[TowerTalk] Question on mast height above tower

[TexasRF at aol.com]

Randy, a quick look at your proposed system shows that at 90mph, the  total 
force applied to the top section is 666 foot pounds. 195 fp for the 40m  
ant, 315 fp for the 6m beam and 150 fp for the Triband beam.
 
If that force was from a single large antenna it would be the equivalent of 
 22 sq ft mounted one foot above the tower top.
 
This is with NO safety factor and does not include loading for the 2" mast  
or feedlines.
 
This is more than the tower rating and you will have to decide if the risk  
is acceptable. The bottom line is that the system would fail with a wind 
speed  somewhat less than 90mph, maybe 70mph or so.
 
The mast proposed is an overkill. My computer program shows that a 2" OD  
mast made with 1026 DOM, .125" wall will fail at 130 mph. This material is  
available at Texas Tower of course!
 
Hope this helps.
 
73,
Gerald K5GW
CEO Texas Towers
 
 
In a message dated 8/21/2013 8:25:38 A.M. Pacific Daylight Time,  
RShirbroun at newportlabs.com writes:

I didn't  get any comments or suggestions to my previous post, so I'll try 
again.
If  this is a dumb question, feel free to tell me!

I have just erected a  new HDBX 40 in place of my 30 year old HBX 48,
sacrificing 8' of height to  gain some strength and load capacity (and 
peace of
mind!).  The stock  rotator plate allows 2' of mast below the top plate and 
I
will be using the  heavy duty Yaesu thrust bearing (along with the Yaesu 
heavy
duty 2800  rotator plus the absorber plate).   I have added steel angle  
braces
to reinforce the rotator plate.

I'm looking at using a 15'  chrome/steel 2" mast (in place of my previous 9'
mast), so 13' of the mast  would extend above the tower.  The mast would 
support
a TX38 tribander  just above the top plate, a 6 m beam half-way up, and a 
40m
rotatable  dipole near the top, 12' above the top tower plate and the  
thrust
bearing.  The mast, of course, is very heavy, weighing around  75lbs.  The
tribander weighs 40lbs and with 5 ft2 surface area; the 6  m. beam weighs 
10lbs
with 1.5 ft2; the dipole weighs 10lbs and is 0.5  ft2.

Is this too much mast for this tower?   (BTW - I'm aware  the boom length
exceeds the 10' maximum for this tower, but the HBX 48  handled a similar
tribander for 30 years, with occasional severe  ice-loading, without any
problems.)

Thanks and 73,
Randy,  ND0C



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