Scott,
Maybe you've heard the saying about "The Devil and the Deep Blue Sea". If
you mount the rotor 10' or more down in the tower, when the rotor fails (and
it will some day) you will need to lift the mast and antennas out of the
tower to get the rotor out. If you mount the rotor up in the top section
where Rohn has made it easy to remove the rotor without taking the mast out,
that long mast with almost any antenna mounted more than 5 or 6 feet above
the top of the tower will likely damage your rotor because of the extreme
side load in a high wind. That mast becomes a heck of a lever and the
bearing is the fulcrum. The solution is to get another 55G rotor plate and
a second bearing. Mount the bearing on the second rotor plate and put it in
the tower just above the top of the rotor. This will prevent any side load
on the rotor and the rotor will be mounted in the only position that has
easy access. I recently did an installation of Rohn 55 and a MonsterIR for
a friend and I did it like this.
Good Luck,
Dave - K4SSU
----- Original Message -----
From: "Scott Dickson, W5WZ" <W5WZ@arrl.net>
To: <towertalk@contesting.com>
Sent: Monday, March 23, 2009 7:10 AM
Subject: [TowerTalk] Antenna Mast - guidelines for ratio in tower / out
oftower
> 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.
>
>
>
> 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.
>
>
>
> 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.
>
>
>
> It seems to me that the length of mast in the tower is as important as the
> length of mast above the tower.
>
>
>
> What rules / guidelines / best practices are used to determine where to
> install the rotor?
>
>
>
> --Scott, W5WZ
>
> _______________________________________________
>
>
>
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