In a message dated 12/10/2006 7:11:24 A.M. Pacific Standard Time,
> The windload rating of a tower usually stipulates that the specified
load is applied at some distance above the tower top, like maybe one or two f
eet. If your multiple antenna installation spreads out your antennas over
let's say 15 feet of mast (triband Yagi a foot above the tower, 2L 40M Yagi
ten feet above that and a longboom VHF Yagi five feet above that), then you
can't simply add the antennas' respective areas. You have to take into
the moment (leverage) that these various antennas at these various distances
above the tower top impart to the tower. If you do this (spread out your
antennas) you could very easily exceed the tower's maximum windload.
> Another way of saying this is if your tower is rated at 20 sqft of load
at one foot above the tower top, it is then rated for LESS than 20 sqft if
that one antenna is MORE than a foot above the tower top.
> There is an article by W7NI that was published in the NCJ for Sep/Oct
1982 and Mar/Apr 1983 and republished in Jan/Feb 1992 NCJ titled Antenna Mast
Design. Stan goes over the fairly simple math involved in calculating the
moment that is imparted on towers by antenna wind loads at various distances
above the tower top.
That was the article that got everyone aware of the increased windload
bending moment with masts but unfortunately according to Dick Weber, PE, K5IU,
his calculations and formulas were wrong. A much better and more accurate
discussion of mast forces is in Dave Leeson's, ex-W6QHS, W6NL, book "Physical
Design of Yagi Antennas". It's out of print but you can probably find a copy
somewhere. It's got LOTS of interesting information about lots of tower and
antenna system topics.
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