Scott Dickson, W5WZ wrote:
> I've seen discussion on the importance of selecting the correct material for
> a mast. I don't recall ever seeing a discussion on the ratio of mast in the
> tower versus above the tower.
>
It depends on what the mast has to do as far as material. I only use
DOM where I consider it necessary, otherwise I use structural steel
tubing. My whole array is on structural steel tubing. The thrust
bearing is being replaced by one foot of 2" DOM as the thrust bearing is
for lateral support, not vertical.
I'm doing the math below in my head (calculator is out in my shop) so I
hope I have the numbers correct. OTOH they are for example only.
>
>
> I have a 2" x 0.25" by 21 ft galvanized 1026 DOM mast. It will hold a Force
> 12 C-51XR, (weighs 250 lbs, wind load 18 sq ft, mast torque < 900 in-lbs)
> installed 2 feet above the thrust bearing at the top of the Rohn 55G tower.
> The rotor is M2 Orion 2800.
>
>
At only 2' above the thrust bearing you have very little leverage. With
2 X 1/4 DOM you can use the whole piece (if you can lift it) and just
put the bottom of the 21' into the tower or about 19' below the top of
the tower. The M2 should have no problem supporting the 250# plus the
weight of the 2" DOM. I believe the 18 ft sq is well within the
capabilities of the 2800.
>
>
> Currently, I do not have any other antenna that will be on the mast.
> Perhaps I'll install a 6 meter beam above the C51 in the future.
>
>
>
In that case things can change drastically depending on the size of the
6-meter antenna and the spacing between the antennas. I have my 7L 3Ci
(bout 30' boom) mounted about 15' above the tri bander. The boom to
mast bracket on the 7L C3i is for 1 1/2 mast. However, "Typically" I'd
carry the larger tube all the way to the 6-meter yagi which is what I
did here. That would put the top 17' above the top of the tower and the
bottom only 4' down into the tower which is not a good ratio. There I'd
make a rigid splice so the bottom would still be around 20 to 25 feet
down in the tower. The 25' lets you use a second full length of tube.
However that splice needs to be strong with no play in any axis. Here
DOM is easier to work with and I'd want the heavier wall due to the
leverage right at the splice.
Remember the force inside the tower compared to the force outside the
tower. IE, with a 100# lateral force on the big antenna only 2' above
the top of the tower compared to the rotator 19' down gives a ratio of
19:2 or a factor of 1/9.5. So the side /lateral force on the rotator
and tower would only be 100/9.5 or app 11.5#
BUT by adding another 15' to the 7L 6-meter antenna the forces will be
almost equal on the 6-meter antenna and rotator. IOW the two forces
will add at the rotator but in the opposite direction than at the top of
the tower where a different set of conditions exist. In the case of the
first antenna being 2' above the top of the tower you have 100# allplied
to a 2' arm which is 200 ft pounds of bending moment/leverage. PLUS
let's say the 6-meter yagi has a wind load of 60# on a 17' arm. That
works out to 1020ft pounds of leverage plus the 200 ft pounds from the
much larger antenna for a total of 1220 ft pounds. This force must be
held by the top set of guys. So going much above the top of the tower
can add considerable force to the top of the tower.
Here, the two 12L 144 MHz antennas and two 11L 440 MHz antennas on a 14'
cross boom 30' above the top of the tower with the 7L C3i 15' above the
top literally destroyed a TB3 in less than 6 years. I've mentioned
before, but the VHF and UHF arrays at 130 foot made the mast look like a
blue gill rod that had just tied into a monster bass when the wind would
get above 30 MPH. When they go back up it will be the tribander at 100
with the 7L C3i at 112', OR the tri-bander at 70' on a ring rotator with
the VHF/UHF systems on top.
> It seems to me that the length of mast in the tower is as important as the
> length of mast above the tower.
>
Only kinda, sorta... It's important if the length below is less than the
reach above the top of the tower, or if the wind load is large. 2' above
and 19' inside the tower is good. Were it the other way around the
leverage at the top of the tower the rotator would see 9.5 times the
force at the antenna. IOW 100# on the antenna and 950# on the rotator.
Regardless I'd want the rotator at least 5' down into the tower.
If OTOH you mount the rotator well down in the tower, or even base mount
it, then it becomes advisable or even necessary to install intermediate
thrust bearings to keep the mast from moving off center. These bearings
should not support any weight during normal use, just keep the mast
centered. As the mast will most likely have a different coefficient of
expansion than the tower, conditions may exist where the mast is pulling
up on the top of the rotator which is unlikely to be a good thing.
> What rules / guidelines / best practices are used to determine where to
> install the rotor?
>
Don't exceed the ratings for the top of the tower or the rotator for
either weight or side/lateral forces. When working with long masts use
sleeves or thrust bearings to keep them centered and prevent oscillation.
73 and good luck,
Roger (K8RI)
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