Tried making my first slim jim antenna...fail

[ClemsonSCJ]

So as the title suggests, I decided to try my luck at a slim jim last night and it was an epic fail. Couple questions that I have though cause from the videos I’ve watched it’s kinda hard to tell. The twin lead wire that I have is not ladder line, it’s just got a solid strand of plastic between the 2 lines (I know this doesn’t affect anything, just thought it would give a better idea as to what I have). The 2 biggest things I was curious about is 1) are most twin-lead/ladder line antennas solid wire or stranded wire. I guess I had an assumption that it would be solid wire but when I started stripping it I found that it was stranded. Wasn’t sure if this would affect performance but I have it a try anyways. And 2) does the gap between the 2 lines matter? I know the ladder line I see in most YouTube videos is much wider and the calculator I used to determine all my length had “4.1cm” as the default separation with, but mine is about 0.7cm from the center of one line to the center of the other, with about 0.5cm between them “edge-to-edge”. That seems like a considerable difference but I’m not sure if it matters or not. Anywho, my lack of soldering skills didn’t do me any favors either. When it was all said and done I was getting an SWR of 7.0:1 ( ) so it was certainly an epic fail...or a massive win if you wan to look at it from the Thomas Edison bulb perspective.

 

[ko6jw_2]

I have never had any luck with TV twin lead to make a roll up j-pole. Ladder line is the only way to go. There are all sorts of designs out there most of which work to one degree or another. Ladder line is solid conductor. I think the larger spacing is crucial.

Good luck.

 

[nd5y]

J-poles and slim-jims suck. You found that out the hard way. Twin lead wire spacing matters and so does the dielectric constant of the plastic. You have to take both into account.

Make a 1/4 wave ground plane. They always work.

 

[AK9R]

TV twin lead works fine for roll-up J-poles. I have several for 2m, 222MHz, and 440MHz. They were all made using a particular type of Belden 300-ohm twin lead (maybe 8225). Next to impossible to find these days. It's a matter of playing around with the lengths until you find a combination that works.

Where these roll-up J-poles, and any J-pole, fail is when they are mounted with metallic objects in close proximity of the quarter-wave transformer section. Nearby metal will disrupt the magnetic field set up by the transformer and de-tune the antenna.

 

[popnokick]

Our Club (and myself) have found one type of J-pole that works very well: The J-poles designed by Ed Fong WB6IQN, PhD. In fact, they work so well that his website now says they cannot keep up with production. HOWEVER, his designs have been published in QST in three different issues; the most popular being the dual-band DBJ-1 published in QST February 2003. There are complete build-it-yourself instructions. And if you can't find / don't have the QST issue, the link to his website below includes downloadable PDFs for the DBJ-1 article and the other two antennas.
A couple of caveats apply: 1) The advice to keep his J-poles (ANY J-pole antenna) away from metal objects is good advice. They WILL couple / detune in the proximity of metal objects. 2) If you are going to change the type of twinlead used or the coax feeding the antenna from what is specified in his articles you MUST use a VNA to determine the proper lengths based upon velocity factors and tuning parameters. J-poles are very narrow band antennas... fine for ham band use, but not general scanning.
Here's his website with the files containing the construction articles -
About

 

[ClemsonSCJ]

I have never had any luck with TV twin lead to make a roll up j-pole. Ladder line is the only way to go. There are all sorts of designs out there most of which work to one degree or another. Ladder line is solid conductor. I think the larger spacing is crucial.

Good luck.

So with some help from KC5AKB finding me some alternative plans with slightly different measurements, I made a new one today that I was much more meticulous in my design and fabrication and it turned out much better. Got a SWR of 1:1 on 145MHz this time and never went above 1.3:1 over both 2m and 70cm bands. Final product turned out well I’m quite pleased this time.