[TowerTalk] KT34XA Torque Balance

[Tod - Minnesota]

Mike:

I have been through the same consideration -- with a KT34XA
in Minnesota and a TH7DXX in Idaho. I do not have an
absolute solution, however I will share with you what I plan
to do in Idaho this summer. 

In Idaho (at my QTH) the winds are considerably stronger
than at the Minnesota QTH. The Idaho winds tend to be
considerably more forceful in Spring and in Autumn and much
less in Winter or Summer (Idaho QTH). The extreme winds
quite often reach a steady 40 mph with peaks in excess of 60
mph. (30 mph is described by the local WX folks as 'breezy'
and the term 'windy' reserved for more substantial air
flow).

My plan is to install a 'wind sail' slightly above the
antenna boom.  This will be a single rectangular piece of
metal (probably aluminum; maybe galvanized sheet metal)
mounted on an arm that extends out from the mast. The arm
will be fastened to the mast using a clamp that allows the
length of the arm from mast to sheet metal to be varied.
This way I will be able to adjust the angle of the sheet
metal 'wind sail' relative to the antenna boom and also the
moment arm length. The size of the sail will be governed by
my analysis of the torque that is needed to counter wind
torque of the antenna. There are a couple of QEX or
Communications Quarterly articles that discuss how to
calculate the wind force torque generated for antennas.
Unfortunately, my references are in Idaho and I am in
Minnesota at the moment. Perhaps someone else on the
reflector can supply references to those articles. 

The general principle is that by fastening a sail slightly
above or below the boom and adjusting the sail area, the
length of the arm holding the sail and the angle relative to
the boom one can counter the wind torque placed on the
rotator sufficiently so that the antenna will not slip in
the rotator clamp (except at very, very high winds) nor will
it destroy the rotator gears. The counter torque does not
need to be perfect  --- just enough so that the maximum wind
torque does not exceed what can be countered by the rotator
gears/and or brake. Bear in mind that this does not come for
free -- the force needs to go somewhere and adding the 'wind
sail' will increase the wind loading on the mast and tower
even though the antenna may not rotate in the wind. You are
exchanging rotational torque for wind pressure on the
antenna and tower.

Tod, KØTO

 

> -----Original Message-----
> From: towertalk-bounces at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of
Michael Tope
> Sent: Friday, January 07, 2005 2:06 AM
> To: TowerTalk
> Subject: [TowerTalk] KT34XA Torque Balance
> 
> Has anyone on the list attempted to wind torque balance a
KLM 
> KT34XA? I just went thru the numbers and my calculations
show 
> that the KT34XA should be a great weather vane. The
factory 
> specs show that the boom-to-mast connection is about 20" 
> inches behind the physical center of the boom. Presumeably

> this is done to compensate for the fact that most of
weight 
> from the elements is concentrated towards the back-end of
the 
> antenna boom. 
> 
> My calculations show that the torque balance is decent if
the 
> wind is blowing at 45 degrees relative to the line of the 
> boom, since the wind induced torque from longer section of

> boom forward of the mast balances pretty well with the
wind 
> induced torque of the large element surface area that is 
> concentrated rearward of the mast. When the wind is
blowing 
> perpendicular to the boom, however, there is quite a large

> imbalance due to the 20" offset between the mast clamp and

> the physical center of the boom.  The antenna is at a club

> station, so pointing it into the wind is not an option
(most 
> of the time nobody is around).
> 
>
http://www.its.caltech.edu/~w6ue/Spalding/Spalding_Stack_5a.
jpg
> 
> I am planning on pinning the mast that holds this thing to

> the OR-2800 rotator this weekend (I am tired of it 
> windmilling in the rotator mast clamp and wrapping the
coax 
> around the mast), but I am concerned that unless I
properly 
> wind torque balance the KT34XA, I am going to end up
finding 
> the next weakest link in the system (rotator parts for 
> instance) the next time the Santa Ana winds blow hard
here.
> 
> Anyone been around the block with this thing?
> 
> Thanks and 73, 
> 
> Mike, W4EF..................
> 
> 
> 
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