[TowerTalk] Mast length in and out of tower

[Jim Thomson]

Date: Mon, 8 Oct 2018 13:00:04 -0700
From: David Gilbert <xdavid at cis-broadband.com>
To: towertalk at 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