2m portable Slim Jim antenna
Having not had much luck with the PA0FBK coaxial dual band antenna I decided to build a 2m Slim Jim from 450 ohm ladder line for portable use. I have heard that some people have said it also works on 70cm too so once it's built I'll check that out. I have found a few calculators online but in the end I used the M0UKD one. Below are some images from the construction and the analysis using a nanoVNA. I had issues with the software running on the laptop, unfortunately it kept on crashing. I eventually found an Android app that ran on an old phone allowing me to get some decent quality images of the analysis.
​
2m Slim Jim cut from 450 ohm ladder line. I used tape before cutting to measure and made use of the reinforcing parts for support.
I had carefully measured so that when I removed insulation for the feed point I had the support of the plastic part of the feeder.
​
The bottom of the antenna was in a gap which was useful giving me room to bend both sides of the feeder for soldering together. For aid of soldering they were bound together with a very fine strand of wire.
​
Insulation on both sides removed for the feeder to be attached.
Position of break in antenna marked carefully with tape to allow movement during initial positioning phase.
​
The position of the top of the antenna was marked carefully with tape to allow movement during initial positioning phase. Use the plastic spacing of the ladder line to give you as much mechanical strength near the ends, feed point and break in the antenna
​
The feeder was attached at the calculated point and was spot on! This is only temporary and will be secured.
​
The antenna was strapped to a section of 10m telescopic pole, I used about 5m to get it elevated and away from any objects. This would probably be the length of pole used for portable operations.
​
Really pleased with the first check on the NanoVNA, image shows a sweep from 140MHz to 150MHz and the marker was on 145.500MHz the UK FM calling frequency. SWR 1.26:1 and impedance 58.4 Ohms.
image shows sweep from 144MHz to 146MHz the UK 2m band and the marker was on 145.500MHz the FM calling frequency. SWR 1.01:1 and impedance 49.5 Ohms. These need to be repeated once the antenna is completed and the NanoVNA calibrated on each change of parameter.
Out of interest I checked the antenna on 70cm band. I was quite surprised with my initial findings. The first check shows a sweep from 420MHz to 450MHz and the marker was on 433.500MHz the UK FM calling frequency. SWR 1.49:1 and impedance 74.9 Ohms.
The check shows a sweep from 430MHz to 440MHz the UK 70cm band and the marker was on 433.500MHz the FM calling frequency. SWR 1.49:1 and impedance 74.9 Ohms. These need to be repeated once the antenna is completed and the NanoVNA calibrated on each change of parameter.
​
Heatshrink tape was used to seal the antenna. The tape doesn't give a great finish on the small spaces of the antenna but at least protects it.
​
​
The plastic section of the ladder line was drilled either side of the coax and cable ties applied to give some support to the cable. Heatshrink tape was used to seal the ends and joints of the antenna. I'll more than likely swap out the cable ties for black ones.
​
Final testing of the antenna was carried out by strapping it to the 5m pole. I first tested using the NanoVNA and then the
Yaesu FT-817ND.
​
The actual sections of the pole used were 4.5m and the base of the antenna was at 2.8m.
​
The antenna was fastened to the pole with some small rubber cable tidies that were with some new equipment at work - I always recycle!
​
Tests using the NanoVNA were repeated this time having found a way to download images to the phone.
The NanoVNA was calibrated at each frequency change.
​
Compared to the earlier tests before completion and with careful calibration of the NanoVNA this time we have a minimal difference in SWR, 1.29:1 compared to 1.26:1. Impendance this time was 47.0 ohms compared to 58.4 ohms previously.
​
The NanoVNA was re-calibrated after the change of parameters. Image shows sweep from 144MHz to 146MHz and the marker was on 145.500MHz the FM calling frequency. SWR 1.29:1 and impedance 55.9 Ohms. There was no significant change with the finished antenna.
​
​
The image shows a sweep from 430MHz to 440MHz and the marker was on 433.500MHz the FM calling frequency. Following completion we have minimal change in SWR, 1.43:1 compared to 1.49:1. Impendance this time was 40.3 ohms compared to 74.9 ohms previously.
​
Tests with the FT-817ND on 2m were good. Results into the local repeaters with the antenna shielded by the house were better than expected.
​
Tests with the FT-817ND on 70cm were also good even with the slightly higher SWR. Again, results into the local repeaters with the antenna shielded by the house were also good.
This was a nice little project for any level of experience. If you do not have off-cuts of cable the actual items you need are cheap, 2m of 450 ladder line, a length of RG-58 coax and a plug. Any extras are just to tidy it up such as shrink wrap or self amalgamating tape and re-usable cable ties. The ties are good for securing to the pole and for keeping the antenna tidy for transport. The extra length of ladder line gives you some spare at each end so you can position the cuts alongside the plastic for strength..