While we're on the subject of rotors, I'm evaluating options here.
Right now, I'm using a Tailtwister to turn a Cushcraft 40-2CD (about 6 sq ft
windload, 24' turning radius, 44 lbs, K-factor = 1,056) mounted about six
inches above the thrust bearing at 110' on my Rohn 55 tower. The mast is 12
feet long, 2" diameter, 1/4" wall aluminum. Although not my main topic here,
I selected an aluminum mast for a number of reasons, not the least of which
was the fact that the antenna would be mounted so close to the thrust
bearing. Five feet of mast stick up above the top plate (for attaching the
boom support struts.) The mast runs down through a second thrust bearing
(not engaged) to the rotor, mounted seven feet below the top of the tower.
No problems turning the 40-2CD and no problems in the last 18 months
starting the rotor in the cold (have had that problem with another
Tailtwister, but not this one.)
Incidentally, the same rotor was previously used for about two years in
mast-mounted configuration on an AB-577 to turn a 4-el SteppIR (about 10 sq
ft windload, 23 ft turning radius, 110 lbs, K factor = 2640.) My
understanding is that mast mounting derates the rotor by 50%. Again, no
problems with the rotor.
Hy-Gain doesn't use the term K-factor, but I've read on this reflector that
their Effective Moment is the equivalent. The Tailtwister is rated at 3,400
ft/lbs. It's also rated to handle 20 sq ft of windload when mounted inside
the tower. It appears that the brake wedge is pretty highly rated (9,000
lbs) but I don't know how that figure applies to an antenna of a given size.
I'm not sure which rotor specs are derated when mast-mounted, but if the
Effective Moment is derated then the SteppIR exceeded the Tailtwister's
rating by s significant margin, but apparently with no ill effects.
Here's the issue: I will soon replace the 40-2CD with a Cal-AV 2D-40A (about
16 sq ft windload, 34' turning radius, 163 lbs, K-factor = 5,542.)
Obviously, this is much more antenna that anything I've turned with the
Tailtwister, and it exceeds the K-factor rating by a substantial amount.
However, I found a post from K7LXC saying that the K-factor rating (and
presumably Hy-Gain's Effective Moment rating) are guesstimates by the
manufacturers and are not the result of rigorous field testing.
My instinct is to leave the Tailtwister in place and see how it goes. If it
has trouble turning the 2D-40A, I can always swap in another rotor. Not the
easiest job, but somewhat simplified by the second thrust bearing and having
only one antenna mounted very low on the mast.
Any opinions on the strategy?
If anyone feels that the Tailtwister is sure to fail under the new load,
then I'd like to hear suggestions for a replacement. The two choices that
appeal to me are the Yaesu G-2800DXA or the M2 RC2800PX/AZ with the K7LXC
clamp. The Yaesu G-2800DXA has a K-factor rating of 6,870. I couldn't find a
K-factor rating for the M2, but know it's being used by quite a few stations
to turn antennas larger than the Cal-AV.
I prefer a rotor that can be purchased without the controller because I plan
to use a Green Heron (by far, the best controller on the market.) The
controller is optional with the M2, but not the Yaesu, but net cost with the
clamp will be a little higher for the M2.
Any better choices out there?
73, Dick WC1M
TowerTalk mailing list