The Slim Jim antenna is a variation of the J-pole antenna, also known as 2BCX Slim Jim, that is related to the J-pole the way a folded dipole is related to a dipole.The Slim Jim is one of many ways to form a J-Pole. Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).
The Slim Jim variation of the J-pole antenna has characteristics and performance similar to a simple or folded Half-wave antenna and identical to the traditional J-pole construction. Judd found the Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna. Slim Jim antennas made from ladder transmission line use the existing parallel conductor for the folded dipole element. In the copper pipe variation, the Slim Jim uses more materials for no performance benefit but since the ladder transmission lines are hard to source in India I went with the copper pipe variation. Slim Jim antennas have no performance advantage over the traditional J-pole antenna.
The approximate gain in the H-plane of the Slim Jim is from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd).[6]
Building Materials
- Four 1/4” 90° copper angles
- 1/4” copper pipe of suitable length based on calculation
- 2 metal hose clamps for coax cable feedpoints
- heavy guage solder wire
- SO 259 Connector
- Torch for soldering/welding the antenna together
- pipe cutter, hacksaw and other tools can be handy
- piece of acrylic for reinforcement
Slimjim antenna calculator
The Slimjim antenna calculator above is an awesome visual tool for building your antenna.
Install the Female SO 259 connector
Use PVC pipe cutouts or acrylic pieces to maintain distance between the two parallel copper pipes
Mounting the antenna
Finding a good mounting place for your antenna is a tricky job. People find all kinds of ways of getting around that, few tips being: higher the better and clearer of any immediate obstacles. My antenna was mounted on a pre-installed DTH antenna mount
Final Thoughts
Before mouting the antenna it is a good idea to test it thoroughly by making few QSO’s and checking SWR. Keeping your cable length shorter is also a good way of preventing losses. Slimjim antennas are electrically short circuited so they can be checked from within your shack for mechanical damages, cable fault etc.
NOTE: Higher SWR ratios can Fry your Radio so do check your SWR.