Hard to give any meaningful advice. You haven't told us the tower model,
it's height or construction. Is it lattice or tubular? You haven't told us
the windload rating of the antennas or what bands they cover. You also
haven't said whether you will stack both antennas on a mast above the top
section, or will stack one of the beams on a lower section. Bear in mind
that if you use a long mast to get separation between the beams, the bending
moment at the top of the tower will increase substantially. U.S. Towers
doesn't rate their towers for such a configuration.
My point is that you need a PE to do some calculations to determine just how
far above the windload rating the installation will be. You're making the
assumption that your windload will be safe up to 50 mph. What makes you
think that? If, taking any mast extension into account, your windload is
just a tad above the U.S. Tower rating, then I suppose you're probably OK.
But if you're overloading the tower significantly, a wind speed of well
under 50 mph might bring the tower down or damage it. Again, some
calculations must be done by a PE to determine the max safe windspeed. I
think you might be very surprised how quickly these tower derate.
That said, the windspeed rating for a crankup improves considerably as it is
lowered. For example, my 72' U.S. Tower tubular sways sickeningly in a 20
mph wind when loaded to the max rating (10 sq ft.) But it stiffens up
considerably when lowered to 50'.
My feeling is that you shouldn't overload the tower. Period. If you take it
to the max windload, then you should lower it when not in use. If the wind
kicks up while you are using it, lower it by 10-20 feet. Also, get a
motorized tower. You will quickly tire of lowering it by hand. There are
many sad stories of folks who got lazy and didn't crank it down, only to
lose the tower in a big windstorm.
My motorized tubular crankup, now loaded to about 1/2 the U.S. Towers
rating, stays down except for contests. If the wind picks up during a
contest, I lower it to 50 feet or all the way down. I got sick of doing
this, and not being able to put stacks on the tower, so I'm building a 110'
Rohn 55G guyed tower :-)
73, Dick WC1M
> -----Original Message-----
> From: jknodel M Knodel [mailto:jknodel@msn.com]
> Sent: Sunday, October 15, 2006 1:58 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Am I asking for trouble? - tower loading
>
> I would like to get everyone's opinion on this -
>
> I am planning on installing a US Towers crankup tower. These
> towers are rated for windloads at 50mph and 70mph. I want to
> stack 2 beams on the tower. These 2 antennas would exceed the
> tower's windload rating at 70mph but would be well within the
> rating for 50mph. I plan to install a wind speed meter and
> deligently crank down the tower every time the wind exceeds 50mph.
>
> I cannot install a guyed tower at this location so this is my
> only option.
> The crankup with these antennas and cranking it down in winds
> over 50mph is my plan. Am I asking for trouble in doing this?? Thanks.
>
>
>
>
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