At 03:58 PM 3/3/2006, Mike Harris wrote:
>G'day,
>
>| One more data point.... I had a CrMo mast with three monobanders on it
>at my
>| previous QTH. I had no slippage in the Orion mast clamp. Antennas were
>all
>| KLM (20M5, 15M6 and 10M6).
>
>Maybe they are properly torque balanced. There was quite a thread on this
>subject in January 2005. The David Leeson spreadsheets derived from his
>book "Physical Design of Yagi Antennas" provide a method whereby mast
>torque and appropriate compensation can be calculated/designed. Seems
>more sense to me that trying to make stronger and stronger more grippy
>clamps and bust gears.
Guys--
But Dave later realized that the method he discussed was not the correct
method and that the Cross-Flow method by Dick Weber, K5IU is the proper
way to do it, as published in Communications Quarterly.
Short version is that the elements' lengths and boom positions are
not important
and the only issue is the balance of the boom (and the element clamps to a much
lesser degree).
This is not intuitive at first, but once you realize that the vector
of one-half element
is exactly balanced by the other half of the same element (probably minus some
minor effect of the boom shadowing part of the element) then you
realize that all
element forces are balanced and the mast only is "torqued" by the asymmetry
of the boom.
Again, it is not intuitive, but once you spend some time thinking about it then
you should have no problems. Bottom line is that there is no need for any
"dummy" elements to balance the element load, only a need to either center
the boom or compensate the asymmetry by some additional "boom" area.
But Dave's book is still a great book to have and to read. Lots of good info
there, just that the torque load on the mast is not correct.
73--John W0UN
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