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 ensu

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)