[TowerTalk] FW: engineering help? Antenna weight for US Tower TX472

[Patrick Greenlee]

Excellent point, Matt.  I don't think that has been mentioned here 
lately if ever.  I will be checking my new tower as soon as it arrives 
(real soon now, they say.)  I never had a telescoping tower before and 
had envisioned experimenting with antenna height above ground vs 
reception with different propagation conditions.  My tower is rated 
pretty well (550 lbs dead load and 100+ mph wind with the SteppIR DB42 
and TiltPlate at about 100 ft AGL.

Being in "Oklahoma, where the wind comes sweeping down the plain" I will 
be mindful of any reinforcing for full hoist so as to not over stress 
any not quite full hoist uses.  Is it safe to assume that if there is no 
reinforcing for full hoist then all partial hoists are just as or more 
rigid due to reduced lateral moment?

Now I have a jillion and one things to obsess over. ;)  ;)

Patrick    NJ5G

On 6/11/2015 11:40 PM, Matt wrote:
> Just a word of caution regarding lowering crank-up towers (increasing
> overlap) to reduce bending stress...   It is true that lowering the tower
> decreases the overall moment reactions on the tower from the wind load,
> however that strategy does not consider the stresses on individual tower
> elements.   There are some tower designs that have extra horizontal
> reinforcing bars placed in the vicinity of the pressure points of the
> overlapping tower sections when fully extended.   When the tower is
> partially lowered, these horizontal bars no longer line up with the pressure
> points and the moment forces are transferred into a region of the truss
> without the extra strengthening that the horizontal bars provide.   That is
> OK if the tower is lowered enough so that the developed moment reactions do
> not exceed the strength of the tower section "at that point".   The
> calculation of those localized stresses is not trivial.
>
> A case in point would be to only lower the tower a little, where in it is
> probable that the reduction in stress fails to offset the increase in
> localized stress.   Conversely, if the tower design does not incorporate
> these horizontal strengthen members, then the point is moot.
>
> Matt
> KM5VI
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> Patrick Greenlee
> Sent: Thursday, June 11, 2015 1:20 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] engineering help? Antenna weight for US Tower TX472
>
> Rick, I think you hit the nail squarely on the head. As the overlap goes to
> its minimum the leverage of the extended tower portion vs the overlapped
> region's leverage begins to bind the overlapped portion with side load.
> Some users of the DB42 add on aerodynamic winglets to balance the center of
> effort (or center of lateral resistance or drag, if you prefer) so that
> winds do not create a torque around the vertical axis.
>
> I wonder how much weight, strategically located and fastened so as to not
> change electrical characteristics it would take to make the antenna
> "neutral?"
>
> I have a DB42 on my shop floor in 4 boxes awaiting it's turn on my priority
> list.  Its going on a Tashjian tower with a dead weight carrying capacity of
> 550 lbs.  I too estimate the weight of the load at
> 300 lbs or so without cables and coax.  I haven't gotten past the musing
> stage but it looks like there is plenty of margin to do some
> counterbalancing if I have stiction issues with the raising or lowering.  It
> is power down with a 1/2 horse power motor via a 40:1 so it will come down,
> smoothly or otherwise.
>
> Patrick   NJ5G
>
>
> On 6/11/2015 12:36 PM, Richard (Rick) Karlquist wrote
>>
>> On 6/11/2015 10:07 AM, kr2q at optimum.net wrote:
>>> Hi all,
>>>
>>> 2.  What point is the weak link for the 200 pounds?
>>>
>> Here is the surprising thing I ran into with my HDX-5106, a MUCH
>> bigger tower.  It has a MonstIR on top, figure
>> 250 lbs.  Also a short mast and a prop pitch, figure another 50 lbs.
>> Figure another 100 lbs for cables, so lets call that 300 lbs.  Should
>> be fine.
>>
>> What happens is that when I get within about 5 feet of full extension,
>> I start to get noise and vibrations.
>> The working theory is that this is due to IMBALANCE of weight, rather
>> than just the weight itself.  The sections start to bind a little when
>> the overlap gets small.
>>
>> I removed the antenna to change the cables in the tower, and ran it up
>> and down a few times, and the problem went away under those
>> conditions. The MonstIR has a very flexible boom that exascerbates the
>> imbalance problem.  Once it starts to lean a little, the boom flexes
>> in the direction that makes it worse.
>>
>> What I am doing as a workaround is manually stopping the raising winch
>> just before the problem begins.
>> I know from experience where this point is.
>>
>> Rick N6RK
>> _______________________________________________
>>
>>
>>
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