Formula and Method for the J-Pole Antenna Calculator
A J-pole is an end-fed half-wave vertical antenna with a built-in quarter-wave matching stub, all made from one continuous piece of wire, tubing, or ladder line bent into a J shape. This calculator starts from your operating frequency, finds the free-space wavelength using λ = c / f, then scales the half-wave radiator and quarter-wave stub by a velocity factor to get physical build dimensions, plus a starting estimate for the coax feed-point tap.
How the calculation works
The free-space wavelength is λ = c / f, where c is the speed of light (299,792,458 m/s) and f is the operating frequency. The radiating element is a half-wave section, so its ideal free-space length is λ/2; the matching stub is a quarter-wave section, so its ideal free-space length is λ/4. Because real conductors shorten the resonant length (the "end effect"), both dimensions are multiplied by a velocity factor — typically 0.92-0.97 for wire or tubing, the same range used in the standard dipole formula L(ft) = 468/f(MHz), which is simply the free-space half-wave in feet, 492/f(MHz), times a 0.951 shortening factor. The stub runs alongside the lower part of the radiator, so the total tubing or wire you need to buy is the radiator length plus the stub length, while the antenna's overall mounted height is just the radiator length.
Building and tuning notes
Keep the two parallel conductors (radiator and stub) spaced roughly 1-2 inches apart for rigid tubing builds, or use twin-lead/ladder-line for lightweight roll-up versions — spacing and conductor diameter both shift the exact match point. Mount the antenna vertically with the long radiator on top and the shorted stub at the bottom for vertical polarization, and keep it well clear of metal masts, clamps, or other nearby conductors that can detune it. Because the feed-point estimate is only a starting point, always finish the build by sliding the coax connection up or down the stub while checking SWR — moving the tap point mainly affects impedance match, while trimming the very top of the radiator fine-tunes the resonant frequency.
Real-world applications
J-poles are popular for 2-meter (144-148 MHz) and 70-centimeter (420-450 MHz) amateur radio base and portable stations because they are cheap to build, omnidirectional, and outperform a simple vertical whip when mounted a few feet above the roofline. Lightweight wire or ladder-line versions are common "roll-up" antennas for portable operating, field-day setups, and emergency communications (ARES/RACES), since they pack flat and hang from a tree branch or mast with just a piece of paracord.