[TowerTalk] Rotator Choice for Larger Yagi

[Steve Baughn]

Agreed John. The one down side (I use PP's also) is the cost to have them 
rebuilt if necessary. Last I checked K7NV was getting in the range of $3k to 
rebuild them but considering the amortization over its lifespan isn't so bad 
which brings me to another point-back in the August 1971 edition of QST a 
good friend W8ZCQ (SK) wrote a pretty good article on rebuilding them. While 
it takes a bit of work it is not insurmountable. I rebuilt one off the 
article and have had no problems with it. If anyone would like a scanned 
copy I'd be glad to furnish it. It is nice if you can mount them where you 
can get to them in the event you have to replace a reed switch for the 
controller.

73,
Steve, WD8NPL

-----Original Message----- 
From: john at kk9a.com
Sent: Tuesday, April 26, 2016 7:36 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Rotator Choice for Larger Yagi

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 at contesting.com
Subject: Re: [TowerTalk] Rotator Choice for Larger Yagi
From: jimlux <jimlux at earthlink.net>
Date: Tue, 26 Apr 2016 12:13:38 -0700
List-post: <towertalk at contesting.com">mailto:towertalk at 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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