[TowerTalk] Antenna Mast - guidelines for ratio in tower / outoftower

[Larry]

I also use two thrust bearings above the rotator. I have a 20 foot mast with 
3 antennas above the tower and it worried me that while working on it it 
might get away from me. The two bearings are a relatively cheap solution.

I made a jig (of sorts) that consists of a hand winch that hangs outside of 
the tower that also has a fixed pulley right next to the mast and can raise 
or lower the mast and antennas relatively easy. Of course, one has to be 
careful not to put too much stress on the tower itself with the jig. Wire 
rope from the winch goes over the fixed pulley and is attached to the mast 
so that it pulls almost perfectly vertically. Trying to lift a couple of 
hundred pounds at 100 feet by hand is not my idea of a good time (even if it 
were possible).

73, Larry  W6NWS
----- Original Message ----- 
From: "Dave Johnson" <djohnson at windstream.net>
To: <W5WZ at arrl.net>; <towertalk at contesting.com>
Sent: Monday, March 23, 2009 11:26 PM
Subject: Re: [TowerTalk] Antenna Mast - guidelines for ratio in tower / 
outoftower


> 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 at arrl.net>
> To: <towertalk at 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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>
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