Square feet is not a good way to pick a rotator. A 5' x 5' piece of sheet
metal on the mast is going to be much easier to turn than a 25 sq ft 80'
boom 20m beam. Also many beams, including my homebrew ones are not
balanced. Some rotator manufactures use a k-factor. Finding a rotator with
enough torque to turn a large antenna is easy, but finding one that holds up
over time is the challenge. Most ham rotators are toys. A couple of decades
ago I bought two of Hy-Gain's largest rotator - HDR-300. Each one HDR-300
needed to be taken down for repair it every year or so. I have never seen a
sq ft rating for a prop pitch motor, but they definitely hold up well.
John KK9A
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Rotator Choice for Larger Yagi
From: jimlux <jimlux@earthlink.net>
Date: Tue, 26 Apr 2016 12:13:38 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 4/26/16 11:46 AM, Máximo EA1DDO_HK1H wrote:
PST-61D has 39sqft.
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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