That UST TX472 would be seriously overloaded at 90 MPH. It's rated as
10.3 sqft at 70 mph. I didn't do the numbers, but I wouldn't be
surprised to find that it wouldn't, or maybe would just barely, stand
with nothing mounted on the top at 90 mph.
For 90 mph you will even have trouble with the HXD555 which is rated at
29.5 sq ft at 70 MPH. I have something similar but the wind speed rating
for my area is 70 mph. As an example, HDX555 tower, 3 element SteppIR at
57 ft, XM240 at 66 ft, the tower maxes out at 79 mph with wind gusts to
101 mph. A DOM mast rated at 85ksi is still below its rating for this
situation. But still you are overloaded. If you require city approval
this may be a problem.
Mounting the SteppIR above the XM240 is actually worse because the
SteppIR is slightly more wind loading that the XM240.
If you remove the top antenna, use HDX555, SteppIR at 57 ft, and a 10 ft
mast, the HDX555 tower maxes out at 96 mph with wind gusts to 123 mph.
It's time to do some serious looking at alternate tower and antenna
configurations.
For some free software to make these calculations go to
http://www.arrl.org/qexfiles/
and download the file labled TRANVITY.ZIP. Also download the following
pdf file which gives some good explanations of the calculations.
http://www.arrl.org/qex/1123.pdf
There are also some other programs available for only a few bucks to do
these calculations.
Jerry, K4SAV
Jon M. Knodel wrote:
>Thank you everyone for the replies. They have been very insightful. I was
>not planning on installing the motor with the tower - just the hand crank
>winch. My original idea was to install my 3 element SteppIR along with a 2
>element 40m beam below it on a tall mast (15 or 20 feet). The tower I was
>thinking about is the UST TX472 which has a wind rating of 10.3 at 70 mph.
>These were my thoughts and the reason I raised the question.
>
>I am now thinking, because of everyone's good responses (and I really do
>appreciate the help), that I should not overload the tower at the 70 mph as I
>originally planned. I will probably step up to the stronger (and more
>expensive) HDX series and keep my antenna load within the 70 mph rating.
>
>But this raises another question. The city here has adopted a sustained wind
>speed rating of 90 mph and a gust rating of 105 mph. I have never seen winds
>of these speeds and the civil PE that I work with thinks that they are
>definitely overly conservative. But we do get some good winds in the winter
>here, being right on the coast - probably around 70 mph at times. So my
>question now is this: Since US Towers only specs their towers for 70 mph max,
>is there a way that I can calculate a rating at 90 mph, using the same method
>that they used for getting the 70 mph rating? Does anyone know the UBC
>calculations used to achieve the EIA ratings? Do you think that US Towers
>would provide this information if asked for it?
>
>Thanks for all the help.
>
>...Always wanting to learn more about antennas and towers... Jon, N7XW
>_______________________________________________
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