Thinking of putting a vertical up. I'd go from 2" pvc, to 1", to 1/2" ,
in twenty foot lengths. Will this thing support itself OK? Seems like it
might physically work OK.

I made a vertical colinear antenna using 6m (18 foot) of 50mm Dia (4") PVC
tube. When vertical it is Ok, BUT it has two drawbacks:

1 - It bends when the wind blows
2 - It bends when the sun shines, bends away from the sun.

The "cure" is to put a 12" across X piece at the midle of it, then thread 4
nylon fishing lines from end-to-end, so the line is fastened to each end and
one leg of the X. This is the "ships mast principle" for stopping a flexible
mast bending.

You said yo want to use 2" diameter, and 7 metre lengths (20 foot). Sounds
like you will have a worse problem than me. There is no reason why you
cannot have three of these Xs along the length of the tube so that they
overlap. See the drawing at:

http://web.telia.com/~u85920178/ante...-shipsmast.gif

Hope this helps. Regards H