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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