Fence Top rail.....
My 2 ¢:
All of the galvinized fence top rail I've bought at Home Depot measures 10' 6" with one end swagged for insertion into the other. They make a connector that goes in-beteween but it's for horizontal mounting of the pipe. If memory serves me right, the top raid O.D. is about 1½".
When I found it necessary to go above the original 10' vertical "stick", I cut a 4' long piece of electrical pvc, 1½ I.D. and slid it in-between joints, drilled holes in the upper "brace" and lower "brace" and bolted it together. It works best if you rotate the bolt hole placements 90º. This makes it mechanically secure.
Above 20', this "bolting" gives you a place to put a guy ring to put the guy wires. I believe they still sell them. I don't think that given the top rail's thickness, even guyed, I would not attempt a 50 footer, LOL!
For that you need to consider a metal tower and the neighbors won't like that. :-(
Respectfully,
73,
Don/KA5LQJ
"I reached into the air, and found a friend."