Does anyone have a good well-tuned and matched roll-up slim-jim I could just copy please? (material, dimensions)
I have built at least 5 of them and I have never managed to get it right. I have used some “kitchen” recipes, that is: “Get a 300ohm ladder line, cut X centimeters here and there and solder the coax here and it will work.”. I have used M0UKD’s online calculator. Nope. It always tunes out of band.
I have noticed there are basically 3 popular available materials:
The 300ohm twin lead (narrow), the 300ohm ladder line (narrow) and the 450ohm ladder line (wide).
I managed a not-so-bad match with the wide 450ohm ladder line, but it is not very flexible and the solid-copper core conductor is prone to breaking. The ones from the narrow material are very practical in terms of packing, but have always been just awful. (out of resonance, bad match)
I built several J-poles using the dimensions shown in my roughly sketched diagram.
The antenna is measured with its top at 3m height. Maybe you have to adjust the resonant frequency by varying the top length.
I can confirm that Peters J-Pole diagram works very well with the 300 Ohm twin-lead by DX-wire.de . One remark - and it might be obvious - I had troubles making a good J-Pole and the problem was actually the coax cable which was of poor quality.
Thanks to Peter for the troubleshooting we did together
So a good coax (H155, or for lightweight: RG316 or RG-188) that is having low losses on 2m are mandatory to make it work properly.
https://vk3afw@wordpress.com.
This afternoon I used solid black 300 ohm ribbon to make another one. The lengths were within a mm of the original and SWR 1,2:1 at 146 MHz. Good across all of 2 m according to my fancy analyser.
If not putting in a radome and if you like rule of thumb, calculate a half wave using 300/f. Reduce this by 5% for end effect. Use a Vf of 0.81 if using 300ohm ribbon to find a half wave. Add a quarter wave equal to half the half wave giving 3/4 wave length of ribbon. Notch one conductor at the junction of the quarter wave and half wave. Bare the conductors 1 1/4 inches from the bottom and solder coax on here with screen to side of quarter wave that is cut. Short the bottom and top. Hang and test. You should be happy.
Note that a Slim Jim and a J-pole are different antennas. The Slim Jim is more compact. I made a Slim Jim using aluminium welding rod and trimmed it to resonance on 145 MHz but unfortunately I didn’t keep the dimensions.
Thanks for the replies so far guys! There is a lot of black magic and myths around antennas. The reason why I want to slim-jim so much (as opposed to a J-pole) that various sources say that the lobes of the slim-jim are more “squashed” towards the ground, giving you more gain at low elevation angles. That it doesn’t radiate to the sky so much.
I’m using VHF in 2 kinds of situations: 1.) when my HF equipment fails or the conditions don’t allow for putting up an HF antenna 2.) for summits when carrying HF would be too difficult and VHF is actually more fun, that is, very high mountains in the Alps or Tatras. Especially for those QTHs the slim-jim looks like a winner to me. I feed them with H155 and hoist them up on a 6 meter high telescopic fiberglass mast.
A Slim Jim and a J pole have essentially the same performance. This shouldn’t be a surprise, they are both half wave antennas.
Working with twin-lead, I recommend the Ed Fong dual band J pole. That gives you 2 m and 70 cm in the same antenna. You can buy pre-made antennas from Ed or you can make your own. Scroll to the bottom of this page and he’s added links to the QST articles describing the antennas.
In addition you could find a ready made one via eBay.
Sometimes I think that considering the cost of connectors, twin lead, coax etc etc plus of course your time, its better to just spring for a ready made $26 version!
That looks like the same material David! Do you have the measurements please? Did you use a calculator or did you experiment and did the good old trial and error? I finally managed to get mine working. I had used the M0UKD’s online slim-jim calculator, but I had to add about 4cm to the top and about 2cm to the bottom part of the antenna. When I did it purely by the calculator it was tuning at 148+Mhz.
You can get pre-built Ed Fong antennas, too. Ed points out that it isn’t easy to get the parts in small quantities, especially the good quality twinlead, so it is best to either buy one or have your club make a bunch and sell them.
I tried to build the same antenna.
Diemensions as mentioned in the calculator.
I ended up cutting the 300 ohm ladder from 1.5m and 128 Mhz down until it reached a length of 1.3m and the antenna was even there resonant on 130 Mhz…
I realy don’t know whats going on haha.
Maybe u have an idea.
Ed Fong is now at $64 USD and it is a well made antenna that performs as it should. However, I chose the N9TAX version for about $25 USD a few years ago and is now $31.99 USD. Showed no difference from the Ed Fong version - my club ha a little shoot-out with one vs the other. Ed’s uses 300 ohm which is lighter and easier to wind up and stow which is an advantage for SOTA users vs the N9TAX version using 450 ohm ladder line. I can say that either one works great on a 12M Spyder Pole!!
The thing that’s messing you guys up is the velocity factor of the ladderline. You really need to either use a dip meter or an analyzer here. The dip meter will give a resonant point you can work from. The analyzer can show resonance and then you can work backwards to determine the vF. Also, when attaching the coax, just tack solder till you tweak the location. I find it needs to be a little higher than stated. Also place a few ferrite chokes near the feedpoint to keep the coax from altering the radiation pattern.
What really messes things up is applying the velocity factor to the half wave portion of the twinlead. That section isn’t working as a transmission line (the currents are the same direction in each wire), so the standard velocity factor doesn’t apply. But it does apply to the lower quarter wave matching section.
Because there are so many different types of twinlead lead / ladder line, the best way I’ve found to match one is to cut the radiator to a formula 1/2 wavelength, then adjust the short circuit at the bottom of the matching stub and the coax tap point. In the past I have used pins stuck through the insulation to find the right tap points, but they don’t always make good contact.
For reference, I have 2 J-poles made from flat 300 ohm TV twinlead. one has a 93cm radiator, and total length to the bottom of matching stub (as near as I can tell, given the heat shrink over it) is 135cm. The feedpoint is 4cm above the short circuit. The second is about 1cm shorter in all measurements. These would be set for around 146.5 MHz, but should give a good starting point. Of course, 7 years after the initial post, you probably have already found a solution.
Both of these J-poles just leave the second wire in the radiator unconnected. It makes no practical difference whether the two wires are connected together at either or both ends: the wires are closely coupled, the currents are basically the same in each, and together they will work as a half radiator in any case. In fact, for light weight, I’d replace the upper 1/2 wave radiator with a single piece of flexible wire and just use twinlead for the matching stub. Especially since TV twinlead is harder to find around here these days, so there is no sense using it where it isn’t needed.
I’ve used the M0UKD web calculator and for a while, I just couldn’t get a Slim Jim to tune where I wanted. Finally figured that the vF that DX Engineering stated was off a bit. Bought a dip meter and found the length for a half wavelength and cut it in half. Ended up with .93 instead of .91 and that difference on a 10m antenna was enough to mess it up. Once I got the right vF, his calculator was right on the money. By the way, DX Engineering also sells 300 ohm twinlead that is slightly heavier gauge and works nicely. I’ve used that to feed a medium sized doublet.
