[TowerTalk] Making aluminum tubes telescope smoothly, with good electrical conductivity?

[Kristinn Andersen]

Hi:

 

I am working on an extensible vertical consisting of a 6m/18ft outer
aluminum tube (40mm/1.5" diameter) and an inner aluminum tube that can be
extended out of it with a motor mechanism.  For certain reasons the inner
tube has to be substantially slimmer than the outer one, probably around
25mm/1" diameter.  So, to make a reasonably tight fit I will make a
narrowing "collar" at the top of the outer tube, where the inner one will
emerge upwards.  Furthermore, at the bottom of the inner tube I will fasten
a cylinder that almost fills the outer tube, providing mechanical stability
there.  Sort of like a piston inside the outer tube.  Thus, the mechanical
connections or interfaces between the outer tube and the inner tube are at
the top of the outer tube and at the bottom of the inner tube.  At these key
points I need to ensure:

 

a)       Good electrical contact (so that antenna currents can continue from
the outer tube to the extended inner tube;

b)       Minimal mechanical friction as the inner tube is pushed up and
retracted back down.

 

Are there any suggestions on this?  What metal should I use that slides with
low friction against the aluminum tube wall, does not corrode at the contact
and provides good conductivity?

 

73 - Kris, TF3KX (http://www.simnet.is/net/tf3kx)