[TowerTalk] HOW MUCH MAST NEEDED IN TOWER

[Tim Cotton, N4UM]

Jose:

Your question is hard to answer since you don't state the yield strength of the steel used in your mast and you don't indicate how the 20 square feet of antenna will be distributed along the mast.  You also don't indicate if the square footage of your antenna has been corrected for streamlining.

Assume a high  quality mast similar to Array Solutions 2" (1/4"wall) 4130 Chrome Moly having a yield strength of  100,000 psi.  They indicate the maximum yield moment of that mast is 53689 in-lbs.  

Scenario #1 -
If you assume a wind force of 50 psf (not unusual during a moderate  hurricane) and your 20 square feet (effective surface) of antenna is concentrated at the top of the mast - then the bending moment (ignoring the effects of the wind on the mast) will be 50 x 20 x L where L is the number of INCHES your mast sticks out of the tower.  In this example, L would need to be less than 54 INCHES.  This doesn't take into account the additional moment due to the effects of the wind on the mast itself.  

Scenario #2
If you think the above  figures are too conservative... try this scenario...
40 psf wind, 20x.667 square feet of surface area (correction for streamlining).  This yields a wind force due to antenna of 533.6 lbs.  Let it act over a distance above the tower of 8 feet (96 inches).  This yields a bending moment of 51225 in lbs.  Add the effects of the wind on the mast and you add another 142 in-lbs. The total of 51367 in-lb... is very close to the yield moment of 53689 in-lb and provides for no safety factor.

I'm obviously not an engineer but I would damn scared to go more than three or four feet (at the most) above the tower with 20 square feet of antenna in a hurricane-prone area such as the one you're moving into.  Raising your antenna another 6 to 10 feet is not likely to do very much for your signal but it certainly puts your antenna at much greater risk.

Tim, N4UM