Part of our Building Your First Ham Radio Station guide.
The Slim Jim is close cousin to the J-pole, using the same basic electrical principle, a half-wave radiator fed through a matching stub, but built from ordinary flat twin lead cable instead of rigid pipe or ladder line spreaders. The result is an antenna you can roll up, stuff in a backpack pocket, and hang from a tree branch or window in under a minute, which makes it one of the most genuinely useful field and travel antennas for VHF and UHF.
What you actually need
- 300 ohm flat twin lead, the same type of cable once common for old television antenna feedlines, widely available and cheap.
- A short section of coax with a connector matching your radio, BNC or SMA depending on what you are feeding.
- Solder and a soldering iron for the connection points and shorting straps.
- Heat shrink or electrical tape to weatherproof the exposed conductor cuts.
How the build actually works
You cut the twin lead to a calculated overall length, then cut and remove a small section of one conductor at a specific point to create the gap between the matching stub and the main radiating element, exactly the same principle as the J-pole’s design, just implemented in flat cable instead of pipe or ladder line. A shorting strap connects the two conductors at the very bottom of the stub, and your feedline connects at a calculated tap point up from that bottom short, with the exact tap distance determining your impedance match. Free online calculators handle all of these measurements for whatever specific frequency you are targeting.
Why the flexible form factor matters
Unlike a rigid J-pole, a Slim Jim rolls up small enough to fit in a jacket pocket or the side pouch of a backpack, and deploying it is as simple as hanging it from any convenient point above you, a tree branch, a tent pole, or a carabiner clipped to something overhead. This makes it a genuinely excellent travel antenna, since it takes up almost no space in luggage yet performs dramatically better than a stock handheld antenna once you hang it up high on arrival somewhere.
Weatherproofing for repeat use
Since the whole point is repeated deployment and rolling up, the exposed conductor cuts and connection points need real weatherproofing that can survive being handled and rolled up over and over, not just a one-time seal. Liquid electrical tape or a properly heat-shrunk joint, checked periodically for wear, holds up far better over repeated use than ordinary electrical tape alone, which tends to lose its grip after enough roll and unroll cycles.
A genuinely great first build
Materials for a Slim Jim cost only a few dollars, the whole build takes under an hour, and the performance improvement over a stock rubber duck antenna is immediately noticeable the first time you hang one up and check into a repeater from further away than you thought possible. If you travel with a handheld radio at all, this is genuinely one of the best pieces of gear to have tucked away and ready to deploy.
Still deciding between the two designs? See Slim Jim vs J-Pole: Which Roll-Up Antenna Wins.