[TowerTalk] Rotator Choice for Larger Yagi

Tue May 3 14:14:23 EDT 2016

In my opinion, and in general, turning/braking torque as a spec makes 
more sense than surface area.  In theory, with normal winds, surface 
area of a perfectly balanced antenna would have little to do with with 
how strong a rotator would be required to turn it.  Mass and length 
would be far more relevant.

However, in my somewhat unique case, balance and mass of the antenna 
isn't all that relevant ... but rated torque of the rotator still is.  I 
live on a hillside that blocks the normal flow of wind, and I get these 
monster swirlers that roar down the hillside as the wind is forced to 
come around the ridge line.  I've measured them at over 100 mph on a 
clear day and I watched one literally lift my 16 year old son about two 
feet off the ground.  When one of those hits my tower straight on the 
forces on the boom ADD UP and put incredible torque  on the rotator.  I 
have a PST-61D with stripped gears to prove it, and the total surface 
area of my antennas (OB16-3 and OB2-40) is less than 20 sq ft.  The 
gears were stripped while in a resting state.

I was going to upgrade to a PST-71D or even PST-110D, but WA7NB's litany 
of woe with two different PST-110D's (Hall effect pulse detector issues, 
I think) has me leaning more toward a prop pitch. The gears on the Smart 
GE 2500 NS Rotator by Giovannini look totally awesome but it appears to 
use an AC motor, and that would involve a permit process I'm not anxious 
to pursue.

At the moment I'm still considering what I'm going to do.

73,
Dave   AB7E

On 5/3/2016 9:39 AM, Jerry Gardner wrote:
> 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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