I've done a number of pipe/post installs for various antennas/dishes. I have often taken a piece of PT 2x8 or 2x10 as needed about a foot long. Get a pair of large round or square as needed U bolts and some fender washers. Mount the J pole or whatever mount to the 2X and drill appropriate holes for the U bolts. Of course check everything for square. Get to the desired height on the pole, Put the U bolts around the pipe and through the board, put on the washers and nuts, fine tune the height and direction, and cinch it down. You want it tight, but not so you start bending the board.
If you are in a severe wind area, you might consider drilling a hole through the board and the pipe after installation and run a lag screw in to keep the whole thing from twisting.
Readily available and inexpensive materials, a few minutes of prep, and a very quick install.
And, should the situation require, rapidly removed with little evidence left behind.
Piece of cake.