The GFM Slim Jim

By Techno Walter

The Slim Jim is an omni directional antenna with a theoretical power gain of around 6dB relative to a dipole. It's long been a favourite of GFM and many similar stations as it's cheap and easy to make and it's reliable with predictable results. In other words, it's one of the simplest antennas to get a result with and no great loss when the DTI nick everything.

Slim Jim

Basic Structure of Slim Jim

The Slim Jim was invented by English ham Fred Judd in the 1970's. Apparently so-called because of the acronym JIM standing for J-Integrated Match. Sounds a bit contrived to me, but never mind. It's similar to a J-Pole which is a half wave vertical attached to a quarter wave matching stub. The Slim Jim is a folded version of the J-Pole, adding another half wave electrical length and leaving a small gap between the end of the top section and the matching stub (see diagram on the left). The resulting antenna has an "omnidirectional" radiation pattern in the horizontal plane but achieves modest gain relative to the J-pole and the dipole by "stretching" the radiation pattern in the horizontal domain and sending less of the power up into the air.

I have no doubt that most slim Jims don't achieve anywhere near the oft-quoted 6dB but if you can achieve 3dB with this simple antenna it's well worth doing.

As mentioned above, this antenna is a full wave with a quarter wave matching stub (which is actually a half wavelength of wire) attached to it. The total electrical length of the antenna (in bold black in the diagram on the left) therefore is 1.5 Wavelengths. You can therefore start with a single piece of wire, rod, pipe or whatever, 1.5 Wavelengths long, and simply bend it into shape at the appropriate places, using the following as a guide (refer to the diagram on the left):

A - approx 0.75 of a wavelength
B - approx 0.5 of a wavelength
C - approx. 0.25 wavelength

I'm saying "approx" because the spacing between the two sides of the antenna (E) and the "end gap" will alter matters slightly. The dimensions of these are not critical and the important things are the value of C (quarter wave) and the overall electrical length of the whole thing.

D - slide for best SWR. Bigger D means higher impedance. We usually find the best feed point with 50 Ohm coax is with about a 10cm value for D.
E - not critical, usually around 4 - 5 cm. Depending on what you've made the antenna from, if it's supported on some kind of mast or self supporting, etc.
End Gap - not critical, 2 or 3 cm is fine.

One last point about the lengths. To counteract proximity effects, GFM engineers usually multiply all lengths by the GFM Fiddle Factor of 0.95 - this works for us - so you can use an overall length of 1.5 wavelengths * 0.95.

In the 1980's, GFM used to broadcast from houses, using slim jim antennas suspended in the loft space. These were usually made from a single piece of stiff wire suspended on a piece of timber, broom handles, or some combination of the two. Results weren't so bad (unless you were trying to listen to the radio next door!). The materials you can make these out of is pretty flexible, and depends on your application or the environment within which your antenna is going to operate.

In addition to bell wire, we've also made these out of thin copper central heating pipe (the thin bore stuff, which I think is 8mm) which you can buy on a roll and simply cut to the required length before bending.

The most recent effort uses metre long aluminium welding rods joined together with brass nuts, and supported on a piece of plastic overflow pipe. The feeder is attached to the aluminium using brass fittings liberated from an old mains wiring junction box. The ends of the coax are soldered to one side and the welding rod is slid through the terminal and secured with the screw. As we've still got such an antenna I'm going to describe it here. It was made as follows but the pictures below explain this better than words.

  1. Join together enough welding rods to achieve the required length of 1.5 wavelengths. Hint - you're going to need four. The ends of the rods are threaded with a 4BA die, and will be screwed into home made junctions which are made by threading 3 brass 4BA nuts onto a piece of threaded welding rod and soaking the whole lot in solder. Take care not to burn yourself ;-) After we screwed our rods together, we slid some heat shrink over the junctions and er... shrunk it!
  2. Feed Point The antenna will be fed low down on the section of rod which makes up the quarter wave matching stub. You can't solder to the aluminium welding rods so if you use junction box terminals as we did (see pictures below) then you may need to slide a couple onto one section of welding rod before bending.
  3. Bend according to the required dimensions To make the rounded ends, we bent the welding rods around a piece of 3/4" copper pipe. Practicing first pays off. Luckily those welding rods are ten a penny.
  4. Support Our antenna is supported by a piece of white PVC overflow pipe, about 10 feet long. This was, at some stage, mounted onto the top of an aluminium mast using the sort of clips which would have held the overflow pipe to the wall. We threaded our aluminium sections through holes drilled in the pipe, and if you take this approach you will need to give careful consideration to the order in which you build the whole thing. To stop the aluminium from flapping about, cable ties were used to secure it to the pipe along the length. Extra stiffness was achieved by shoving a smaller piece of plastic pipe inside the length of the whole thing.
  5. Matching Those terminals from the junction box mentioned above, provide an ideal method of setting up the antenna for best SWR - mount the antenna in a convenient place, connect your SWR meter and slide the feed point up and down the matching stub until best match is found. Run as little power as your meter needs to hit its calibration setting for SWR measurement. The feed point will be quite low down the stub. The coax braid attaches to the side nearest the gap, the inner to the longer piece of the antenna, and both sides of the feed should be level with each other horizontally.

Pictures

Here are some pics of the various parts of our slim jim antenna. Click on any of them for a closer look.

The antenna is made from aluminium welding rods supported on a plastic pipe. The ends of each rod are threaded and joined together using junctions made from four 4BA nuts soldered together.

To make the junction, take four 4BA brass nuts and tin the flat surfaces of each with solder. Screw the nuts together onto a threaded piece of welding rod and heat to melt the solder. Above is the result.

Join the sections of antenna together using the brass nut junctions. Screw the ends tightly together, and seal each one with 2 layers of heat shrink as shown here.

The antenna feed is made from brass terminals out of a mains wiring junction box. The coax is soldered to the back of each terminal and the aluminium welding rods pass through each terminal, allowing the feed point to be slid around for best SWR.

The screws are tightly secured into place and the feed point can be sealed, e.g. in a potting box or similar. The 3 loops of coax attached to the mast below the feed point are a simple coax balun to reduce radiation from the feeder.

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