Hi Andrew,
The process is called metal spinning, its done on a mandrel in a lathe
or similar machine. Somehow I do not think its an easy process to get
the hang of in a few tries.

What I would do is make a sleeve with a cut in it (so its diameter can
be adjusted) and clamp that between the bigger and smaller tube. You
may have to experiment to roll the sleeve out of just the right
thickness alu for a good fit.

If I were to build a yagi big enough for the 20m band I would not use
alu tubing at all. Instead go for the fibreglass fishing rod blanks and
run a 2mm copper wire inside for the element. That would be much
lighter still and also stiffer, as the fishing rod is tapered. The
blanks come quite long, the ones I used for the cubical Quad I'd built
some years ago were over 3 m ( 12' I think) long. You could always
extend them at the centre with an alu tube epoxy glued (or sikaflexed)
to the inside of the fibreglass blank.

Have fun

Klaus VK6AKS (not active)


Andrew VK3BFA wrote:
Hi All,
new to this metalworking stuff, so please excuse my stupid newby
questions. I am trying to build an HF Yagi antenna from telescoping
aluminium tubing(to reduce windloading and weight) - trouble is,
Imperial size tubing was a good fit into the next size ID, so
telescopic sections could be made easily. All thats available now is
Metric sizing, and its too sloppy a fit into the next size up. HOW do I
roll/reduce the diameter of the bigger tubing so its a nice
interference fit onto the smaller size down? - whats the machine
called, can I homebrew one (I have basic hand tools, a welder, lots of
"stuffit - lets try this"...misguided enthusiasm -- well equiped
here...) - any tips, references. pointers appreciated.

PS I built the tower its going on, 8.5 metres, teaching myself to weld
as I went. Welding gets better as you go up.......so, the next logical
step is to build the antenna that goes on top of that.........

Andrew VK3BFA.