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?
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.
In light of the new antenna windload figures, I'm wondering how to interpret
all this. Here's how I use the configuration now:
Even though winds above 40-50 MPH are extremely rare at my QTH (and only get
up that high once a year or less), and the tower is shielded by trees on
three sides (but it's exposed to the dreaded Nor'easters), I decided that it
would be best to operate the tower fully extended only during contests or
when chasing rare DX, and then only if the windspeed is less than 20 MPH
(actually, I start to get nervous when the wind gusts over 15 MPH.) At all
other times, the tower stays at 50 feet, unless the wind starts to gust over
about 25 MPH. Then it goes all the way down to 22 feet. I always crank it
all the way down when out of town. The tower is motorized and has remote
control, so it's not a big deal to crank it.
I don't think I've had to lower the tower all the way down due to "high"
winds more than a handful of times this year. I check the weather forecasts
every day, and leave the tower at 50 feet unless the prediction is for winds
above 20 MPH. When the tower is at 50 feet, I have a windspeed alarm set for
25 MPH and always crank it down when the alarm goes off. So, the biggest
risk is that a freak, unexpected wind will exceed the tower's rating when it
is at 50 feet when I am away during the day (I work at home, so am rarely
away for more than a few hours on any given day.) I haven't rigged up an
automatic lowering circuit triggered by the alarm, but that's realtively
easy to do and something I've considered.
Up to now, I've been comfortable with this setup, even though my various
height and windspeed selections were not arrived at through any scientific
means (I have no idea how to compute the windload capacity of the tower at
50 feet -- it just looks a heck of a lot more stable in high winds than when
it's extended to 70 feet.) So, I'm wondering whether I need to rethink the
limits in light of this new information. Any suggestions?
73, Dick, WC1M
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