> I have decided to put up a 72' crank-up
>tower. My immediate plans are to mount my Force12 C-3E tribander (5.9 sq.
ft
>area) and 13 element 2M beam (1.8 sq. ft area). My county of Marin, CA is
a
>75 mph wind zone.
>After talking with a few locals about their installations I had decided on
a
>US Tower model TX 472 rated at 18 sq. ft max allowable antenna area @
50mph.
>Problem is that this rating drops to 7.3 sq. ft at 70 mph and I calculate
>about 5 sq. ft at 75 mph. Upgrading to a TX 572 rated at 30 sq. ft at 50
mph
>solves the problem but at twice the expense and seems like overkill for
what
>seems to be common practice for supporting a small tribander and a few VHF
>goodies. Obviously with the approach of a winter storm which roll in here
>from the Gulf of Alaska with some forewarning, thoughts would drift towards
>cranking down the crank up. Still even if I could get a permit through
these
>figures are a worry.
I think a lot of people would tell you that you're close enough to the
ratings and not to worry. Judging from the responses I got when asking the
same question about putting a TH7 on my MA-770MDP, a lot of hams overload
their crankups. Several boasted about having big tribanders *and* a 2-el 40M
beam on their tubular crankups! Your installation is considerably more
modest, but still over the published ratings. Therefore, don't listen to
anyone who tells you it's completely safe.
However, for what it's worth, the other day I was talking with Bruce,
president and owner of U.S. Tower, and he told me that the recent changes in
the codes for windload calculations have raised the ratings on their towers
to 70 MPH. In the past, he has told me that the old 50 MPH ratings were
conservative and that 60 MPH was closer to the truth. I didn't ask if they
have any plans to redraft the engineering specs or publish new figures, and
somehow I doubt that this will happen anytime soon. You might call and ask
him for his opinion on this and your specific installation (don't bother
asking anyone else at U.S. Tower -- you probably won't get an answer.) Of
course, this is all anecdotal, which won't help you get a permit.
U.S. Tower does have engineering calculations for their towers at 70 MPH for
$25 (refundable if you buy a tower.) Judging from what the specs showed for
my MA-770MDP -- a drop from 10 sq. feet @ 50 MPH to about 2 sq. feet @ 70
MPH -- your calculations are probably pretty close. Even if you stick with
the old ratings, you're painfully close to the limit with that setup.
Perhaps a smaller 2-meter beam would do?
With a 9.5 sq. ft. (old code) TH7, I've obviously got considerably more
windload on my tower than the old specs said it could handle at 70 MPH, and
a much bigger difference (7.5 sq. ft.) than what you are contemplating
(about 3 sq. ft. worst case.) While it's tempting to tell you that it's
probably safe, if you have ever seen one of these crankup towers sway and
buckle in a strong wind you might not agree. The first time I saw that, I
wished I had been able to put up one of the 30 sq. ft. jobs (non-tubulars
were nixed by the XYL.) I, for one, don't want to even think about exposing
my expensive tower to that kind of risk. For that reason, I only put it up
to the full 70' when chasing rare DX or contesting, and when winds are
relatively calm (max 20 MPH.) Most of the time it lives at 50 feet, where
the windload rating improves enough to set my mind at ease. Also, the tower
doesn't sway or buckle at 50 feet unless the winds get pretty strong. After
watching it for many hours, I feel quite comfortable that it could survive
70+ MPH winds when at 50 feet. Still, when the winds get over 25-30 MPH, or
are forecasted to do so, or when I plan to be out of town overnight, I just
crank it down all the way. No sense in pushing my luck.
IMPORTANT: I am convinced that for the crank-down strategy to work, the
tower has to be motorized and have remote control. It takes a heck of a lot
of cranks (hundreds of turns) to lower one of these towers. It won't be too
long before you get less vigilant and stop cranking the tower down when you
should. Also, if you decide that cranking the tower down is acceptable, be
sure to get an annemometer with a wind speed alarm and monitor it closely.
Also monitor weather forecasts every day. If there's even a hint of high
winds coming, crank the tower down.
So, either spend the money on the motor drive or get the bigger tower. Or
both, if you can afford it. You won't regret it. A tower like this is
nothing to pinch pennies on.
73, Dick, WC1M
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