Construction and Radiation Pattern of Slim Jim Antenna

Designed in the 1970s by G2BCX, the Slim Jim (Slim J-Integrated Matching) is essentially a vertically polarized, end-fed folded dipole. It is a favorite among amateur radio operators for its outstanding performance on VHF and UHF bands, outperforming standard ground planes and J-poles.

The Radiation Pattern

While the Slim Jim is perfectly omnidirectional in the horizontal plane (radiating equally in all directions), its true advantage lies in its elevation pattern (take-off angle).

Because the antenna is a folded dipole, the currents in its two parallel legs are in phase. This phase reinforcement squashes the RF energy downward, rather than letting it bleed off into the sky.

• Standard 1/4 Wave Ground Plane: Radiates at an angle of roughly 25° to 30°.
• Standard J-Pole: Radiates at an angle of roughly 20°.
• Slim Jim: Radiates almost parallel to the ground at roughly 8°.

This flattened, low-angle pattern focuses your transmission precisely where you want it — at the horizon. This gives the Slim Jim approximately a 1.5 to 2 dB advantage over a standard J-Pole, and nearly 3 dB over a ground plane.

Construction: The 2m “Roll-Up” Slim Jim

The most popular way to build a Slim Jim is using 450-ohm ladder line. It is incredibly cheap, rolls up perfectly into a backpack, and can be hung from a tree branch in seconds.

Here are the target dimensions to build one tuned for the 2m band (146 MHz):

1.Prepare the Ladder Line:

Cut a 60-inch length of 450-ohm ladder line. Strip about 1/2 inch of insulation from both ends. Twist and solder the parallel wires together at both the top and the bottom to create closed loops. Your total length from end to end should now be roughly 58 inches.

2.Cut the Notch:Measure from the bottom loop.

Measure up 19 inches from the bottom on only one of the wires. Cut out exactly a 1-inch section of that wire. The long continuous wire is your 1/2-wave radiating element; the side with the notch forms the 1/4-wave J-match stub.

3.Attach the Feedpoint:

Strip your 50-ohm coax (like RG-58 or RG-8X). Solder the center conductor to the long (un-notched) side of the ladder line. Solder the coax shield to the short (notched) side. Temporarily attach them about 3.5 to 4 inches up from the bottom loop.

4.Tune the SWR:

Hang the antenna in the clear, away from metal objects and your own body. Apply low power and check your SWR. Slide the feedpoint up or down slightly (usually just fractions of an inch) until you hit a 1:1 match on your target frequency.

5.Choke and Seal:

Once tuned, permanently solder the feedpoint. Add a ferrite choke (like a mix 31 snap-on, or 5-6 turns of the coax itself) just below the feedpoint to block common-mode currents from running back down your coax shield. Weatherproof the connections with marine-grade heat shrink or liquid electrical tape.