[TowerTalk] Concentric Masts
K8RI on TowerTalk
K8RI-on-TowerTalk at tm.net
Thu Dec 21 23:15:42 EST 2006
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
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
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.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
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