The long section radiates as a half-wave antenna. The shorted parallel section provides a feed position with impedance close to 50 Ω.
Antennas · F1SYH
J-Pole antenna
The J-Pole is a vertical half-wave radiator fed through a parallel quarter-wave matching section.
VHF
UHF
½ λ
Operating principle
Main characteristics
Typical gain
≈ 2,15 dBi
Impedance
Adaptée sur section ¼ λ
Polarization
Verticale
Difficulty
Facile à intermédiaire
Dimensions and design
The radiating section is near ½ wavelength and the matching section near ¼ wavelength. Physical dimensions require correction for conductor diameter.
Construction and tuning
Step 1
Build the two parallel conductors.
Step 2
Short their lower ends.
Step 3
Attach the feed points symmetrically.
Step 4
Move the feed points gradually to tune SWR.
Step 5
Add a common-mode choke.
Advantages
- No horizontal radials.
- Coaxial feed.
- Simple wire or tubing construction.
- Useful portable VHF/UHF antenna.
Limitations
- Feed-point position is critical.
- Common-mode current may occur.
- Metal supports can disturb the pattern.
- Gain claims are often overstated.
History and origin
The J-Pole developed from parallel-line-fed antennas and the earlier Zeppelin antenna concept.
Safety
The bottom is DC shorted, but RF voltage is not zero throughout the matching section.