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[TowerTalk] For Chrome Moly and tower/wind load experts out there - Moun

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Subject: [TowerTalk] For Chrome Moly and tower/wind load experts out there - Mountain top tower installation
From: <trappaport@austin.rr.com>
Date: Sat, 15 Aug 2009 10:20:11 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
With inputs from many, I have settled on making the mountain-top 65' towers out 
of heavy duty Rohn 45G  SR (solid rod) with 2 guys per tower, using three 20' 
sections (and a 10' base section having 5' submerged in reinforced concrete). 
After much analysis by many people, this appears to be the most rugged yet 
climbable choice for a mountain top location, as it is sturdier than hollow 
tubed 55G in ice, and comparable to 65G in strength but easier to climb and 
outfit. With my 2 big antennas on top, it is engineered to withstand > 110 MPH 
winds and 1/2" radial ice on mountain tops, and hopefully *much* more than 
that. Does anyone have advice/experience/words to the wise regarding 45G SR 
during installation or over the long term?....The professional tower installers 
will be using a crane to install.

Now, the question of mast pipe arises, and the yield strength in high winds and 
ice becomes a concern to all I talk with.. Chrome Moly seems to be the mast 
material of choice, and 4130 N tubing(Normalized) appears to be readily 
available and strong, with 105,000 typ. PSI yield strength. Much of it is 
imported from China, hopefully meeting stated specifications. I will have two 
65' towers on a mountain ridge, and will put 2 big antennas (and maybe a 
smaller VHF/UHF) on each tower: a 14 ele. Force 12 C31XR at 2' above the tower 
top (67' elev) and a Magnum 240N at about 8' above the tower (73') elev, and 
the mast pipe will go to a top height of 75' (county maximum limit).  I learned 
all about Chrome Moly from Charles Volgesang who runs Dillsburg Aeroplane works 
in Pennsylvania..He is not on email or the web, but knows how to use a fax 
machine, and he and his wife have dedicated their life to this, and have 
millions of ft. of this stuff at a reasonable price. 
 
Tower companies have expressed deep concern about the deflection issues and 
yield strength with such big antennas up on a mast on top of a mountain (its a 
huge moment arm with much torque, especially when loaded down with ice), and I 
have learend alot about the new 222-G tower specs that specify mountain top 
winds as being 3 times the normal level. The amount of ice on the antennas is a 
big concern for loading. Counties around the country may be adopting this new 
222G tower regulation within a year or two, and this may create havoc for hams, 
as new, more rugged steps may be on the horizon where building permits are 
required. Some tower comanies have recommended I get 5" OD steel mast pipes, 
which will not work for my rotors that take a maximum of 2.5" OD mast!
 
HERE is the question: I have both 2.0" and 2.5" Chrome Moly Mast tubing 
available, each about 18' long.  The 2.0" Mast pipe (I have learned that the 
word "tube" is the preferred term by experts, instead of "pipe") has a wall 
thickness of 1/4", and the 2.5" mast tube has a smaller 0.12" wall thickness. I 
suppose there is a book with a formula for yield strength of tubes of different 
inside and outside radii, made of the same material, its probably an undergrad 
ME question, but I was hoping some of you out there might know off hand the 
answer to this question:

Which is better/stronger/wiser to use for ice/wind strength on a mountain top 
tower, and why?
 
a. The 2.0" OD  0.25" wall thickness Chrome Moly 4130N tube, weighing 100 lbs 
for 18'
b. The 2.5" OD  0.120" wall thickness Chome Moly 4130N tube, weighing 75 lbs 
for 18'

And a second question: would it be wise to put a little 4' or 5' lightning rod 
above the mast pipe to attract electrons, and if so, is there a cheap/easy way 
to affix the lightning rod to the top of the mast, and what should it be made 
of...I am thinking of a sacraficial metal element that might offer protection, 
etc. and would be very hard to see ("invisible") so as not to exceed zoning 
height requirements.

Thanks to all... 73 Ted N9NB   trappaport at austin dot rr dot com
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