That, I don't know. I've only built a 2 meter "J", from instructions I
found on another page, and attached the coax about 4.5 inches from the
bottom.
Bob
k5qwg
Based on other Two Meter J-Pole designs, I would guess that the Co-Ax connects
about two or three inches from the bottom of the "J" matching section, with
shield connected to the main element and center conductor connected to the
matching arm. For Six Meters, perhaps the connection would be about six inches
above the bottom. If you used stainless steel hose clamps to attach the cable,
adjustment would be easy.