[TowerTalk] Rotator Choice for Larger Yagi

[jimlux]

On 4/26/16 11:46 AM, Máximo EA1DDO_HK1H wrote:
> PST-61D has 39sqft.
>
>

>>
>> The one thing that still concerns me about the Orion, however, is that it's
>> only rated to 35 sqft. The OB17-4 is 27 sqft, so there's not a lot of
>> margin there.
>>



It's not clear to me what a "square foot" rating for a rotator would 
be.. Inertia loads would be in some sort of mass *length^2.

Square foot would be for wind drag forces:  Unless you're talking about 
the "side" (radial) load on the bearings (which depends a LOT on the 
mast length and whether there's other bearings or mounting points.

But for "turning in the wind", you'd need to know an area and a radius 
from the axis of rotation to turn that into a torque (e.g. will it 
overpower the brake or rip the teeth off the gears).


Maybe they're using "square feet" as a shorthand for "size of antenna 
and polar moment of inertia".  Square feet cross section is given for 
most antennas, polar moment is not.   Since most antennas are fairly 
similar in construction, knowing cross sectional area (square feet) 
probably correlates well with overall size and mass.

(unless you use solid steel bar as your boom, and silver plated steel 
bars for the elements. <grin>)