When attaching leads to a copper tubing J-pole, does the solid center conductor go to the short side or the long side ?
The J-Pole is not a well-engineered antenna and that's why you don't see many of them in commercial applications. It is good engineering practice to install a balun at every BALanced to UNbalanced junction in the system. There are two of those junctions in a J-Pole antenna and not a balun in sight unless one rolls an ugly balun at the feedpoint which is a mechanically difficult implementation. A J-Pole is basically a Zepp antenna for vhf/uhf and shares the same common-mode problems. One will have common-mode signals on the coax no matter which conductor goes where.
Has anyone tried feeding a J-Pole with twin lead attached at the 300 ohm feedpoint?