It has really been interesting reading the discussion on the K-factor and
Effective Moment on the reflector. While I agree that our EM and Yaesu's
K-Factor are not perfect, they are much better than the older "square
footage wind area". The EM rating was developed from what was commonly
available from all antenna manufacturers - weight and turning radius. It
would be nice to have a physical model of every Amateur antenna on the
market so that we could compute the actual wind torque, but this seemed
unlikely since we also manufactured and sold competitive antenna products.
The weight of antenna corrolates nicely to the diameters of the antenna
elements and boom, and the turning radius also corrolates to both long boom
and long element length antennas, so this is a good approximation with what
is available. We looked at the failure history of our rotators versus what
antennas were placed on them. We also asked Craig at the Rotor Doctor
(C.A.T.S.) for similar information. There were two parts that usually fail
when a rotor is overloaded - the main ring gear and the brake wedge. The
ring gear failure occurs when you try to turn large antennas in high winds.
The brake wedge fails either when the brake is engaged during rotation or
during extreme storm conditions. From this failure history we derived the
EM numbers. At the same time, we also improved the ring gear and the brake
wedge so that they were less likely to fail.
If someone would like to propose a better specification, that can be
applied to all antennas with the limited data published by the
manufacturers, I would consider adding this to our rotator spec's (and
antenna spec's).
BTW, the "torque dampener" that has also been discussed is standard in our
larger commercial rotors, the R-3503 and R-3502. The R-3503 is used in our
"twin-tower" RLPA systems that have been sold since the 70's and are used
at the regional air traffic control centers nationwide. This RLPA is a 19
element 4 - 30 MHz LP with a 72' boom and a longest element of 87'. Weight
of just the LP-1001 antenna head is 1695 lbs. The "torque dampener"
consists of 2 die springs in parallel with 2 similar-length urethane pads.
When the antenna stops or starts rotation, the die springs compress, then
the pads compress. There is also an electrical delay built-in to prevent
changing directions and starting/stopping immediately. This RLPA system
has a conservative rating of 140 MPH (no ice) and 100 MPH (0.25" radial
ice).
73 Roger Cox WB0DGF
Telex Communications, Inc.
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