[TowerTalk] Antenna Mast - guidelines for ratio in tower / out of tower

[Scott Dickson, W5WZ]

I've seen discussion on the importance of selecting the correct material for
a mast.  I don't recall ever seeing a discussion on the ratio of mast in the
tower versus above the tower.

 

I have a 2" x 0.25" by 21 ft galvanized 1026 DOM mast.  It will hold a Force
12 C-51XR, (weighs 250 lbs, wind load 18 sq ft, mast torque < 900 in-lbs)
installed 2 feet above the thrust bearing at the top of the Rohn 55G tower.
The rotor is M2 Orion 2800.

 

Currently, I do not have any other antenna that will be on the mast.
Perhaps I'll install a 6 meter beam above the C51 in the future.

 

It seems to me that the length of mast in the tower is as important as the
length of mast above the tower.

 

What rules / guidelines / best practices are used to determine where to
install the rotor?

 

--Scott, W5WZ