I just started building my first trellis this weekend. I'm going to be using metal conduit piping. For the base, I bought a 1" 10' piece and cut it in half. I then hammered the 5' piece into the ground, emptying out the dirt plugs as i went so it would be hollow inside.
Then I have two 1/2" diameter pipes, 10' each connected together that slide into the 1" base. They slide into the ground about 3 or 4'. I only did one of these this weekend just to see what happened and see how stable it was. The base was pretty sturdy, but the 20' pole was way more flemsy then I was expecting.
My plan is to have 2 of these long poles 20' apart, then connect them at the top with another 20' length of 1/2" pipe that will hold the hop plants. To control the flembsiness, think I should be okay just running 2 support lines from the tops of the poles going out at an angle away from the structure and held down with stakes. Hopefully, if I can balance all the wieght on the poles evenly, they'll remain straight.