In a message dated 98-08-19 12:46:45 EDT, dick.green@valley.net writes:
<< The new numbers could have some serious implications for some of us. Let's
translate this discussion into English for the non-engineers among us.
>>Obviously, the new computation standards have significantly increased the
published windload ratings on some Hy-gain antennas. For example, the TH-7
was previously rated at 9.4 sq. ft., but is now rated at 11.5 sq. ft. That's
better than a 22% increase. The question was asked before, but not answered,
so I'll ask it again: Have the new computation standards also increased the
windload handling capacity of popular Amateur tower configurations, or have
many of us overloaded our towers by relying on the old formulas?
This is an important point and my particular area of interest as well.
What are the implications vis-a-vis Rohn projected area antenna specs - either
round-member or flat member antennas? What will enable us to make direct
comparisons between antenna and tower figures?
>> A year ago, when I bought my TH-7 (then rated by Hy-Gain at 9.4 sq ft
windload area) for placement on my U.S. Tower MA-770MDP 72' tubular rotating
crankup (then "conservatively" rated by the manufacturer at 10 sq. ft. @ 50
MPH), I thought I could rely on the configuration being safe at wind speeds
up to 50 MPH (assuming no ice loading.) I knew that the configuration was
unlikely to survive winds much above 50 MPH. In fact, U.S. Tower's own
engineering calculations showed a (conservative) derating of the fully
extended tower to less than 2 sq. ft. at 70 MPH! I suspect that the original
50 MPH rating was done long ago, before the computational standards changed,
whereas the 70 MPH rating was done relatively recently.
According to Bruce Kopitar, KB6JAH, the President of US Towers, the
towers are really rated at 60 MPH while the literature indicates 50 MPH
ratings.
Bruce Horn, WA7BNM, has done some calculations and written an
interesting little paper on crank-up specs and windloading. It illustrates how
a crank-up can be de-rated for different windspeeds and loads. The tower is
one of the 89-footers but the implications are similar for their other crank-
ups. Also, Roger Cox, WB0DGF, of Hy-Gain wrote an article that addressed some
of the same topics about ten years ago. Both are interesting reading and any
crank-up owner or potential owner would be well advised to have copies of
both. They are available for a buck each from Tower Tech, Box 572,
Woodinville, WA, 98072.
A BIG TowerTalk Thank You!! to Kurt and Roger for their work in working
on this topic and present the information to us.
Cheers, Steve K7LXC
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