Mast support

[Chris R. Burger]

I guess down here we've had to reinvent a lot of wheels that other 
people can buy at the corner shop, but one approach I've standardised 
on seems to be less than universal in the States.

There's a lot of talk on the Reflector of mast bending and 
overturning moments, but none of the obvious solution.  I use twin 
thrust bearings on all my towers.  The twin bearings are spaced about 
500 mm (20 inches) apart, and carry all the weight and overturning 
moment of the antennas.  The rotators then carry no weight and no 
lateral force.  They only supply torque.

I have a set of bolt-on steps on each mast, to enable me to get to 
the top antennas if needed.

Because of the absence of any lateral play in the mast, alignment is 
pretty critical.  However, it's a small price to pay.  I have the 
ability to remove rotators without moving the mast, as there's a 
short piece of stub mast between the rotator and the main mast.  The 
joint can be removed, after which the rotator comes out without any 
hassle.  The mast remains supported by the bearings.

A new idea that's being incorporated in my new station's design is 
to use two concentric masts.  The inner mast has a concentric 
rotator, while the outer mast has a motorcycle drive sprocket to an 
offset rotator.  This way, you can have an independently rotatable 
top antenna without much additional cost.  The rest of the stack is 
turned through the motorcycle chain.  On the one tower, I intend 
putting a spotting tribander and 6 m beam above the 40/80 beams in 
this way.  To claim originality would be unfair; this idea is the 
result of a brief visit to IQ4A.

Chris R. Burger
ZS6EZ