[TowerTalk] W6NL 40m Moxon

[Grant Saviers]

I think there are two factors, one is the tower loading  and the other 
is the rotator loading.

Each antenna, plus the mast itself, contributes a bending moment on the 
mast from the wind load.  The mast calculators translate these moments 
into a single number used to determine the total bending load and thus 
the material stress to see if the mast will bend at the point it exits 
the tower support.  That same number can be used to see if the tower 
will withstand the load.

The rotator loading is much more complex since it involves the system 
dynamics - the rotational inertia of the mast and antenna assembly, 
perhaps a bending moment applied to the mast/rotator connection, the 
dead weight load, and the dynamics of the antennas + mast + tower.

One thing I've observed is even in a hard blow, wind is not very 
"steady", my towers are in a turbulent wind field which causes all sorts 
of motions by each antenna and coupling of these oscillations thru the 
tower to other antennas.  I would guess there are few ham towers so 
isolated to not be in a turbulent wind field.  Each element half is a 
mass + spring, as is the boom an assembly of those mass + springs and 
the mast a further assembly.  Then the tower has a torsional rigidity 
and inertia, a yet bigger mass + spring.  So it is a very complex 
dynamic system.

There may be some uniform wind load unbalance from the projected boom 
and element areas x distance that if in a uniform wind field would cause 
a net torque in one direction on the rotator.  It seems to me to be 
unlikely that that this static force is large enough to overload most 
rotators for most yagis properly positioned.

What seems likely IMO is the dynamics of heavy wind loading cause the 
slipping of mast clamps, boom to mast clamps, broken brakes, stripped 
gears, etc.  No stock rotator I know of has zero backlash, even my prop 
pitch moves angularly a bit in turbulent wind. "Bouncing" against the 
brake or against the gear backlash is probably the most significant 
force a rotator has to withstand (assuming that the start/stop is 
sensibly ramped up/down to manage the kinetic energy - as in larger 
Yaesu, GH controllers, Orion). Any "slop" in the boom/mast or 
mast/rotator or rotator/tower mount connection means more energy can 
accumulate in the angular oscillation before something in the rotator 
tries to stop it.  Worn rotator mounting holes are a sure sign.

Yaesu makes a crude stab at specifying the rotational inertia limit, 
some others have no comment.

For "tube sleeve top" towers, very little bending moment should be 
passed thru to the rotator.  For stock tower "thrust bearings" (one of 
the most misused terms in ham radio) the bearings I've used/seen will 
rotate in the horizontal plane enough to pass thru some moment.  As to 
dead load, the rotator specs I've read put those values way above any 
likely mast loadings.  In fact the advice is to insure sufficient dead 
load to keep the rotator bearings fully engaged to resist any applied 
moment.

To hazard a CW based guess of failure distributions: mast slip > boom 
slip > rotator failure > mast failure > tower failure.  The first three 
probably driven by the system dynamics, that latter two by the static 
loading.

and then "more towers are killed by falling trees than trees by towers".

So computing wind load areas to more significant digits, may not be 
worth it, IMO.

The 4'x4' plate is interesting dynamically.  I've seen roadside signs 
oscillate almost +/- 90 degrees in a 40 mph wind.  If the 4x4 plate did 
that the rotator would be likely destroyed in short order.   And slender 
structures in the wind can have vortex shedding induced oscillations.  
Recall Galloping Gertie.

Grant KZ1W




On 4/13/2016 6:55 AM, Stan Stockton wrote:
> If calculating wind load for the purpose of deciding which rotator to use, I would think the boom length would factor in.  Surely a square plate 4'x4' mounted to the mast would put less stress on a rotator than a 50 foot boom antenna with the same 16 SF calculation?
>
> At any rate if the difference in the various ways of calculating the wind load mentioned makes the difference in what you believe will survive, I would think you are cutting it too close.
>
> Stan, K5GO
>
> Sent from Stan's IPhone
>
>
>
snip..