[TowerTalk] Rohn 25 & Mast lingth
Roger (K8RI) on TT
K8RI-on-TowerTalk at tm.net
Wed Apr 19 01:15:05 EDT 2017
I respectfully disagree on the advantage of going farther down with the
mast. The farther you go (within reason) the less moment / leverage is
applied to the top plate and bearing. 25 G benefits from spreading the
I agree that with a relatively small tribander there would be little
advantage depending on the wind and wind area of the antenna. 2 feet
above the top of the tower is the same as doubling the wind load. 5 feet
gives a 5X increase. Going down inside the tower translates the bending
moment to a side thrust. OTOH if the mast is too flexible, or too long,
then going down too far will let the mast flex inside the tower,
increasing the bending moment.
For most ham installations these are of little concern. They can become
problems with large antennas and arrays.
My installation was an experiment in loading
http://www.rogerhalstead.com/ham_files/Tower29.htm The rotator is
mounted between 15 and 20 feet below the top with a thrust bearing
located between the rotator and top thrust bearings, These bearings
only keep the mast centered and support no load. The mast is 2" OD
structural steel tubing
The 144/440 antennas were close to 30 feet above the top bearing with
the 7L C3i 6-meter Yagi about 15' above the top plate, and the TH-5
about 2 feet above the top plate. Definitely a bit over loaded. This
arrangement "destroyed a TB-3 and TB-4 thrust bearings.
I have a relatively inexpensive DeWalt (sp?) cut off saw that uses a
relatively large diameter abrasive disk. It makes fast work of steel
masts. Don't use one of these on Aluminum. The disk will load up with
Aluminum and possibly disintegrate from the imbalance. For Al, just use
a regular cutoff saw. Don't be tempted to speed up the cutoff as you
will then have to debur the piece. OTOH you may need to anyway.
Removing burs from Chrome Moly tube will likely require an abrasive
Alloys and coolant. It depends on the alloy and cutting speed. The
harder you push (Within reason), the faster it cuts generating more
heat. I don't bother with coolant on steel masts and can cut off a 2"
mast in just a few seconds. Harder alloys should be cut slower. Cut
with little pressure and a lot of patience can be done without cooling,
but it's a good idea to rig a garden hose with a low flow into the
cutting area as most will succumb to a desire to cut faster.
It's a good idea to do this on the garage apron, or driveway as abrasive
cutting is messy. With water, it can be a big mess. Also, slower cutting
leaves a cleaner cut. Be careful! any burs are likely to be very sharp.
73, Roger (K8RI)
On 4/17/2017 5:26 PM, Grant Saviers wrote:
> I had 3x 25G sections on a house bracket and about 14' of 2" x 0.375"
> wall 6061T6 exposed mast to a TH7DX, which probably is in the large
> size category for tribanders. Survived several hurricanes in E. Ma.
> Probably 80kts at my house. Although it was plenty strong enough, I
> would not use Al mast again, it is too bendy. I could tilt the tower
> over and went with Al to save weight against the raising tackle.
> There is no structural advantage to more mast inside the tower beyond
> 3 ft or so. More doesn't matter either way.
> How you reinforce the wall the bracket attaches to is important.
> Besides internal cross bracing of 2x8's on the studs, I structurally
> nailed a 4x8 sheet of 3/4 ply down as the attic floor. That
> diagonally tied from floor to wall bracket with a 6' long steel
> angle. End walls on roofs have just a few nails top and bottom in the
> studs. Structurally, a floor membrane should take the tower loads.
> This is a bit harder if the room inside the bracket is finished space
> and your decorating police have a vote.
> You don't want to cut an alloy mast at home except with an abrasive
> saw or a cobalt tipped bandsaw blade and plenty of coolant.
> Grant KZ1W
> On 4/17/2017 13:54 PM, Clifton Keely via TowerTalk wrote:
>> I am in the process of erecting a Rohn 25G to 30 feet with a house
>> bracket. My question is how long should the mast be within the
>> tower? I have a thrust bearing top plate and a good rotor for the
>> load, but should the mast extend 2 feet below the top plate or 6 or 8
>> or 12 feet. Would there be a real advantage to letting the mast go
>> below the next tower joint down (10 ft). DX sells a high budget mast
>> they they won’t cut, it is 22 feet long. I don’t think I want to
>> have a mid sized triband 10 or 12 feet above the top plate/thrust
>> bearing. My plan is to place the antenna at about 5 feet above the
>> top plate/bearing Speak to me someone who knows. Please. All advice
>> greatly appreciated. Clif AA6FE.
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