Slim Jim Antenna for VHF and UHF: How It Works and How to Build It
The Slim Jim antenna is one of the most popular home-built antennas on the VHF and UHF bands. It is a vertical, omnidirectional antenna made from a few metres of twin-lead or ladder line, it costs very little, it can be rolled up for transport and it gives repeater-quality coverage from a window, a balcony or a mast. It is often described as a J-pole in a different shape, and that description is correct, but the shape changes how it is built and where the feedline is attached. This guide explains how the Slim Jim works, what gain is realistic, how to build and tune one, and how to avoid the problem that spoils many home-built vertical antennas: current on the outside of the coaxial cable.
Where the Name Comes From
The antenna was introduced by Fred Judd, G2BCX, in 1978. According to the Wikipedia article on the J-pole antenna, the name came from its slim construction and the J-type matching stub, with “JIM” standing for “J Integrated Matching”. The same article says that the Slim Jim is related to the J-pole in a similar way to how a folded dipole is related to a dipole. In both cases the radiating element is folded back on itself, and the folded part makes the shape more compact and changes the impedance.
To understand it, start with the J-pole. A J-pole is a half-wave vertical radiator fed at its bottom end, where the impedance is very high. A quarter-wave shorted stub, built as a parallel line next to the radiator, matches this high impedance to the lower value of the feedline. The coax is connected at a point on the stub where the impedance is about right. The Slim Jim does the same job, but the radiating half wave is folded into a narrow loop at the top, and the matching stub is the lower section of the same folded structure. Because of this, the whole antenna fits in a single strip of ladder line, with no separate parts to join.
How It Compares With a J-Pole and a Ground Plane
A few claims about the Slim Jim are repeated on the internet more often than they are measured. The same Wikipedia article reports that Judd claimed a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane. It also says that other tests and analyses show no performance advantage over a conventional single-wire J-pole. The gain in the horizontal plane is given as roughly 1.5 to 2.6 dBi, or -0.6 to 0.5 dBd, which is close to that of a half-wave dipole. For comparison, the average free-space gain of a J-pole is near 2.2 dBi. If you are not sure how dBi and dBd differ, our guide to dBi versus dBd explains the conversion.
| Antenna | Typical gain | Construction | Strong point | Weak point |
|---|---|---|---|---|
| Quarter-wave ground plane | About 0 to 2 dBi depending on radials | Vertical element plus radials | Simple and robust | Needs radials and a mount |
| J-pole | About 2 dBi | Tubing or ladder line, two parallel conductors | No radials, good DC grounding of the stub | Sensitive to the mast and nearby metal |
| Slim Jim | About 1.5 to 2.6 dBi | One piece of ladder line or twin-lead folded as a loop | Cheap, light, easy to roll up | Needs trimming, needs a choke on the feedline |
| Collinear J (two half-wave sections) | About 4.6 to 5.2 dBi | Longer, with a phasing section | More gain at a low angle | Taller and harder to build |
The result is that a Slim Jim is not a gain antenna, in spite of what some sellers imply. It is a good, low-cost vertical with a pattern like that of a dipole, and its main advantage is practical: it is made from a few dollars of ladder line and can be built in an evening. For more VHF antenna options, including the ground plane and the collinear, see our comparison of 2 metre antenna types.
How the Radiation Works
Inside the Slim Jim, the current distribution is what you would expect from a vertical half-wave dipole. The current is maximum near the middle of the radiating section and zero at the ends, while the voltage is high at the ends. Radiation comes mainly from the upper half-wave part of the loop, where the currents in the two sides of the fold flow in the same direction and add together. In the lower matching section, the currents in the two conductors flow in opposite directions, so they cancel each other’s radiation, which is the property of a good transmission line. This is the reason the matching stub does not disturb the pattern, in principle, and in practice some residual radiation from the stub slightly changes the pattern, as the Wikipedia article notes for the J-pole.
The vertical polarisation and the low takeoff angle make it suitable for FM repeaters, simplex contacts and local nets, where the other stations also use vertical antennas. For SSB and weak-signal work, where horizontal polarisation is the standard, a Slim Jim is the wrong choice.
Building a Slim Jim From Ladder Line
The popular version uses 300 ohm twin-lead or 450 ohm ladder line, because the cable is cheap and the insulation is weatherproof if the cable is the type with solid dielectric windows. The general plan for any band is as follows. Dimensions depend on the frequency, the type of line and its velocity factor, so use a design calculator or a proven plan for your chosen cable and then trim. Do not trust a single set of numbers copied from a forum for a different line.
- Decide the frequency. For a repeater antenna, centre the design in the middle of the part of the band you use, for example 145 MHz on 2 m. The wavelength in free space is 300 divided by the frequency in MHz, which gives about 2.07 m at 145 MHz.
- Cut the line slightly long. Make the overall length a few centimetres more than the design says, since it is easy to shorten but impossible to lengthen.
- Prepare the folded section. Cut a notch in the conductors at the top and at the bottom of the radiating loop, as the plan shows, so that the signal path goes up one side and comes back on the other, forming the half-wave loop. At the bottom of the matching section the two wires are shorted together.
- Mark the feed point. Prepare a place for the coax a short distance from the bottom, on both conductors of the stub, where the plan specifies. The centre conductor goes to one side and the braid to the other.
- Seal and hang. Cover the cut points and the feed joint with self-amalgamating tape or heat-shrink with adhesive, and hang the antenna from a non-metallic support.
Keep the antenna away from conductive objects. The J-pole article gives a useful rule for the matching section: parallel conductors should be kept at a distance of two to three times the spacing between them from other conductors such as downspouts, metal window frames and flashing. A Slim Jim hung against a metal wall or inside a metal frame behaves differently from the design and will not tune properly.
Tuning With an Analyzer
You do not need a lot of equipment, but a simple antenna analyzer or a NanoVNA makes the job much easier. Our guide to the NanoVNA and antenna analyzers describes the calibration. With the antenna in its final position, sweep the band and look for the dip in SWR. If the dip is below the frequency you want, the antenna is electrically too long, so trim a little from the top of the loop. If the dip is above the frequency you want, the antenna is too short, and the only remedy is to build another or to add a small piece at the top. Move the feed point slightly along the stub to adjust the match: shifting it toward the shorted end lowers the impedance and shifting it away from that end raises it.
Take notes after each change and avoid trimming more than a few millimetres at a time. Repeat the measurement with the antenna in the same position, as the surroundings change the result. When you are satisfied, repeat the sweep at the highest and lowest frequencies of the band to see the bandwidth. A very good dip with a poor match at a nearby channel suggests that the antenna is picking up current on the coax, which brings us to the next point.
The Feedline Choke
A Slim Jim, like a J-pole, is a balanced antenna fed with an unbalanced cable. The J-pole article says that these antennas can be fed with a coax cable if the feedline includes a means of suppressing currents on it. Without a choke, RF flows on the outside of the braid and the coax becomes part of the antenna. The result is a distorted pattern, a changing SWR whenever you move the cable, and possible RF in the shack. A choke is simple: wind 5 to 8 turns of the coax in a coil of about 15 cm diameter close to the feed point, or use several ferrite cores on the cable. Our guide to common mode chokes explains the types and how they work. Place the choke at the feed point, not near the radio, because the aim is to stop the current where it starts.
Mounting and Weatherproofing
- Use non-metallic supports. A fibreglass pole, a PVC pipe or a nylon cord keeps the antenna away from conductors. Do not tie it with wire.
- Keep the ladder line off the wall. The matching section has a field that extends outward, so use stand-offs of a few centimetres if you hang it on a wall, and keep it clear of gutters.
- Protect from water. Moisture on the open windows of a ladder line detunes it. Solid twin-lead without openings is less sensitive, and a PVC tube over the antenna keeps rain off and also supports the structure.
- Mount it high and in the clear. At VHF and UHF the horizon is limited by the line of sight, so every metre of height helps more than any change in the design.
- Take care with the mast. A metal mast close to the antenna spoils the pattern. Mount the Slim Jim at the top, with the whole antenna above the mast.
Common Problems and Their Cures
A resonance in the wrong place usually means a mistake in the length of the loop or the measurements of the line, and even a small change in length moves the resonance noticeably at VHF. A high SWR across the whole band suggests a bad connection at the feed point or a short at a cut point. An SWR that changes when you touch the cable points to a missing choke. If the antenna works well on the bench and poorly on the roof, check what is nearby: a metal gutter or a vent pipe will change the tuning, and the fix is to move the antenna or to retune it in place.
The Bottom Line
The Slim Jim is a folded J-pole made from a single strip of ladder line. It is a vertical, omnidirectional antenna with a gain about that of a dipole, and the claims that it outperforms other designs are not supported by measurements. Its strengths are the low cost, the light weight and the ease of construction. Build it a little long, tune it with an analyzer in its final position, keep it away from metal and add a choke at the feed point. On 2 m and 70 cm it is an excellent first antenna for repeaters and simplex.
Frequently Asked Questions
What is a Slim Jim antenna?
It is a vertical, omnidirectional antenna that is a variation of the J-pole. The radiating half-wave section is folded into a loop and is combined with a matching section in a single piece of ladder line or twin-lead.
How much gain does a Slim Jim have?
Measurements reported for the Slim Jim show about 1.5 to 2.6 dBi in the horizontal plane, similar to a half-wave dipole. It is not a high-gain antenna.
Is a Slim Jim better than a J-pole?
Not in performance. Some tests find no advantage over a conventional J-pole. The difference is mainly in construction: the Slim Jim is made from one strip of line and is easy to roll up.
Do I need a choke with a Slim Jim?
Yes, in most cases. A choke at the feed point stops current on the outside of the coax, which would otherwise distort the pattern and the SWR.
Can I use a Slim Jim for SSB or weak-signal VHF?
It is vertically polarised, so it suits FM and repeaters. Weak-signal and SSB work on VHF and UHF normally uses horizontal polarisation.
How do I tune a Slim Jim?
Build it slightly long, then sweep the SWR with an analyzer in the final position and trim the top of the loop a few millimetres at a time. Adjust the feed point along the stub for the best match.