Question for antenna gurus: Wind Load

tkiuntke · Post by **tkiuntke** » Wed Aug 02, 2006 4:54 am

I am wondering if anyone of the Batboard Gurus has a good rule of thumb concerning antenna wind load and its effect on a tower load? For example, how do you judge when an antenna should or shouldn't be put on a tower or a smaller antenna should be used? I've looked at the formulas used to calculate total load for a tower but my eyes quickly rolled back in my head due to overload. Thanks for any suggestions.

Toby
KC0FRU

Post by Pj » Wed Aug 02, 2006 8:51 am

Although a generic answer, the tower should have came with (and usually clearly advertised and known) what wind load the tower is capable of.

Base station antenna's are also rated for their wind load.

Basically, if you have the information for the tower you have and the antennas, its very simple. If you don't, a call to the manufacturer of both will produce the correct information.

Bruce1807 · Post by **Bruce1807** » Wed Aug 02, 2006 9:44 am

It also depends on what you are trying to achieve.
If your tower is rated at 150 mph and you put a bigger or more antennas than what it was designed for then the rating goes down.
The tower manufacturer should be able to conduct a load test for a few thousand dollars.
They require the antenna type, height, leg the antenna is on,feeder,
face or leg for the feeder and whatever else is on the tower.
I just had one done for $2250 including a PE stamp

440roadrunner · Post by **440roadrunner** » Wed Aug 02, 2006 2:45 pm

Don't forget to pick the proper rating for ice and snow in your area. By that I mean the additional wind load caused by ice

Jim202 · Post by **Jim202** » Wed Aug 02, 2006 6:08 pm

Tower and antenna wind loading go hand in hand. You
start with the antennas and see just how much wind
loading they have. Next you design the tower to carry
at least 125 percent of the load you just came up with.

Bear in mind that the tower wind loading will vary with
how it is constructed, how and with what type of guy
cables are used.

As someone mentioned, don't forget to take into
account ice loading if that is a chance in your part of
the country.

Also make sure you know what the wind speed rating
is for the tower location.

Don't forget to add a fudge factor in for the coax
transmission cables. They add some wind loading
to the tower that will vary depending on how they
are attached to the tower.

As one well know old ham ( W1BU Sam Harris ) once
told me, "If it stayed up through the winter, the
antenna wasn't big enough". Sam was know for
putting up these massive antenna arrays for his VHF
and UHF long haul communications that he loved to
have. Even had some moon bounce antennas in the
yard. Several of his towers rotated, not the antennas on a rotator.

Jim

N4DES · Post by **N4DES** » Wed Aug 02, 2006 9:45 pm

Best bet is to hire a PE (Professional Engineer) who has experience
in this type of calculation and let him do the work and take the risk.

I use Pate Engineering out of Tampa Florida to do all of mine. It is well
worth the $ especially when engineering for 140MPH winds.

Bruce1807 · Post by **Bruce1807** » Wed Aug 02, 2006 9:56 pm

Our manufacturer runs our tests but they have an independent PE verify and stamp the paperwork.
We try a little harder Mark and rate all our towers at 150mph.
Same reason as you rate yours so high. Who knows when the big one is comming.
Well at least it won't be Chris

N4DES · Post by **N4DES** » Wed Aug 02, 2006 10:20 pm

yep I agree on Chris, at this point we ducked another one....we have one tower that we keep rated to 150 which is at the County EOC the rest we use the current standard for the area which is 120 sustained and 140 for 3 seconds.