[TowerTalk] Question on mast height above tower

[Gene Fuller]

These numbers looks like calc's for bending moment at the top of the tower.

What about torque ?

Would it be possible to put the rotor plate lower in the tower and pick up 
some extra restraining moment with "overkill" in the mast ?

However, this still wouldn't help the torque problem.  If he's stuck with 
this tower and the given antenna family, how about a leg anchor plate and 
torque bars at the top of the tower ?  This would also help the bending 
moment problem. Pretty soon a more heavy duty tower, or less ambitious 
antenna system, starts looking good !

With some extra $$$ and a little engineering there will  be a good answer.

Gene / W2LU

----- Original Message ----- 
From: <TexasRF at aol.com>
To: <RShirbroun at newportlabs.com>; <towertalk at contesting.com>
Sent: Wednesday, August 21, 2013 12:41 PM
Subject: Re: [TowerTalk] Question on mast height above tower


> 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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