Run the calculations.
This article might help:
http://www.w9smc.com/BH/2007/k9cc%20article.pdf
You will need this table to take your wind load to PSI.
http://bristolite.com/interfaces/psi_wind.aspx
See how much the force that a beam of X windload places on the mast at the
point where the mast exits the top of the tower if it is mounted Y feet above
the tower.
Basically it is the windload of the antenna (at max desired windspeed)
multiplied by the moment arm (length of mast between top of tower and antenna)
minding the units. That will tell you if you have good enough pipe or need to
go to CM. I did these calcs 10 years ago and have them written down for a
XM240 mounted 1 foot above the tower and a KT34XA mounted 10 feet above the
tower on the same mast. I ended up using CM but can't remember off the top of
my head what the numbers were. They are in a notebook somewhere and I could
dig them up but a quick recalc shows that it would be almost 30000 in lb at the
point where the mast exits the top of the tower (neglecting the load of the
mast itself).
Here's the calcs:
XA 9 sq ft windload mounted 10 feet up on mast
XM240 5.5 sq ft wind load mounted 1 ft up on mast
Antenna Loads
XA 9 sq ft X 25.6 lb/sq ft = 230.4 lb
XM240 5.5 sq ft X 25.6 lb/sq ft = 140.8 lb
Load translated back down to the top of tower
XA 230.4 lb x 10 ft = 2304 ft lb
XM240 140.8 lb x 1 ft = 140.8 ft lb
Both antennas together are 2444.8 ft lb
Converted to in-lb
29337.6 in lb
That doesn't count the load of the mast so you have to add that in there too.
Now look and see if your mast is capable of that load with some safety factor.
This is the kind of stuff where if you are really serious it might be worth
getting a PE involved to take in account all the factors, but the calcs aren't
too difficult and will give you a better warm and fuzzy feeling.
Earl
N8SS
Disclaimer: I'm a EE not a ME/CE.
----------------------------------------------------------------------
Message: 1
Date: Tue, 12 May 2015 16:36:13 -0700
From: "Richard (Rick) Karlquist" <richard@karlquist.com>
To: Patrick Greenlee <patrick_g@windstream.net>,
towertalk@contesting.com
Subject: Re: [TowerTalk] Mast inquiries
Message-ID: <55528E6D.5050207@karlquist.com>
Content-Type: text/plain; charset=windows-1252; format=flowed
On 5/12/2015 12:22 PM, Patrick Greenlee wrote:
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
Patrick NJ5G
I have a similar installation. The original MonstIR on a homebrew
tiltplate turned by a K7NV prop pitch on an HDX-5106. The antenna is
3 feet above the tower, just enough room for the tilt plate and
some cable loops. I started out with 0.12 wall mast. It stayed up
through various storms, but I felt it was marginal. I changed to
0.25o inch. I don't believe it is CM, just decent mast material
(as opposed to water pipe). With this mast I can sit on the end (top)
of the mast and bounce up and down with the tower tilted over and it
moves very little. Not scientific but it has worked for me.
Rick N6RK
------------------------------
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