[TowerTalk] Broken masts/ here's the mast software url. !!

[Jim Thomson]

Date: Thu, 6 May 2010 10:50:56 -0400
From: "Mark Robinson" <markrob at mindspring.com>
Subject: Re: [TowerTalk] Broken  masts

I notice that Texas Towers carries a line of steel masts.

A friend recommended the 20 foot steel 0.25 mast. He said that if I nested 
about 7 feet or so in the tower and kept the rotor lower it would add 
strength to my setup. I don't want to scrimp on the mast. We can get some 
horrendous local storms here in the summer


I am trying to figure out how much that mast will weight.


http://www.texastowers.com/steelmasts.htm



Mark N1UK

##   94-96 lbs.    The Texas towers mast are made from  1026 DOM   [87 ksi]

##  In my case, with the huge load of 4 x ants... I went to 2" OD  x 3/8" thick ,
heat treated 4130 chromolly steel, 107 ksi.   I'm stuck with using a 2" mast.   My
1st choice  would have been a 3"  mast.. or a 2.875" mast.   Either of those, in 1/4" wall
is one helluva lot stronger than 2" od x 3/8"  wall. ..[and weighs the same 140 lbs]

## In my case,  I have  5'  8"  of mast inside the tower, with 2 x thrust bearings. The small K7NV
PP  rotor hangs below the rotor plate [normal for any PP.. and same idea as the HDR-300.  This allows
even more mast inside the tower.  This leaves 14'  4"  of mast above the tower.   Since the mast will be installed
inside a HDX-689 tower, the same crane that installs the tower, will also install the mast.  After that,  climbing
steps to be installed on the mast. 

##  Ok.. here is the url to the mast strength software.  BEWARE, the sq footage  you enter for EACH yagi, 
must be the PROJECTED AREA OF A CYLINDER !!    IE: length x width for the el's.   Now 99%  of most ant 
makers  will use the old eia standard... and  take the projected area and multiply it by .67... to arrive at 
EFFECTIVE windload.    IE:  10 sq feet  of projected area = 6.7 sq feet of effective.   That's all fine and dandy,
and ant maker's  like to quote the lower number.. which is valid.    Just  DON'T use the lower number  EFFECTIVE
area they quote.. and  stuff it into this mast software.. other wise you will applying the drag coefficient TWICE. 

## The software wants to see  projected area.. and applies it's own  drag coef.   In the case of my F-12 yagi's,  I have
to take the F-12  effective area specs.. and multiply em all by 1.5  to arrive at the original Projected area. 

##  IE:  F-12  sez ..'10 sq feet'     The projected area of that ant is 15 sq feet.    15 sq feet is what is then entered into
 the mast software.  

## also BEWARE. The ant area specs will  be  for  EITHER the boom, OR ALL the els.  It's one or the other.  It's always
the GREATER of the two... which is normally the els for a hf yagi.  In some cases, the two areas will be equal.. or very close. 
IF the boom area is geater than the total el area, then DON'T enter the boom area into the software,,, if the yagi will be
STACKED with other yagi's.  In the case of stacked yagi's,  you enter the area of the ELS.. since all els, of all yagi's  will
be broadside to the wind.    This will result in the worse case senario.  Then do another set of mast cals.. this time using
the area of the booms  for each ant.   You will see that most of the time, the windload of all the els  will be greater.

## also BEWARE, when you add something like a 80m rotary dipole to the top of a mast, the 80m dipole will be IN line  with the
boom's  below it.    [ IE:  at right angles to the yagi els]   In this one off case, the normal sq footage of the 80m dipole is NOT
entered into the spreadsheet.   With all the yagi els  broadside to the wind, the 80m dipole is pointed into the wind... like looking
into the end of a pencil. In this case, I enter  .1 sq ft.     However, I also enter the projected  sq footage of the 8' wide x 1" diam
cross support, that is 3' above the 80m dipole... and is used to support the LL wires... which IS broadside to the wind. [ EF-180B]

## In the bottom right hand corner of the spread sheet, you can increase the size of the spreadsheet, to make it much easier
to see. 

##  also BEWARE, the 'mast' the spreadsheet wants to see.. is ONLY the portion of the mast ABOVE the TOP of the tower.... IE:
the Un- supported portion of the mast.  A  10'  x 3"  diam mast  = exactly 2.5 sq feet of projected area.   3" / 12 =  .25'
.25' x 10' = 2.5 sq feet.   The software will do all this for you.... and THEN apply it's own drag coef.  

## The mast spread sheet will provide hrs of entertainment.. and will be a real eye opener.  Then you can do cool stuff... like
move yagi XXX  up the mast by 1-3'.. and see the results.  The other one is  trying different  wall thickness's... and also different
yield strengths [ksi]    Run of the mill  water pipe is typ 35 ksi.  6061-T6  is 40 ksi.    1026 DOM  will range any where  from a low
of 60 ksi.. to as high as 87 ksi.. and sometimes 90 ksi.   UN- heat treated 4130 chromolly steel is typ 70 ksi... and when heat treated,
will  range from 104 ksi.. to as high as 125 ksi.  Also beware,  you can buy  41340 chromolly steel  at most steel places... and 95% of it
is  UN- heat treated   plane 70 ksi stuff.    Reason is... the UN-heat treated stuff is easy to drill, cut, bend, weld, etc, etc.  After all that is
done.. THEN the finished piece is sent in to be heat treated.   You CAN'T  weld heat treated 4130 chromolly steel.... you will ruin the 
heat treating.  The result is the portion of steel  surrounding the weld  will be  down to  70 ksi.   Same deal when you try and weld
6061-T6 AL.... it will drop down to T-0... then eventually work harden  back up to T-2.

## The latest  EIA-222 F /G  specs are more stringent than the old 70's era  EIA-222 rev D  specs.  A 204BA that's good for 100 mph
under the REV-D specs, will only be good for 80-85 mph, when using the newer 222- rev F/G  specs.  The newer specs are used for
this mast software.. are are conservative. 

## also beware,  when you think it's  '60mph' in the back yard.. at face level.... rest assured, it's probably 15 mph faster.. when measured
at top of tower.  Install one of those  solar powered, wireless peak reading  wind speed devices at the top of the mast.. and a 2nd one
10'  above the  grnd... and you will  be in for a rude awakening... when comparing the 2 x readings.           



http://thebont.com/spreadsheets/AnalysisOfAntennaMastStrength.htm


later.... Jim   VE7RF