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300 Ohm Twinlead 6 Meter Slim Jim Antenna Project
Here is the answer to my question: My experimentation in creating a 6 metre 300-ohm Slim Jim antenna is here for you to try... I have finally made a prototype of the antenna, and it appears to work! I have attached some pictures for you to look at. Essentially the first versions I made were unstable. Using standard very light weight 300-ohm TV cable I had problems managing the very fine wire, both exposing it to make the connections and soldering the wire to the feedline of 50 Ohm coax. However, in the end I used a couple of short lengths of dowel to strengthen the feedpoint and top of the antenna to make it a bit more robust. I think ultimately the antenna would benefit from being mounted inside some PVC tube to help with weather proofing and rigidity. The final measurements (METRIC) I used are as follows: (The 6 metre Slim Jim antenna is centered on 52 MHz as I am only allowed to Tx in this region of 6 metres !!) The formulae will obviously work at which ever frequency you desire. Copy these lengths and convert to inches, feet, or any other conversion here! We have done the conversons for you to inches and metric. Overall length: 4155mm (163.58 inches) Short element (from base): 1338mm (52.67 inches) Top element (from top): 2740mm (107.87 inches) Leaving a gap of 77mm. (3.03 inches) The feedpoint is 209mm up from the base. (8.22 inches) At the design frequency (52mhz) the SWR of the Slim Jim is almost flat, with it being around 1.5 to 1 at each end of the band. I have it 'inserted' about 15 metres up in a pine tree at the moment.... suspended by a cord - using the top mounting configuration (dowel with a hole through it and the ribbon cable) to suspend it. Whilst I haven't made any Dx contacts with it, I will let you know whether it is a 'radiator' or a very long dummy load... The cut out section is wrapped in tape at this stage to help with strength. See pictures and diagram below. I am currently using an Icom 706 MKIIG radio rated at 100 Watts on SSB. The fact that having it suspended in a pine tree (RF vacuum) is not ideal, but my QTH here has no large masts at this stage. Given that the antenna is 4 metres plus (in excess of 12 feet) you need space and if I had the opportunity to suspend it from another non-conducting structure, I would ! Once I rack up some DX (local or otherwise) I will let you know, but given that I have only a local 6 metre operator a few blocks away, and no other operators close by, that may be a while. Maybe someone else might be crazy enough to try it like me. When I pull it down I will silicone up the ends of the 300 ohm line to weather proof it as much as possible, as well as the feedpoint. Hopefully it doesn't change anything, but as stated, it has almost flat SWR at 52.300MHz... I am sure this would travel up and down depending on how you made it. At the feed point, I only exposed a short amount of wire to connect to the coax, because everytime I exposed a lot of wire, it became brittle and I had major hassles with connection. The draw back of a small exposure of wire is that if I need to tune, I basically can't !! I would have to expose more wire, bit by bit. I have seen other 300 ohm twinlead with thicker wire... That may be worth a shot and would be much more workable than the thin, cheap stuff I have used in this project. Anyway, all the best. Have a go at this 6 metre version of the Slim Jim for yourself and have a G'day on 6 metres! 73 de Geoff VK6HOG, WESTERN AUSTRALIA Editor's notes follow: These are the formulas that you can use for designing the antenna for 6 meters or any other band for that matter. They follow: Lengths in inches 3/4 wave (longest section) = 8415/MHZ 1/2 wave section = 5610/MHz 1/4 wave section = 2805/MHz * 1/4 wave freespace = 2953/MHz * distance that antenna should be from mounting boom, mast, or tower. Example: 89.5MHZ = 8415/89.5 = 94.02" 89.5MHz = 5610/89.5 = 62.68" 89.5MHz = 2805/89.5 = 31.34" 89.5Mhz = 2953/89.5 = 32.99" Metric Formulas: for mm: 220.5 / MHz X 980 = Overall mm length 11.1 / MHz X 980 = Feedpoint up from bottom in mm (Approximate) 71 / MHz X 980 = Lower Wire Breakpoint(Length from wire cut point to base of antenna) Example: 220.5 / 50.0MHz x 980 = 4321.8 mm 11.1 / 50.0MHz x 980 = 217.56 mm 71 / 50.0MHz x 980 = 1391.6 mm Either of the formulas above should get you very close to the proper lengths for the antenna with a bit of experimentation as usual with any project. They may yeild lengths that are longer than needed for the final lengths and should allow room for tuning. As far as the gap spacing, I have not found any formula in my research for calculating it. As with any antenna, scaling can probably be used. For instance if the 2 meter gap is 1 inch as an example, then since 6 meters is 3 times as big, then the gap should be 3 times as large. This seems to hold true with Geoff's experiment with his 6 meter Slim Jim antenna. Another example, if the gap for a 4 meter antenna is 1/2 inch, then the gap for a 2 meter would be about 2 times smaller, or 2 x .5 = 1 inch. You should have no trouble running 100 watts with it. It should also be noted that when I built the Slim Jim antenna from 1/2 inch diameter aluminum tubing, that the free space distance was accidentally not used in the installation by error. No change in swr was noted either with or without using the freespace distance from surrounding metal. Since I have no way of checking the difference in the "8 degree" pattern other than over the air, with or without the freespace distance, only further experimentation will tell. 73 N4UJW Hamuniverse.com (PICTURES BELOW)
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