[TowerTalk] Rotator Choice for Larger Yagi

[Jerry Gardner]

Is there a better way to determine if a rotator can handle an antenna based
on its size rather than just its wind area? Lots of people here have
recommended the M2 Orion 2800, which on paper is rated for 35 sqft, but
when I asked the tower installer who will be putting the antenna up whether
he thought the Orion could handle the OB17-4, he said that antenna will
tear an Orion up and strip the gears on the output shaft in short order.

The OB17-4 has a 39' boom, 17 elements, of which the longest is 48', and
weighs 220 pounds. I've noticed that some rotators don't list a sqft rating
at all, but give turning and braking torque in Nm. Is there a way to
calculate how many Nm would be required to rotate an antenna and keep it in
place? I do have an RT-21 controller, which has slow ramp-up/ramp-down to
ease the load on the rotator by starting and stopping it slowly.

The various vendors aren't much help with this as they all say their
rotators will handle the load, while at the same time saying their
competitor's product won't.

73, Jerry


On Tue, Apr 26, 2016 at 12:13 PM, jimlux <jimlux at earthlink.net> wrote:

> 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>)
>
>
>
>
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