On qrp-l.org, there’s been a discussion of the J-Pole antenna. There were various short, incomplete explanations of how it works, some comparing it to a Zepp antenna. I can see the similarity to a Zepp, but must also admit that my understanding of how it works is incomplete.
Fortunately, Gary, N3GO, has done a very thorough analysis of the J-pole titled “From a J to a Zepp: The truth and its consequences.” While I haven’t puzzled through it all yet, it looks like a very thorough analysis, and includes a number of observations that the author says appear to be contrary to popular opinion:
- Adjusting the feedpoint “tap” is not the proper way to tune a J-pole.
- 300 ohms is the optimal transmission line impedance to use in the construction of J-poles.
- 300 ohm TV twin-lead is minimally susceptible to RF current induced in the feedline coaxial cable, enabling the J-pole to perform well without a balun.
- The “stub” portion of the J-pole is electrically longer than a quarter wavelength.
- The J-pole “shorting bar” can be connected to an earth ground reference if and only if a balun is employed at the antenna feedpoint.
- The velocity factor is the most critical transmission line parameter to consider when designing J-poles.
- J-poles are easy to build, and tune (even by the inexperienced). As a consequence, they are easy to reproduce.