[TowerTalk] Concentric Masts

[K8RI on TowerTalk]

I worked out and pretty much built such an arrangement, BUT in my case the 
interaction between antennas would have been too great and I abandoned the 
project although the top UHF and VHF arrays are mounted on an inner mast 
while the 6-meter and tri-bander are mounted on the outer mast.

This is another one of those: "It looks simple until you try to make it 
work", things

There are also some issues with holding the external rotator 
straight/vertical and keeping the chain tight.  The side thrust on the 
external rotator due to the leverage from the sprocket and chain could be 
considerable.
With a small VHF antenna this is not much of an issue, but it becomes one 
with larger antennas and can be a major issue with really large wind loads. 
I would expect a 2-L 40 meter antenna would fit into this category even if 
the elements are linear loaded.

IF the external rotator is going to handle much of a load, be it from a 
large antenna, high winds, or both then the system should have a spacer or 
arm going between the mast and the top of the rotator.
Basically you need a steel or heavy Aluminum bar with a bearing (or sleeve) 
to fit the mast one end and to fit the drive shaft to the sprocket at the 
top of the rotator at the other end.  Better to have the brace beyond the 
sprocket rather than between it and the rotator.  The expensive and probably 
the easiest way would be thrust bearings on the mast and the drive shaft 
bolted to a strong flat plate between the mast and rotator.

Going back to your drawing I'd just mount the flat plate with the two thrust 
bearings above the sprocket and chain as shown.  You might need to allow for 
adjusting the spacing between the two bearings to some extent as well as 
adding an adjustable idler sprocket to maintain chain tension. A spring is 
often used for this, but that might not be a good way to go with this kind 
of setup as you might run into resonances and oscillations in the wind.

However...I'd expect to see considerable interaction between a 2-L 40 and a 
tri-bander with only 10 feet of vertical seperation. Some one needs to run 
that through a program to see what comes up.

One other problem that comes up with chains is rust.  Really grease it up 
good and build a cover to keep the rain off.

Good luck.

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com