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Mast Strength

To: <>
Subject: Mast Strength
From: (John Brosnahan)
Date: Mon, 02 Dec 1996 10:14:27 -0600
At 09:38 AM 12/2/96 -0500, you wrote:
>I am erecting a quad antenna at the top of a Rohn 45, tilt over tower. To
>clear the top guy wires, I need to extend the  mast some 6-7 feet above the
>top of the tower. The mast I intended to use is a Rohn M200, which is 2"
>outside diameter with a 16 ga. wall (.065). It appears to be mechical steel
>tubing, low carbon, electric welded (seam visible inside). According to the
>specification books at hand, it would be either hot rolled or cold rolled,
>but since it is galvanized, there is no way of even guessing. This is tubing
>designed for mast service, not conduit or pipe.
>Question: Is this mast adequate for 6 sq feet (35#) of antenna, 7 ft above
>the thrust bearing? Location is north Florida, Wakulla cty, about 15 miles
>from the Gulf of Mexico.

Without knowing the exact material it is impossible to say for sure.  But if one
makes an assumption that the yield strength of the material is 60,000 psi
then the survivability of the mast is 70.4 mph.  (It does not scale linearly
on yield strength--doubling the rating to a 120,000 psi material will only
get you
to 99.6 mph.)  (goes as the square root!   2x strength gives you sqrt 2 (1.4)
times the rating)

Going to a 0.125 wall mast will get you to 93.3 mph and going to a 0.25 wall
will get you to 119.9 mph.

All of this assumes that there is no additional antenna area due to coax
or switches or matching, etc.  

My suggestion is that 0.065 wall mast is just too light weight.  There is no
margin of safety built into these calculations--only that it will fail at 70
assuming that 60,000 psi is the strength of the material--probably a reasonable
estimate but it might be lower strength (or possibly a little higher).  I just
wouldn't do it in Florida without a lot of safety factor and without knowing
for sure
what the yield strength is.  I'll run the numbers again if you can get the exact
psi rating from Rohn.

gl es 73


John Brosnahan
La Salle Research Corp.
24115 County Road 40
La Salle, CO 80645 USA

voice 970-284-6602
fax   970-284-0979

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