[TowerTalk] Antenna wind load vs tower specs.
Bob Shohet, KQ2M
kq2m at kq2m.com
Sun Apr 29 18:17:16 EDT 2018
I have two towers:
100’ of Rohn 45 on a pier bin base plate
and 130’ of Rohn 45 on a pier bin base plate.
The 100’ Rohn 45 has FOUR 5L HG155CA’s plus FOUR 5L HG105CA’s with guying at the 30’, 60, and 90’ levels and a Rohn flat top section above that with a rotator and two thrust bearings for the mast.
There is a low fixed 5L 155CA at 30’ (just above the first set of guys) and then 5L 155CA’s at 60’ and 90’ BOTH on sidemounts just above the guys. I have a flat top section above the top guys through which goes a 14’ Chrommoly mast with a 5LHG105CA at 100’ and 5L HG155CA at 109’ on the same mast.
The other 10’s are fixed at 23’, 37’ rotatable on a sidemount, 65’ fixed and then the top rotatable @ 100’
All the fixed antennas are clamped directly to the tower.
The 130’ Rohn 45 has THREE HG205CA’s (LJ205CA’s) plus a Cushcraft 40-2CD fixed SE at 65’ with guying at the 40’, 80’ and 120’ levels. I used regular guys at 40’ and starguys at 80’ and 120’ and a Rohn flat top section is above that with the rotator and two thrust bearings out of which sticks a 10’ Chrommoly mast on which the top 20 resides. (and on several days this past week on which two golden eagles “MA and PA eagle” were repeatedly “having fun”.
The other two HG205CA’s are fixed and bolted to the tower with angle iron at 54’ and 80’.
These towers have survived direct hits from F0 and F1 tornados and the two Hurricanes Irene and Sandy. Sandy was the most vicious of all with exceptionally violent sustained wind gusts of 110+ mph, although I have seen straight line wind gusts from violent T-storms here approaching 120 mph. Innumerable ice-storms and blizzards besides.
The point is that if your tower is guyed robustly, and you do not overload it (“overload” is a relative term), and you use a pier pin base plate to help reduce the torque on the legs, you should be able to safely put another 10’ section and rotator and antenna above the top set of guys.
I personally would not put 15’ of tower above the top set of guys, but again, it is relative. Some would say that my 10’ above the 90’ plus a rotator, 14’ Chrommoly mast and 2 5L monobanders is much more wind/ice load than 15’ of tower plus a small monobander.
Others might be more concerned with the TOTAL tower wind/ice load from all tower sections, rotator, antennas, cables, guy wires and everything else. I believe that that would actually be the correct way of measuring it – although more complex to calculate.
Wind/ice load and torque factors are also relative. If you get a straightline wind of 100 mph coming directly at and parallel to a tower guy wire
tower---->guy wire <------wind
is the torque and wind/ice load greater or less than if the wind comes at the tower at an angle that is BETWEEN the guy wires?
> guy wire
> guy wire
The answers are not simple but important to understand and figure out – with the caveat that every qth has different topography, different weather and every tower installation is different which would provide different answers even if everything else was identical (which it never is).
From: Ed Sawyer
Sent: Sunday, April 29, 2018 5:06 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Antenna wind load vs tower specs.
I have seen these depictions of the guy wires so low on the tower however I
have never built or seen one done that way. Typically the top guys are
somewhere on the last section. Personally - I like the guys at the 5 ft
below the top point. That gives the most strength vs access to the top
I have to think that the top guys being 15 ft below the top of a tower that
is using 10 ft sections and only 80 ft tall is seriously reducing the load
Curious as to opinions in this area.
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