Hi Alan,
Actually I read it as you meant it and didn't even realize you had said mast
instead of tower. <:-))
> On 06/10/05 12:04 pm Alan NV8A (ex. AB2OS) tossed the following
> ingredients into the ever-growing pot of cybersoup:
>
>> Roger:
>>
>> Maybe you've had somebody do the calculations and ascertain that
>> everything is safe, but I definitely do not like the way your system
>> sounds from your description: that's an enormous load on a long piece of
I'm afraid this is going to get long, the figures are going to be coarse,
and a bit of rule of thumb. The steel mast is quite springy, so there is a
lot of give that does not translate directly to the tower as side force, but
it sure appears to be hard on those small VHF and UHF antennas, yet they
have held together for over 3 years now.
There's a lot of calculating required. Particularly when you push the edge
of the envelope as I am doing (sorta sounds like a pilot doesn't it <G>)
Multiple antennas, pivot points, and multiple attach points require a lot
of number crunching, something that should be done in far smaller
installations.
For instance every one should calculate the arm and moment if they mount the
antenna above the top of the tower. They should also figure it for towers
that are not guyed at the top and a real biggie... They should calculate the
side thrust on the rotor down in the tower when the antennas are mounted
above the top of the tower.
One that is rarely thought of is the force transmitted to the tower at the
end of the mast and the relationship of that point to both the top pivot
point (top thrust bearing, or bushing) and the position in relation to the
guy lines above and below the anchor point (rotator or thrust bearing).
For the side force a the top, just multiply the force by the arm, but that
arm ends at a pivot point. That creates a force multiplied by the ratio of
the arms to the anchor point and in the opposit direction. Take an
instalation of a tribander on a mast 15 feet above the top of the tower and
the rotator 5 feet below the top. That is a 3:1 arm, or if you have a 100#
of wind force on the antenna there is 300# side force on the rotator in the
opposit direction. This is a gross simplification, but with large arrays,
multiple antennas at different heights above the top of the tower and a
rotator mounted well down in the tower can use a bit of paper figureing out
the forces. <:-))
73
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>> tube sticking out the top -- and on the top section of tower above the
>> uppermost guys.
>>
>> I'd rather go higher with the tower itself and keep the mast to a much
>> more reasonable length: 2" CM tube is almost as expensive per foot as
>> additional AN Wireless mast sections,
>
> Ooops! I mean "tower sections," not "mast sections."
>
> Alan NV8A
>
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