I know this has been asked a 1000 times but I need a specific answer from someone who has built one of these antennas.

im using this calculator to get me close to my frequency, and then a NanoVNA to tweak it. https://m0ukd.com/calculators/slim-jim- ... alculator/

But iv noticed the feedpoint calculation is well off with no balun. Its probably double what it says on the calculator.
My VNA is reading 1.5 swr with no balun. But im now using a 4:1 Balun to get it near perfect.

My understanding is that it needs to be fed higher up because the impedance is higher. But with the balun attatched, the best feedpoint seems to be the same. Could the balun not be working ?

Im using this method to create a balun. Im not even sure if its correct or not. Image below.
The coax length is a 1/4. taking the VF into account. RG213 0.66.

XXL wrote:I know this has been asked a 1000 times but I need a specific answer from someone who has built one of these antennas.

im using this calculator to get me close to my frequency, and then a NanoVNA to tweak it. https://m0ukd.com/calculators/slim-jim- ... alculator/

But iv noticed the feedpoint calculation is well off with no balun. Its probably double what it says on the calculator.
My VNA is reading 1.5 swr with no balun. But im now using a 4:1 Balun to get it near perfect.

My understanding is that it needs to be fed higher up because the impedance is higher. But with the balun attatched, the best feedpoint seems to be the same. Could the balun not be working ?

Im using this method to create a balun. Im not even sure if its correct or not. Image below.
The coax length is a 1/4. taking the VF into account. RG213 0.66.

Hi XXL. I made a roll up version using ladder line and had a similar experience.

This antenna seems very sensitive to adjustmehts in length and feed point. I spent hours making adjustments to mine and eventually got SWR down to 1.2:1.

I didn't make a balun. I wound the coax (4-5 turns ~45mm dia secured by zip ties) to make a choke. The choke sits about 6" from the feed point.

In summary, tuning the damned thing took far longer than making it. Good luck.

Yeh I was using it with a few turns of coax, but wouldn’t that just be a guess rather than the correct turns/coax amount etc for the frequency? Or would the difference be so small it wouldn’t be noticeable?

It was a guess using the suggestion on the calculator as a guide. I tried 4 then 5 turns and it made no real difference so I stuck with 5. Important to keep it tight and to ~40mm diameter.

I remember building one out of copper pipes, welded the lot and it was well out of frequency by about 5mhz according to my swr meter. That was using the calculator.

I suggest connecting your wires/pipes loosely and trimming them till the swr meter approves.

I now use a 5/8 vertical at almost 1:1 with near to nothing being reflected. The signal is booming. I imported it from a European ebayer but it only handles 300w lol

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I built the 'Slim Jim', the ribbon feeder version using that calculator over Christmas and it worked pretty well. I didn't use an antenna analyser, so my resonant frequency may have been a little off, but by moving the feed point up a bit I got an SWR of < 1.5:1.

You don't need 4:1 impedance transformation, the antenna is designed for a 50 ohm feed, you just need a choke to help minimise feeder radiation; I just used about 7 turns of RG58 on a bit of 20mm diameter PVC waste pipe. The main coax feed was with higher quality coax.

I fixed the finished aerial to a 10 metre flag pole with cable ties and tape, it was just for a bit of fun, nothing serious. I've attached a couple of photos.

In the end I used the 4:1 coax balun. But the feedpoint was roughly in the same place. I managed to get 1:5:1 at site.

That's good enough TBH.

My J-poles were usually around 1.35 : 1 after a lot of messing with them. They worked really well, though.