[TowerTalk] Mast Insertion Depth

[K7LXC@aol.com]

In a message dated 98-10-05 23:11:35 EDT, djl@andlev.com writes:

> Is there a rule of thumb about what percentage of the mast should extend
>  down into the tower to the rotor, vs. what percentage should extend out to
>  the top of the tower to hold the antennas?

    Hmm, interesting question. The short answer is no. 

    The long answer is that there are two primary forces at work. The
(relatively) smaller is the bending moment of the mast on the top of the
tower. This is a function of mast length and antenna load and is normally
absorbed by the thrust bearing or whatever is at the top of the tower. An
extreme example of this would be 22 feet of mast above the top of the tower
and 2 feet of mast inside down to the rotator. Most of the forces are at the
top and little actually makes it to the rotator. The length of mast in the
tower doesn't make any difference in this stress.

     The larger force is wind induced torque on the mast, ALL of which is
transmitted directly to the rotator. This force then goes through the mounting
plate to the tower legs. For BIG loads, you would like to have the rotator
plate close to the guy brackets so that the torque is transferred to the guys,
thus relieving the tower structure of the force. In this case, the longer the
mast the better because the long rod acts as a torsion bar spring and absorbs
some amount of the stress. Imagine a 50 foot tower with a 50 foot mast. The
amount of force at the bottom of the mast is smaller than the force at the top
of the tower. 

      Since the longest mast available is 24 feet, your decision is probably a
combination of antenna load and spacing along with SOME amount of mast inside
the tower.

       I guess I didn't really answer your question and didn't give any real
engineering data. Oh, well. Perhaps a more engineering enlightened
TowerTalkian can add to this.  

Cheers,   Steve   K7LXC

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