several reasons:
1. jacking the antenna further up the mast than you can work standing on top of
the tower is a real pain.
2. lowering an antenna jacked up on the mast to work on it and then putting it
back is even more of a pain.
3. the lever force of the antenna working against the top section of tower gets
larger as you extend more mast out.
4. The side force on the rotor gets larger as more mast is above the tower than
in it. and remember most rotors are only bolted to the mounting plate with 4
relatively short bolts into a cast housing.
5. if you have to take the rotor out for repair the further into the tower the
mast is the easier it will be to control it when the rotor is removed.
6. if the mast should bend it becomes an expensive and dangerous job to remove
some of these can be helped by putting an extra bearing below the tower top to
control the bottom of the mast and take some of the side force.
Al Williams wrote:
>
> Two recent postings have 8' of a 21' mast and 6' of a 12' mast inside the
> tower. Why so much, if the
> bottom end of the mast is bolted to a sturdy rotator and mounting plate?
>
> I am planning to put a 11 sq ft windload antenna at the top of a 20' Chrome
> mast that will bolt to a
> Orion 2800 rotator sitting inside the LM470 tower. The antenna to be is a
> Force12 Magnum 2/2
> for 80 & 40 so I want as much height as can be safely achieved.
>
> I will appreciate comments on how far to insert the mast and also whether
> the 80-85 foot height
> is adequate for the 80 beam.
>
> k7puc
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
--
David Robbins K1TTT
e-mail: mailto://k1ttt@berkshire.net
web: http://www.berkshire.net/~robbins/k1ttt.html or http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
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