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J-Pole

Vertical Antennas

An end-fed half-wave vertical matched by a quarter-wave parallel stub.

Band: VHF/UHF
Gain: ~2-3 dBi
Polarization: Vertical

Photos

Real-world photo of a J-Pole in use — Real-world example. Source: Wikimedia Commons (CC BY-SA 3.0; Crcwiki).

Radiation / wave patterns

Idealized radiation pattern of the J-Pole — Idealized azimuth radiation pattern (illustrative, generated). Radial scale in dB.

How & why it works

The J-pole is an end-fed half-wave radiator with a quarter-wave parallel-line matching stub at the bottom that gives the structure its J shape. The shorted stub transforms the high end-feed impedance down to 50 ohms and the half-wave radiator needs no radials, so the whole antenna is a single mechanical piece that is easy to build from copper pipe or twin-lead. It produces an omnidirectional vertical pattern with a touch more gain than a quarter-wave ground plane.

Real-world uses

2 m / 70 cm amateur base and portable antennas, and emergency 'roll-up' antennas.