[TowerTalk] Question on mast height above tower

[Brian Alsop]

Another possibility.
Put the rotary 40m about 1' above the tribander and at right angles to it.

The extra few feet of height lost won't amount to much.  Also you would 
still have the advantage of pointing it at the signal of interest.

That will help with the load, but won't impact the torque.

73 de Brian/K3KO

On 8/21/2013 17:56, Gene Fuller wrote:
>
> 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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