Multiple antennas and Swing Arm Design

[Gary E. Jones, Ph.D.]

This comment is really a response to N4ZR and deal with his concern with so
many yagis on one tower. 

Well, my QTH is 70 miles north of the Gulf of Mexico in Mississippi (part
of Hurricane Alley). I have had a three yagi 100' stick of Rohn 45 and a
two yagi 78' stick of Rohn 25 up through Hurricane Frederick.... sustained
winds in Hattiesburg were in excess of 80 MPH. Lodge pole pine trees
(60-70') were down everywhere and snapped off in the middle. All my yagis
and towers made it through without a scratch. All yagis are heavy duty or
stock. 

Now, my current Rohn 45 has the 2 element 40, over the 5 element 20, with
the swing arm with a 5 element 15 and 5 element 20 in a 20 meter 5 over 5
stack configuration 15M = 65" and 20M at 55') and the Th6 at about 35 feet.
The TH6 sits at the first set of guys. It has also been through very high
wind storms with no problems. The one hurricane we have had since this has
been up gave us gusts into 60 - 80 MPH with narry a problem. 

Obviously, I try to do things right. And, the difference between Rohn 45
and Rohn 25 is a rather substantial difference. I use Rohn guy brackets at
all locations, and torque arms, and 1/4" EHS all around. Rohn 45 can take a
lot more load than 25. 

My big swing arm (12' with 2 big yagis on it) sits right on top of the set
of guys at 50'. The swing arm uses 2" angle iron across two faces of the
tower, vertical legs 1 and 2 on the bottom of the arm, and vertical legs 2
and 3 on the top of the swing arm. The arm is made of 1 1/2" schedule 40
water pipe with a cross brace in the center (cold galvanized... wish it was
hot dipped). I will try to draw a diagram of it below. I have never doubted
that this tower is going to stay up... it is rock solid. The swing arms are
twisted with HAM-IVs and there is a TB-3 thrust bearing at the top of the
swing arm to take all the lateral torque of the swing arm and the dead
weight of the swing arm and the antennas if the rotor needs to be replaced. 

My new tower will be 140' of Rohn 45 with a 5 element 15 monobander at the
top (150'), 2 element 30/ 2 element 40 (shortened) FORCE-12 duo-band yagi
at 140' at the top of the tower, a 12' swing arm at 70-80' with a 5 element
12 meter (Hygain) at 80' and a 5 element 10 meter (Hygain) at 70' and at
40' on a single antenna swing arm will be a 5 element 17 meter monobander
(Hygain). My belief is that the yagis will go long before the tower is in
trouble. 

  

Diagram of the 12' swing arm...
______________ Thrust Bearing (TB-3)
-----------||      2" angle iron 
           ||______________
           |-------------|
           ||         // ||
           ||        //  ||
           ||       //   ||
           ||      //    ||
           ||     //     ||
           ||    //      ||
           ||   //       ||
           ||  //        ||
           || //         ||
           ||//          ||
           ||/           ||    swing arm itself
           ||\\          ||
           || \\         ||
           ||  \\        ||
           ||   \\       ||
           ||    \\      ||
           ||     \\     ||
           ||      \\    ||
           ||       \\   ||
           ||        \\  ||
           ||         \\ ||
           ||          \\||
           ||___________\||
           ||--------------
           ||          
           ||
           ||
          /  \
          |   |  Rotor
          |   |
          |___|________________  2" angle iron
           --------------------


That should give you a good idea of what they look like. 



							Gary      W5FI 


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