[TowerTalk] Antenna wind load vs tower specs.

Jim Thomson jim.thom at telus.net
Mon Apr 30 15:26:45 EDT 2018

Date: Sun, 29 Apr 2018 13:11:55 -0500
From: <kstover at ac0h.net>
To: "'Steve Walter'" <stevewalter90 at gmail.com>,
<towertalk at contesting.com>
Subject: Re: [TowerTalk] Antenna wind load vs tower specs.

<I'm planning one now and am using the Projected Area number on the JK site.
JK Mid-Tri = 16sqft
JK 12/17 Jr =  7.8sqft (I use 8)
JK 402T = 9.3sqft ( I use 9.5)
20 foot out the top 3" mast 5 sqft
<All that is 38.5 sqft and I round it up to 39.
<Using the RevG numbers from the JK site gets me to 40 sqft of antennas plus
<5 sqft of mast. So, 45 sqft total.
<It's not going on a Rohn tower. I'm getting a AN Wireless HD70 which is
<rated to carry up to 52.5  sqft at 100mph (RevG, Exposure Category B, soil
<type clay).


##  That 52.5 sq ft rating  will be the top of the tower, or  1 ft above the top of the

##  For loads above the top of the tower, on a 20 ft mast protruding above the top of the
tower,  its a simple set of math calculations to  easily determine if you are within  the max
wind load rating of the tower.   No rocket science here folks. 

##  Kurt Andress, K7NV  has it figured out, and he is correct.  http://lists.contesting.com/_towertalk/1999-10/msg00368.html
The same simple maths can also be used for yagis  that are mounted below the top of the tower, like on a side mount or ring rotor,
or fixed to side of tower  etc.   Leeson also had it figured out. 

##  Think of the free standing tower as a huge  70 ft  Torque  wrench.  Then add another 20 ft to the end of it, so its now 90 ft. 
See where Im going with this ?  The difference is.... instead of applying one big windload via  one big  yagi  at  70-71 feet,  you 
are now applying   3 x different windloads, at varying levels on the 20 ft mast, like in your case,  70-71 ft for the Mid –tri....
then 80 ft, for the 12/17 Jr...... then  90 ft for the 402T....and  80 ft for the average height of the mast.    

##  100 mph =  25.6 lbs per sq ft......per the generic formula.    Force   @  100 mph  =  Projected  area  X  1.2 
That equates  to .... 
16 x 1.2 =  19.2  sq ft for the mid tri    =       19.2 X 25.6  =      491.52  lbs      X  71 ft   =   34,897.92  ft lbs  at base of  tower
7.8  x 1.2  =  9.36 sq ft for the 12/17 jr.   =    9.32  X  25.6  =  239.616  lbs   X  80 ft   =    19,169.28  ft lbs at base of tower
9.3 x 1.2  =   11.16  sq ft for the  402T.  =   11.16  X 25.6    =  285.696 lbs.     X  90 ft   =  25,712.64 ft lbs at base of tower
5.0 x  1.2  =  6.0  sq ft  for the mast  =      6.0  X 25.6  =    153.6 lbs         X  80 ft   =  12,288.00  ft lbs at base of tower

##  Total ft lbs at base of tower =  92,067.84   ft lbs. ... at 100 mph. 

##  Ok, lets compare that to say   52.5 sq ft  at  just 71 ft level...... IE:  1 ft above top of tower....and also in a 100 mph wind. 
  52.5 sq ft  X  25.6  lbs    X  71 ft  =    95,424.00   ft lbs  at  base of tower. 

##  Per my above calcs....  Your tower will just handle your proposed 3 x yagis  + mast... with a tiny bit to spare. 
But then you also have to factor in any coax,  remote switch boxes, baluns,  climbing steps on mast, surface are of rotor
etc, etc.   I would also be concerned about the  top portion of the tower, where the mast sits inside the tower. 
Per leeson, take the  calculated load of the yagis  + mast... this time at the TOP of the tower, where mast enters top of tower
and then divide by  distance between top  of tower..and whatever is the 1st item below the top  of tower.    IE:   IF  1st item
below top of tower is say a 2nd thrust bearing..and say 2nd bearing is 6 ft below top of tower, then  divide by 6.  If 2nd bearing
is  only 4 ft down, then divide by  4.  If no 2nd bearing used at all, and instead the rotor is  down say  4 / 6 /8  ft... then divide by 
4/6/8 ft.  Then you will know what the resulting side force is on the.....  top bearing.....and whatever is  the 1st item below the top of the

##  This will no doubt be a huge side force...and the top of the tower has to be able to handle it... or the top section of tower has to be
modified and beefed up a bit to handle it. 

##  Note, say  2 x thrust bearings are used.... and a rotor.    Say the 2nd bearing is down 4 ft....and  rotor is   down  6 ft.   The resulting side forces
are between the 2 x bearings.   The portion of the mast below the 2nd bearing...... between 2nd bearing and rotor, does not enter into the equation. 
In this example, the   top 4 ft of tower  has to be able to handle  the side loading.  BUT the  portion of the tower, that resides below the  4 ft level from
http://k7nv.com/notebook/topics/windload.html    top... also has to be super strong.   IE: the entire top section has to be stupid strong when using  
20 ft of mast protruding out the top of the tower. 
Strong CM mast is one thing, but the entire top section has to be designed, or modified a bit to handle the worse case wind loading.   

###   Max the mast can handle is  =   yield strength in psi  X  section modulus.    Section modulus is a function of   OD, ID,  and wall thickness.   

##  IF it was me, I would use the  40m version of the mid tri,  then install the JK-801  at top of mast...and inline with the boomS below it.   Then the
wind wont even... see the  80m rotary  dipole.. when the 2 x yagis  have their els  broadside to the wind.     Another option  would be the  40m version 
of the mid tri..and  the  80m rotary diplole 6-10 ft above it.    Then a  LOT shorter mast could be used... IE:  dump the  17  /12 M yagi  from the mix. 
You would not require the  3.0  x .375 wall  CM mast either.  A  3.0  x  .25 wall  CM  mast would be ample...aprx  7 lbs per foot.   A  3 x .375 wall 
mast will weigh aprx  50 % more... like  10.5 lbs per ft.  30’ of that size is gonna weigh aprx  315 lbs.   Toss in the weight of all the yagis, prop pitch
rotos, coax, any mast steps, two thrust bearing plates and bearings etc, etc, etc,... and the total weight  will be sky high.  Is the proposed tower
even capable of that much weight ??    Then toss in any possible heavy wet snow or ice loading.   

##  The upper HF bands will be going down the tubes   soon.   IMO,  you would have more fun with the  JK-801..esp since its swr is dead flat from 
3500-4000 khz.... but thats just me.   I have no interest in 10 or 12m.   80-40-20-17-15m   would be my focus.. but again, thats  my personal opinion. 

##  This proposal of yours will amount to a large expense...and a one shot deal... so  it needs to be well thought  out.   Having yagis on an essentially
dead  10 + 12 m band makes no sense to me.  Inverted vee  at  68 ft  for 80m ... vs a 80m rotary dipole at  80-90 ft is apples and oranges.   How much
fun do you want to have in the long  run ?    If you were hell bent for 10M..for say contesting or some other application, a simple 10m yagi, or  10/12 m
yagi could be side mounted to the proposed tower, when those bands are actually open.... during the next cycle. 

Later... Jim   VE7RF       

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