For dimensions see PDF |

This 10m one, made from 450 Ohm slotted ribbon cable from Moonraker and secured on an 8m fishing pole, was inspired by Jim Bacon G3YLA (also a fellow Norfolk ham) who brought one along to our annual “Radio by the Seaside” event.

It worked so well (beating a Rybakov vertical by about 6 S points) that I thought it needed more attention.

The result is attached - it took a lot of fiddling to optimise the length, cut out and feedpoint, but now you don't need to!

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ReplyDeleteNot sure yet - I would imagine they will be very similar. Hope to do a back-to-back test this week (weather permitting).

ReplyDeleteAlso need some signals to hear!

Steve G0KYA

I want to use this design on 6M any dimension hints or links to something already done..tnx gene wd5fdl

ReplyDeleteYou could take the dimensions of this one and scale for 51 MHz. That is, if this was designed for 28.5 MHz just multiply everything by 0.559. It will be in the ballpark.

ReplyDeleteSteve G0KYA

I've been thinking of making a 10 m slim jim for some time. Thanks for the construction guide. The dimensions I was planning to use were slightly different to the one here. I'll give it a go ... 5W into a slim Jim. Chris. G4KDX

ReplyDeleteHi. any chance please someone could give me the adjusted correct figures to resize this antenna for 27.5 instead of 28.5mhz? it would be greatly appreciated as I've had the new 450ohm cable hear waiting for some time for me to get on with it. thanks, peter

ReplyDeleteThe "Star Trek" Bill Shatner???

DeleteThe lengths and dimensions will change depending on the wire you use to make it, and how far you have the base of the antenna above anything metal. Bare copper wire has a faster velocity factor than PVC insulated wire, so you can reduced the lengths required by using the latter. For instance, to work out the total height of a Slim Jim made with bare copper wire you would use the formula: 300 divided by the frequency in mhz (the calulated length), times 0.98 (the velocity factor of bare copper wire), times 0.75 (because it's 3/4 wave in height). For 27.5mhz that will be almost exactly 8m (26.3 feet) high. But if you use PVC insulated wire you need to use the formula: 300 divided by the frequency in mhz, times 0.96 (the slower velocity factor), times 0.75...That gives a total height of 7.85m (25.7 feet)...About 7 inches shorter which reduces the height and how much wire you have to use. To find the height up from bottom to place the bottom of the gap simply multiply the calculated length by 0.96 and then by 0.25 Because it's a 1/4 wave high stub). The length of the gap can be critical to tuning. For 2m you need about 1" (25mm)...I would start at 1" and test it (use an analyser and try finding the feedpoint...If the tuning is difficult to acheive then the gap is probably too small, so snip a little bit off the top wire (about 5-6mm) and test again as before...Repeat until both the radiation resistance and SWR are the lowest you cab obtain. Note, only cut bits off the top wire (the wire above the gap) not from the stub below it. If you cut too much off the top wire you can lengthen it again by soldering a short piece of wire back on. It should really be an air gap, so if possible avoid using an insulator like PVC to hold two ends of the wire above and below the gap together as it will also change the tuning. It is better to use a mechanical means to hold the two ends parralel to each other making sure there is just air in the gap between the two. Another critical factor in making tuning easy is making sure the antenna is as far above any metal objects as possible...For 2m I recommend 1 wavelength, which is about 2m (6 feet)...It is still possible to do this on 27 or 28mhz, by mounting the antenna on a fibreglass tube mast, at least 30 feet long or longer...Here in the UK such tubes (properly called "Pultrusions") can be bought from R B J Reinforced Plastics in Rickmansworth. 73 Alf (G7VGG)

DeletePete Thomas, OH2EUU wrote to me to say; Dear Steve, just to let you know how much I appreciate your 10m Slim JIM instructions. In the end I had to add 17cm in total length and found the best feedpoint to be at (centre) 24cm and (sheath) 25.5cm rather than your 13cm. This achieved a centre resonance of 28.500 MHz with an SWR 1:1.28.

ReplyDeleteSo his total length was 7.48m. To make one for 27.5MHz, basically multiply all the figures by 28.5/27.5=1.036

So length 7.75m, cut out at 2.393m and feed point at about 25.9cm. Should be ballpark figures for 27.5MHz.

Steve G0KYA

Thanks A lot Steve for the help, it has greatly encouraged me. I couldn't get the maths but the figures you give me an approximation to start from.my major concern was trashing lots of 8 odd meter lengths of line before getting it right and I am not rich. I'll post the results when I have got somewhere. peter

ReplyDeleteHi,i have finally built the antenna and put it up. total length is 7.75 meters, the feedpoint with the lowest swr across the usa fcc cb band is 335mm from the bottom, and the cutout is 260cm from the bottom. swr is 1:1 on ch1 rising to 1:1.2 on ch40. contacts on ssb on a quiet band 30 miles so far. it is working but how well time will tell. thanks all. Peter

ReplyDeleteI did the same. I built one for the CB band using 25 feet of 450 Ohm ladder line., center frequency at 27.2 MHz. It actually is resonant at 27.4 MHz with an SWR of 1.14:1 Works very well.

ReplyDelete