[TowerTalk] Mast inquiries

[Patrick Greenlee]

On the HRO Web site:

*Detailed Product Description*
US Tower offers three different types of masts. The M-5, 10, 15 and 20 
masts have .120" wall thickness. The M-15R and 20R have steel pipe 
inserts in the lower half of the mast which provides extra strength 
where most needed without adding unnecessary weight in the top portion. 
The M-15R and 20R have a thickness of .250".


On the UST web site there is also listed CM alloy tube  that is 2 inch 
OD and 0.375 wall thickness.

Can someone please give me an explanation of the comment above regarding 
the extra strength where needed in the lower half of the mast?  Does 
this assume the lower part of the mast is still above the thrust 
bearing?  I can see how the leverage of the part of the mast above the 
top bearing would try to fold the mast at the upper bearing. I see how 
lateral force on the part of the mast above the thrust bearing is 
distributed to the rotator acording to the comparative lever arm 
lengths: 1. rotator to thrust bearing distance and 2. thrust bearing to 
the antenna's center of lateral resistance ( like CG but for 
aerodynamics.) If their statement is true in the case where the 
reinforced part of the mast is below the thrust bearing then I need it 
explained to me.

Also, while on this topic, how does the reinforced-at-the-bottom mast 
compare to the CM tube with .250 wall.

Any thoughts on the efficacy of upgrading to the .375 wall CM tubing?  
Is it overkill and a waste of $ or just slightly belt and suspemnders 
and worth considering?  My application is a SteppIR DB42 with 80m and 6m 
accessories plus a NN4ZZ TiltPlate atop a Tashjian HD LM354  turned by a 
Kurt Andress PropPitch motor with the antenna height above ground of 94 
ft or so due to mounting the Tash Tower on a 40 ft steel structure. I 
intend to only have the mast a couple feet or so above the top of the 
tower unless I use the .375 wall CM tube for the mast.  Note: the tower 
site is more or less at heart of tornado alley a few miles from 
locations that repetitively are in the news for tornado damage.

Thanks for your consideration.

Patrick    NJ5G