Just my 29 cents worth... I don't think using real thick
copper tube, coax shield, etc, will really be worth the trouble,
vs using a standard wire wound coil on a form. There is some
increase in performance , but overall it will be fairly small unless
the
wire used in the standard coil is very thin. Once you get to about 1mm
thickness, you will have fairly decent performance. 2mm is even better,

and any increase using a thick tubing will basically be a waste of
time.
I think anyway... The spacing of the wires, and keeping water, etc
from between the coil windings is more important. You can wind a
coil using 14 gauge wire and have a very high Q coil, if wound right.
Even 18 or 16 gauge won't be too bad as long as there is the proper
spacing between windings. Again, just my opinion from building various
mobile antenna coils... I'm pretty picky about my mobile antennas, but
I don't bother using "fat" coil conductors. To me, not worth the extra
trouble, weight, etc. I don't think the extra performance is that great
vs any regular wound coil , that has the proper pitch. With the thinner
wire coils, it's using a close winding pitch, with the wires nearly
touching that makes for excess loss. Not really the thin wire in
itself
unless it's super thin like magnet wire. And yes, I avoid braid for
anything
carrying rf. Even my grounding straps are solid.
MK