Table of Contents
What Is a J-Pole Antenna?
The J-Pole antenna is an end-fed omnidirectional antenna consisting of a 3/4-wavelength radiating element and a 1/4-wavelength matching stub connected at the bottom. Its name comes from the J-shaped profile when viewed from the side. Originally designed for use on Zeppelin airships (hence its other name, the Zepp antenna), it has become one of the most popular antennas for VHF and UHF amateur radio operation.
The J-Pole offers approximately 3 dBi gain (slightly better than a dipole), excellent omnidirectional coverage in the horizontal plane, and does not require a ground plane. The matching stub provides an impedance transformation from the high impedance at the end of the half-wave section to approximately 50 ohms at the feed point, making it easy to match to standard coaxial cable.
Design Formulas
Common Band Dimensions
| Band | Frequency | Radiator | Stub | Total Height |
|---|---|---|---|---|
| 2m | 146 MHz | 58.2" | 19.4" | 58.2" |
| 1.25m | 222 MHz | 38.3" | 12.8" | 38.3" |
| 70cm | 440 MHz | 19.3" | 6.4" | 19.3" |
| 6m | 52 MHz | 163.7" | 54.6" | 163.7" |
| MURS | 151.82 MHz | 56.0" | 18.7" | 56.0" |
Construction Tips
- Material: Copper pipe (1/2" or 3/4") is ideal for VHF. Twin-lead wire works for portable versions.
- Spacing: Keep the two parallel elements about 1-2 inches apart for VHF frequencies.
- Feed point: Connect the coax shield to the shorter stub element and the center conductor to the longer radiator at a point 2-5% up from the bottom shorted end.
- Tuning: Adjust the feed point height and stub length for best SWR. A SWR of 1.5:1 or better across the band is achievable.
Frequently Asked Questions
What is the gain of a J-Pole?
A J-Pole antenna provides approximately 3 dBi of gain, which is about 0.85 dB better than a half-wave dipole. This modest gain comes from the slightly longer radiating section compared to a standard dipole, which provides a small amount of compression in the radiation pattern.
Does a J-Pole need a ground plane?
No, the J-Pole is a self-contained end-fed antenna that does not require a ground plane. This makes it excellent for portable operations, attic installations, and situations where mounting a ground-plane antenna is impractical. The matching stub acts as a balun, keeping RF off the feedline.
What is the velocity factor?
The velocity factor accounts for the fact that radio waves travel slightly slower along a conductor than in free space. For bare copper or aluminum tubing, use 0.95. For insulated wire, values range from 0.66 to 0.82 depending on the insulation material. The velocity factor shortens the physical antenna dimensions compared to the free-space wavelength.