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Re: [TowerTalk] Antenna spacing question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna spacing question
From: TexasRF@aol.com
Reply-to: "Tower and HF antenna construction topics." <towertalk@contesting.com>
Date: Wed, 27 Jan 2010 09:38:28 EST
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
 
Hi Jim  and Pete, measuring the deflection of a mast is not the data  that 
matters in wind survival. With a given diameter and wall thickness, the  
deflection will be about the same for all steel alloys.
 
What is needed is the stress that will be applied so it can be compared  
with specifications of the material used. Measuring the stress is not a home  
project but we have plenty of computer software available to accurately 
compute  the stress.
 
I think Steve, K7LXC has a program that will do the job.
 
73,
Gerald K5GW
 
 
 
In a message dated 1/27/2010 8:26:30 A.M. Central Standard Time,  
Jim.thom@telus.net writes:

Message:  2
Date: Wed, 27 Jan 2010 05:51:06 -0500
From: Pete Smith  <n4zr@contesting.com>
Subject: Re: [TowerTalk] Antenna spacing  question
To: "Tower and HF antenna construction  topics."
<towertalk@contesting.com>
Message-ID:  <4B601A9A.2040305@contesting.com>
Content-Type: text/plain;  charset=ISO-8859-1; format=flowed

Isn't schedule 80 water pipe?   And if so, aren't you inviting a bent 
mast by trying to put a 2-element  40m yagi 10 or more feet above the 
tower top?

73, Pete  N4ZR
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## Not necessarily.  The HDX-689 crank up  tower uses  sched 80  for the 
legs
of ALL  5 x  sections.   It's all  46 ksi steel.   I agree though,  his is 
probably
sched 80 water pipe,,.. and probably  1.9" OD   x   1.5" ID  x .200" wall 
...
which is typ  just 30-35  ksi.  The cheap and dirty way to test a mast is to
clamp it to your  deck.. with XXX feet  sticking out in mid air, 
horizontally.

Then  place XXX lbs of weight on the very end.. and measure the deflection.
If  the yagi is say 7 sq feet... and a 70 mph wind....[20 psf].... = 140 
lbs.  
7 sq feet  and a 100 mph wind..[40 psf]... =  280 lbs   etc.  Add  a safety 
factor

I once had  a 204BA   mounted  11' above the same tower as his....
[ with one straight  section added in the middle, and also guyed].... on 
the  same 
1.9"  OD  sched 80 pipe   [1976]   It also had   climbing steps  welded to 
it ! 
It took  65 mph winds.... but I  would never do it again. Climbing that mast
freaked me out.  If he  used the  bigger 2.275"  OD pipe in sched 80... or
even the  2.875"  sched  80 pipe, [ .276" wall]  he would be  ok,
depending on  local wind condx.   His  40m yagi may  well  screw up his
15m array.  

I added  several more  #5 straight sections of  Delhi.. [ right to the 72' 
top of tower]
and  12 x guys wires  [ 4 x per level x 3 x levels]    and then 2 x  HB  
monobanders... 
+ a prop pitch. 15m-26' boom    20m-36'  boom.   Prop pitch was way down 
inside the tower.
The top 2 x  sections had a slight twist in em, after 68 mph gusts!    
Delhi's  are very
limited as to what you can do with them.  I  gave up on the delhi.. and 
went to  30"  wide
commercial   L+R tower [ 350 lbs  per  20' section] ... and never looked 
back.  

The Delhi's  were originally made for  deep fringe   TV  reception. [late 
50's]
One large yagi, just above the top  bearing.. and perhaps some smaller 
stuff... 4-6' 
above that.... and  it's  maxed out.  We had a local fellow here, who  
overloaded
a  64'  delhi  [ no guys]  and it   folded right in half, during a 
windstorm ! 
The 64' Delhi is rated for 3 sq  feet.... NOT  10-15 sq ft. 

later... Jim    VE7RF



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