Also known as: j-pole, j antenna, end-fed half-wave with stub
A J-pole antenna is a vertical, omnidirectional antenna consisting of an end-fed half-wave radiator matched to the feedline by a quarter-wave parallel stub at its base — the two pieces together forming the shape of the letter J.1 Its defining virtue is that it needs no ground plane or radials: the stub does the impedance matching that a ground plane would otherwise provide, making it a rugged, self-contained vertical popular for VHF/UHF base and portable use.
How it works
A half-wave radiator is a fine antenna, but fed at its end it presents a very high impedance — thousands of ohms — hopelessly mismatched to 50 Ω coax. The J-pole solves this with a quarter-wave matching stub: two parallel conductors shorted at the bottom, which act as a quarter-wave parallel-line transformer. At the shorted base the impedance is low; it rises smoothly toward the top. Somewhere a short distance up the stub there is a point whose impedance equals 50 Ω, and that is where the coax is tapped — one conductor to each side of the stub. From there the stub steps the low tap impedance up to the high feedpoint impedance of the half-wave radiator, matching it without any ground plane.
Because the radiator is a full half wavelength worked against the stub rather than a quarter-wave monopole worked against a ground plane, the J-pole is electrically complete on its own. The radiation pattern is essentially that of a vertical half-wave dipole: omnidirectional in azimuth, vertically polarized, with a low take-off angle favouring the horizon. Tap position and stub spacing are adjusted for lowest SWR.
The best-known relative is the Slim Jim, a folded variant built from a single length of 300 Ω twin-lead (“J Integrated Match”), which packs the same idea into a flat, rollable ribbon — a favourite portable antenna. The J-pole descends historically from the end-fed Zeppelin (“Zepp”) antenna trailed from airships.
Relevance to SDR
For an SDR listener the J-pole is an easy, cheap, ground-plane-free vertical that beats a stock whip and rivals a ground-plane for fixed VHF/UHF monitoring. Built from copper pipe or twin-lead, it hangs from a single support with nothing radial sticking out, which is handy indoors, in an attic, or clamped to a mast. Its vertical polarization matches land-mobile P25, DMR, and NXDN signals, and its omnidirectional pattern hears all trunking sites at once.
GopherTrunk is a receive-only decoder and needs no special antenna; a well-tuned J-pole simply raises the signal-to-noise reaching the front end. Like any single-band resonant vertical it is narrower in coverage than a discone, so cut it for the band you care about most.
Sources
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J-pole antenna — Wikipedia, for the end-fed half-wave radiator, quarter-wave matching stub, coax tap point, and the Slim Jim variant. ↩