A near half-wave radiator is combined with a quarter-wave matching section. The folded geometry produces a narrow antenna.
Antennas · F1SYH
Slim Jim antenna
The Slim Jim is a folded vertical antenna derived from the J-Pole. It is especially popular for portable VHF and UHF use.
VHF
UHF
Portable
Operating principle
Main characteristics
Typical gain
Proche d’une demi-onde
Impedance
Adaptation intégrée
Polarization
Verticale
Difficulty
Facile
Dimensions and design
Total conductor length is near ¾ wavelength. Dimensions must be corrected for the velocity factor of the material.
Construction and tuning
Step 1
Choose ladder line or parallel conductors.
Step 2
Create the gap in the radiating section.
Step 3
Short the lower end.
Step 4
Connect coax at the calculated position.
Step 5
Tune the feed position with an analyzer.
Advantages
- Very lightweight.
- Easy to suspend.
- Low cost.
- Useful emergency or hiking antenna.
Limitations
- Must be kept away from conductive surfaces.
- Velocity factor strongly affects dimensions.
- Common-mode current may occur.
- Limited mechanical strength without support.
History and origin
The Slim Jim was popularized by Fred Judd, G2BCX. Its name refers to its narrow shape and integrated matching section.
Safety
Do not mount the antenna directly against a metal mast or conductive structure.