowerTalk] Rohn 25 & Mast lingth

[Grant Saviers]

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.

Grant KZ1W

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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