Homebrewing a Slim Jim Antenna

 Slim Jim antenna, as the name implies, has a very slim design and hence very little wind loading. JIM stands for "J Integrated Matching" as it uses a J type matching stub. Slim Jim antenna was described by Fred Judd, G2BCX in 1978. The antenna has a lower takeoff angle and hence reported to have better performance than 5/8 wavelength ground plane antenna. People have made it using half inch copper pipe, ladder line and many other options! Approximate gain in the horizontal plane has been mentioned as around 2  dBi.


Slim Jim antenna should be insulated from the antenna mast with a 'free space' of at least quarter wavelength. Antenna itself can be supported on a PVC pipe. Though it can be made for any band, it is quite popular as a 2m VHF antenna which can be made easily with good performance. It is almost like a vertical asymmetric folded dipole with a gap between the half wave and quarter wave regions. The exact feed points on the long and short parts of the antenna are determined by trial and error, checking VSWR.

For measurements, I am using the online calculator for Slim JIM and J-Pole by M0UKD, available at: https://m0ukd.com/calculators/slim-jim-and-j-pole-calculator/ for the measurements of my Slim Jim antenna project. 

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