N4KG response below.
On Sun, 12 Dec 1999 23:41:13 EST K7LXC@aol.com writes:
> In a message dated 99-12-12, K7LXC@aol.com writes:
> > > FWIW, the practice of using weight times boom length is also a
> pretty crude "measurement".
> > Since both Hy-Gain and Yaesu both use it (Effective Moment
> and K Factor), it's the de facto standard.
> Oops - I neglected to mention that these are just factory
> estimates anyway.
> Cheers, Steve K7LXC
> Tower Tech
I agree that the Wt X Boom (or 1/2 boom) length is better than
specifying boom length alone or even sq ft of antenna load. For
typical tribanders this is probably acceptable because for the
most part they are similar size antennas.
When pushing the load limits, a better measure of capacity
is needed. The problem with boom length only specifications
is that they do not account for element size and weight. The
problem with Wt X Boom specifications is that they do not
account for element WEIGHT DISTRIBUTION. A 2L Yagi
has ALL of it's element weight at the ends presenting a
worst case scenario. A multi-element multi-band antenna
has it's element weight (and often boom weight) distributed
along the boom. Thus two antennas with equal boom
lengths and equal weight can have VERY different torque
requirements. Examples: 2L40 vs. 3L20 or 2L80 vs. 5L20.
It should be a simple matter to determine rotor stall torque.
Simply attach an arm (boom) to a mast and measure the
force in a restraint applied perpendicular to the boom.
Force X distance (from the mast) = torque. If the imbalance
is a problem, apply two restraints at each end of the boom
and sum the forces. Any Mechanical Engineer worth his
salt should be able to make this determination.
Have any rotor manufacturers ever done this?
de Tom N4KG (who majored in Electrical Engineering)
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