Date: Mon, 8 Oct 2018 13:00:04 -0700
From: David Gilbert <xdavid@cis-broadband.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Mast length in and out of tower
<For what it may be worth, I suspect that DX Engineering's calculator is
<pretty conservative.? It says my setup won't pass at 100 mph, and that
<my OB2-40 alone (no OB-16-3 at all) at thirteen feet just barely does.
<Dave?? AB7E
## Thats cuz the DX engineering online mast calculator uses the extreme
stringent
222- Rev G...per DX eng...which includes simultaneous seismic + ice loading
etc, etc.
## EIA specs are now using EIA-REV H as of 2017.
## I use the DX engineering superb yagi mech software for both the booms,
eles,
and also the mast. You can choose any spec you want. Most folks,
including JK ants
etc, will use the .. NO spec, which IMO, makes the most sense, since then a
height is not
required, nor how much terrain is covered by trees... IE: exposure
conditions within XXX
feet of the tower in question.
## NO spec is more stringent than the obsolete 222-C spec. NO spec is just
max gust speed
applied to various points on the mast, factoring in the wind load for each
yagi.
## IF you installed an accurate wind speed indicator at the very top of the
mast, then entered the
max wind speed gust into the software, it spits out dead on accurate
results..which is plenty good
enough for 99% of most applications. The DX eng yagi mech software will
also factor in ICE
loading if you desire..and how thick an ice layer.
## Think of the mast as a long Torque wrench, with loads applied at various
points along the mast.
Total bending moment at the bottom of the mast where it enters the top of the
tower can not exceed the
max bending moment of the tubing used. And this all assumes the top of the
tower is strong enough etc, etc.
## In its basic form, max bending moment is just section modulus of the mast
tubing X yield strength in PSI.
Bigger diameter tubing with a thinner wall is a lot stronger than small
diameter tubing + a thick wall. Reason
is the bigger diam tube with the thinner wall has a bigger section modulus
vs the smaller diameter tube + thick wall.
This assumes both masts have the same yield strength.
## case in point would be to compare a 3 inch OD mast with a .25 inch thick
wall VS a 2 inch OD mast..
with a .375 wall. They both weigh..and cost the same, yet the 3 inch mast
has a much bigger section
modulus.
## 222- rev f-g-h have taken the basic concept to an extreme..and then
some. Thats why your mast that
wont pass muster using the 222-G spec at 100 mph, will easily handle your
100+ mph winds. Most 100 mph
rated yagis using NO spec, are only good for 55-65 mph using the more
stringent G spec. Even if you assume
the wind at the top of the tower is 20-30 mph faster than at ground level, the
maths still dont add up correctly.
## The dx eng mast calculator, while tech correct for the more stringent G
spec, I suspect is more of a subtle
marketing tool. Having said that, local regulations may well want you to use
the more stringent specs for a given
installation. The software is cheap, and easy enough to use...and is an eye
opener, esp when entering real old
yagi designs from 30-50 years ago. It will also explain why masts have
folded over, in the past 40 years.
35 ksi water pipe just doesnt cut it in the real world..with yagis way up the
mast.
Jim VE7RF
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