How a J-pole works
A J-pole is a half-wave vertical fed from its bottom end. The end of a half-wave element has a very high impedance, thousands of ohms, so it can't connect straight to 50 Ω coax. The J-pole solves this with a quarter-wave matching stub: two parallel conductors shorted at the bottom. The impedance along the stub rises from nearly zero at the short to very high at the top, so somewhere in between there's a point close to 50 Ω. That's where you connect the coax.
A Slim Jim works the same way, but the half-wave radiator is folded back down on itself with a small gap. It's slimmer and lighter, and easy to make from ladder line or twin lead and hang inside a fibreglass pole.
The dimensions
Everything is a fraction of a wavelength, shortened by the velocity factor of whatever you build it from:
Matching stub = ¼ wavelength × velocity factor
Radiator = ½ wavelength × velocity factor
Overall length = ¾ wavelength × velocity factor
The spacing between the two conductors isn't critical, but it changes the stub's impedance and so moves the feed point. 2 to 3 cm is typical on 2m. The feed point given here is a starting point. Expect to slide the coax connections up or down a few centimetres to find the lowest SWR.
Tuning tips
- Set the length first, then the feed point. Measure where the SWR is lowest. If it's below your target frequency, shorten the top of the radiator. Then move the feed point to bring the SWR down.
- Move both feed connections together. Keep them level with each other as you slide them up or down.
- Add a choke. J-poles are end-fed, so current likes to flow on the outside of the coax. A few turns of coax in a 10 cm coil just below the feed point, or some clip-on ferrites, will steady the SWR and keep the pattern low.
- Mount it clear. Keep the radiator clear of metal. A J-pole made from copper pipe can be clamped to a metal mast at the shorted bottom, which is at zero voltage.
Twin lead and ladder line have a lower velocity factor because the plastic slows the wave down, so antennas made from them are noticeably shorter. Different brands vary, so trim to suit.
Questions
How long is a 2m J-pole?
About 1.47 m overall at 145 MHz in copper pipe, with a matching stub of about 49 cm and a radiator of about 98 cm above it. The feed point starts around 8 cm up from the shorted bottom.
Is a Slim Jim better than a J-pole?
Not noticeably. Claims of extra gain for the Slim Jim are mostly myth. Both are half-wave verticals with similar performance. The Slim Jim is thinner and easier to make from ladder line, while a copper J-pole is more rugged.
Can I make a J-pole for 2m and 70cm?
A 2m J-pole often works on 70cm too, because it's close to three half-waves long there, but the pattern lifts upwards and the match may need a tuner. A dedicated dual-band design or a separate 70cm antenna works better.
What velocity factor should I use for ladder line?
Around 0.91 for 450 Ω windowed ladder line, and around 0.82 for solid 300 Ω twin lead. Different makes vary, so build slightly long and trim.