Once again, the question of sizing rotators to a given antenna(s) has come
up. I'm not an engineer, so please excuse me if the terminology in what
follows is not 100% correct, but I think the question is worth asking.
The K-factor as defined by Yaesu is a step in the right direction, but we
should be able to go further in determining whether we have enough rotator.
As I understand it, the product of turning radius times total weight is a
conservative and easily calculated measure of the mechanical
inertia/momentum that a rotator must overcome to start and stop turning an
array. The trouble is that it doesn't tell you very much about real worst
case, when wind is opposing either the starting or stopping of rotation.
Force 12 quotes a worst case mast torque for each of their antennas, based
on a 70 mph wind blowing at the angle that is calculated to produce the
most torque. Why couldn't one combine such a figure with a MEASURED
mechanical inertia (the real equivalent to the K factor) to come up with a
single "rotator load factor" for a given antenna?
My pair of Force 12 antennas on a Yaesu G-1000SDX have a "K-factor" of 1127
foot-pounds total, well within the G-1000's rating of 2020 foot-pounds.
The total maximum wind torque is 40.8 foot-pounds. The rotator is rated
for a braking torque of 434 foot-pounds, and a rotating torque of 43.4-79.5
Since both numbers are comfortably within spec, this rotator should have no
trouble turning or stopping this array, and in fact it doesn't. but I'm
uncertain how far one really can push it, and many people will have more
aggressive antennas than I do.
Question for the MEs out there -- how would you combine wind torque and
measured inertia/momentum to produce a real number, that could be compared
with rotating and braking torque specs to determine if a given rotator will
do the job reliably?
73, Pete Smith N4ZR
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com