BTW, based on your SWR readings I somewhat doubt it's a resonant length of coax causing your issues. A short or high impedance causing an issue is possible on the J-pole issue, hard to say there. But measuring SWR in this case is a solid way to know what your load is doing. IOW, it doesn't matter if an antenna is effective or not at radiating RF but if SWR is low then you can reasonably conclude you are not getting RF reflected, at least none at the measurement point (e.g. it's possible for load reflected energy to be absorbed by the feedline in theory, although you'd see issue like melted insulation or RF interference).
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