Most of the instructions and plans you see online for J-Pole antennas often include the admonition to use a choke balun with the antenna. The purpose of the balun is to limit the amount of RF radiation coming back down the feedline and into the radio. Even though limiting the return of RF energy is a good thing, I still get the question ‘Is the choke balun necessary?’
To answer this question, you need to look at the antenna itself. The J-Pole is a half wave antenna by design. As a half wave antenna, it doesn’t require any special grounding or a groundplane to operate. The purpose of the groundplane is to ‘balance’ the antenna, that is to give the antenna something to push against as it releases the RF energy. For example, with a quarter wave antenna, not all the RF energy will be released up the antenna itself unless it has an effective groundplane to direct that stray energy.
Half wave antennas don’t need the groundplane, but they also have the tendancy to send excess RF energy back down the feedline, increasing your SWR and limiting sensitivity. The choke balun, which can be as simple as six turns of coax or ferrite chokes near the feedpoint of the antenna will stop the excess RF energy from returning down the feedline.
If the antenna is perfectly tuned and is mounted in a location where it isn’t couple with any nearby structures, then the choke isn’t necessary. I’ve used J-Poles with and without the choke and a well tuned antenna mounted in a good location will work fine without out it. But as I usually say, the choke will never hurt and always help. Therefore I recommend the choke balun for any new J-Pole antenna installation.