How To Build A DIY 2M Roll-Up Slim Jim Antenna (Including How To Crimp A PL-259 UHF Connector Onto RG8X Coaxial Cable): Step-By-Step Instructions
Sure, the Slim Jim and it’s cousin, the J-Pole, can be easy beginner antennas to make. But there are so many things to take into consideration when building one that it becomes an all out lesson in antenna theory if you let it. Lets start with the basics: How does the darn thing work anyway??? I don’t know either…
The best references I found for this build are here (https://m0ukd.com/calculators/slim-jim-and-j-pole-calculator/) and here (https://www.youtube.com/watch?v=igHQ3EsE5Bk). They both make assumptions that you can handle basic antenna building tasks. I am going to go in more detail in the post. Furthermore, I am only going to describe here how I made it and what my results were. Modify this in any way you see fit…mine did not work out perfectly as you will see.
MATERIALS
450 ohm window line (twin lead) – 5 feet
RG8X coax cable – 1 foot
PL-259 (“UHF”) Crimp Coax Connector (one)
Various sizes of heat shrink tubing
Solder
Measuring tape (metric is handy)
Extra wire (I made some mistakes and needed wire for jumpers)
Chalk for marking ladder line
Cable ties of various sizes
Screw-on wire connectors (two)
Lead wires with alligator clamps on one end and bare wire on the other (two)
EQUIPMENT
Utility knife
Coax crimp kit (Mine is Lands Precision or LP, available at Ham Radio Outlet)
UHF/VHF antenna analyzer (Mine is RigExpert AA-650 Zoom)
Heat gun (for heat shrink tubing)
Solder Iron
Wires strippers and cutters
Multimeter (continuity meter)
Sewing pin
MISCELLANEOUS ITEMS
Coax jumper (I used both a 25 foot and a 50 foot professionally terminated lengths of RG8X)
SO-239 to SO-239 coax adapter
RF choke (I tried an LDG RBA-1:1 balun, but snap-on ferrites, or coiled coax may work)
PROCEDURE
STEPS #1 – #7: PREPARE THE TWIN LEAD
1. Establish the velocity factor for the twin lead. I was able to establish that my flea market 450 ohm window line was JSC twin-lead 1317 by researching its markings and located it at Seminolewire.com. It’s velocity factor was published as 89.8%. This value is needed for the next steps.
2. Use M0UKD’s Slim Jim Calculator (here), and sketch out the dimensions of your project. If you refer to the graphic below, my values in centimeters were: A=141.4, B=92.9, C=46.4, D1/D2=4.6, E=2.1. Keep in mind, that at least for me D1/D2 at 4.6cm did not produce a useable VSWR and the actual dimensions I ended up with were D1=9.8cm and D2=8.8cm. My notes are copied here as well.
Of note, the formulas are: A= 3/4 wavelength x velocity factor + size of gap; B= 1/2 wavelength x velocity factor; C= 1/4 wavelength x velocity factor; D= wavelength/40 x velocity factor; E= wavelength/100.
3. Measure and mark each section of the twin lead with chalk. Measure twice, cut once!
4. After measuring and marking the window line, strip enough wire at one end (both sides) so that you can fold them toward eachother and solder them together. Be careful not to make the mistake that I did. Having never worked with window line before, I managed to mangle the wires and needed to solder on an additional wire jumper in order to complete the loop. Both wire cutters/strippers and a utility knife come in handy. I started with the end that would be away from the feedline attachment.
A multimeter to check for continuity, wide heat shrink tubing, and a hot air gun can make this mistake seem to disappear! The top portion of the twin lead is the corrected mistake.
5. Do this same thing (except better!) with the other end of the twin lead. Notice that the second time worked much better for me and no jumper was required. The overall length of your twin lead should still be what you calculated for “A” in the diagram above.
I interdigitized the copper strands and soldered them together. Essentially this makes a loop of wire jointed together by plastic bits.
6. Cut out the “gap” section (i.e. “E” in the diagram above) in the wire from one side of your now-looped twin lead.
7. Strip the insulation from where the feedline will attached (“D1” and “D2” on the diagram above).
(Note: In my case, the M0UKD calculator gave me a very poor match at the calculated “D” of 4.6 cm. I ended up around D1=9.8cm and D2=8.8cm, which gave a far more acceptable VSWR. It took a lot of experimenting and testing to find the closest match. The picture below shows the initial wire stripped at around 4.6cm. I left plenty of exposed wire because I knew I would have to test it to find the right location. I ended up having to go back and strip more wire to get a match.)
STEPS #8 – #20: BUILD THE ANTENNA FEEDPOINT
So, it is now time to build the feedline attachment point to the coax. I am sure there are many ways that this can be done, but I decided to utilize the separated center conductor and braid of an approximately 8″ length of RG8X coaxial cable as my connection wires that I would solder directly on the antenna at point “D1” and “D2”. I would incorporate strain relief for this coaxial cable attachment piece so that there was limited pull on the soldered joints. Finally, I would terminate the coaxial attachment with a PL259 crimp connector. I would have prefered an SO239 crimp connector termination, but I did not have one readily available.
I also used the Lands Precision (LP) coaxial connector crimp kit for this.
8. Slice open the outer insulation of the coaxial cable a few inches using a utility knife, leaving the underlying braid and dielectric intact. If you are careful and leave the outer insulation intact, you can use it to cover up the dielectric and center conductor later on. Make sure to remove approximately 1/2″ of dielectric and braid from around the end of the center conductor.
9. Use a sewing pin to tease out the copper braid.
10. Twist the copper strands of the coaxial cable braid into a single wire.
11. Use screw connectors to temporarily secure the center conductor and the coax braid to lead wires with alligator clips. You may want to assure continuity from the alligator lead to the coaxial cable at this point with multimeter. You will use the alligator clips to test various locations around D1 and D2 to get a good VSWR match.
12. Use the coax cutters provided in the crimp kit to get a nice clean cut of the end of the coaxial cable. Somewhere around 8″ long will be fine; just pick where you would like the coax connector to be in relation to your antenna when choosing a length.
13. Prepare the coax stripper tool. The orange “V Block” settings for RG8X coax in this cheap, popular stripper tool are shown below. There are also ways to adjust the blade depth. Experiment a bit with a scrap piece of coax if it is your first time using this.
14. The picture below shows the coax cable inserted at a proper length into the stripper tool. Each connector has published lengths on where to cut, but I just measure the coax connector against the coax and give it my best guess. Also in the picture below you can see the three pieces of the coax connector. The two larger pieces screw in together to form the connector and the smaller metal cylinder is the “ferrule” that you will crimp.
15. After the coax is clamped into the stripper tool, spin the tool around the coax about five times. Pull the tool from the coax. The view below of the coax after being cut by the stripper tool is what you should now see.
15. Pull the braided shield back and out of the way of the underlying dielectric (white insulation).
16. Next, arrange the various connector parts on the coax cable in preparation for crimping. The order you see below is: clear heat shrink tubing, ferrule, outer ring of connector, inner portion of connector inserted over center conductor. The outer ring connector and the ferrule probably should have been switched for a more accurate picture.
17. Slide the inner aspect of the connector over the center conductor and push it down so that it is flush with the black insulation of the coax (without any white dielectric exposed). Then fold the exposed braid of the coax back up and now over the inner aspect of the connector. Push the ferrule up over the braid and the inner connector.
The exposed braid should now be covered entirely by the ferrule, and sandwiched between the inner portion of the connector and the ferrule. The coax center conductor should extend toward, but not past, the tip of the connector. You can see the copper strands in the correct position within the connector tip tube below.
18. Using the crimp tool, crimp the ferrule onto the coax cable.
Make sure to use the correct die for the coax you are using. You may need to do some searching to track down this information. For RG8X, the die is labeled “.255”.
Insert the coax assembly into the crimper so that the ferrule is completely engulfed by the die, and is flush with the base of the inner connector. Make sure everything is tightly pressed together with no gaps. Go ahead and crimp!
Here is sort of what the crimped connector should look like. I got nervous and crimped mine twice, thus distorting it a little bit.
Here is heat shrink tubing being slid up over the ferrule to complete the assembly. I use a small heat gun to shrink it.
19. Now solder the center conductor of the coax to the center conductor tube of the connector.
Take care not to get solder on the outside of the assembly. I try to plug the entire cross-section of the tube with solder.
20. Creative use of shrink heat tubing is used to protect the entire feedline assembly, including replacing the outer sliced black insulation back around the dielectric. A hole is made in the outer layer of heat shrink tubing to allow exit of the coaxial braid. Eventually, the coaxial braid is covered with heat shrink tubing as well as seen in later pictures.
STEPS #21: TEST THE ANTENNA TO LOCATE BEST FEEDPOINT ATTACHMENT
21. Screw-on wire connectors join lead wires (with alligator clip ends) to the center conductor (yellow) and braid (white). Refer to the diagram showing the different segments of the antenna earlier in this post. The center conductor of the coax should be connected to the side without the gap, while the braided coax shield should be attached to the side with the gap. Use different positions of the alligator clips to find an acceptable VSWR. I mentioned it earlier, but the M0UKD calculator placed the length of “D” at 4.6 cm. This did not produce a very good VSWR for me. In fact, I needed to strip additional wire (as you can see in the picture below) and eventually settled on the center conductor attaching at 9.8cm (D1 in the graphic) and the coax braid at 8.8cm (D2 in the graphic).
22. Hang the antenna in a place where adjustments to the positioning of the alligator clips can be made for repeated testing. Below is the VSWR graph from my RigExpert AA-650 Zoom antenna analyzer that represented the best location for soldering the feedpoint assembly to the antenna. The frequency is shown on the horizontal axis with each vertical blue line represents a ham band, The VSWR is shown on the vertical axis. The yellow trace is the VSWR reading from the antenna analyzer. One pink asterisk is at 2M, and two pink asterisks is at 70cm ham bands.
Zooming in on 2M shows a slightly better SWR than the full spectrum, at between 1.5 and 3 across the band.
The VSWR of 70cm was below 2 across nearly the entire band.
STEPS #22 to #25: COMPLETE THE ANTENNA
22. Solder the center conductor and the coaxial braid to the proper locations on the antenna.
They should look like this:
23. Melt (with a solder iron) or drill holes in the top and bottom solid plastic portion of the antenna. The top one will be to put a cable tie loop as an attachment point for the antenna. The bottom one will to provide strain relief for the feedpoint assembly.
24. Secure the feedpoint assembly to the antenna using cable ties.
25. Creative use of heat shrink tubing can be used to cover the exposed metal portions and various connections of the feedpoint assembly. If you look at the end of the antenna on the right hand side near the connector, there is an additional piece of heat shrink tubing there as well.
Here is the final Slim Jim antenna!
BALUN, CHOKE, AND FINAL TESTING
I intended to use this antenna primarily for 2M. I attached it to a pole (as shown below) with orange paracord. I used three separate configurations: one without a balun or choke, another with a 1:1 commercial balun (LDG RBA-1:1), and finally a choke at the feedpoint insertion composed 6 loops of RG8X cable-tied together (pictured below). I then hung the antenna freely out the 2nd story window of my house.
The VSWR trace of the antenna without a choke or balun was between 2 and 3, with a small portion dipping below 2 across the 2M band. The coax was 25 feet of RG8X.
The VSWR trace of the antenna with the commercial 1:1 balun (composed of 12 parallel turns of enameled wire around an unspecified toroid; see picture) is shown below. The majority of the 2M band registered a VSWR between 2 and 3 with the balun at the feedline attachment. Again, the coax was 25 feet of RG8X.
Interestingly, the VSWR was the best, under 2 for most the band with a maximum of approximately 2.5 at the lower end of the band, using the coax choke. This time, however, I used 50 feet of RG8X cable, and perhaps the longer cable run accounted for the better VSWR due to power loss along the feedline.
ON AIR TESTING
What can I say? It works. I need to use it more, but I was able to make contacts at approximately 10 miles away on a 2M SSB net using my Yaesu FT857-D at both 20 and 50 watts. The output power and the SWR reading on my radio both seemed acceptable. I need to spend more time with it, especially outside the city, away from noise and buildings, to see if it is an adequate antenna for portable ops or not.
Let me know if you make one, what difficulties you encounter, and if it works!
KM1NDY