I have made several "roll up" J-pole antennas out of spare RG-8X coax and 300 ohm TV flatlead. I get low SWR almost every time, following this video by Dave Tadlock KG0ZZ: https://youtu.be/ql6zWMZ1dEU However, every time I have tried to fit one inside a plastic PVC pipe, the SWR shoots up to 5.5 or more. I'm certain that the antenna isn't getting kinked up inside; so is it the PVC material itself? I'd have thought that plastic wouldn't affect the radiation pattern significantly.