This antenna with the adjustable plate for tuning can be built from type M 1/2in copper tubing for around $13 and the coax pig tail would cost around $12. The antenna, not including the coax would cost approx $25.
In the picture above I used hose clamps to clamp the coax pig tail directly to the antenna. W4TFX, Jay and I tuned the antenna for 146.400 and we had acceptable SWR of 1.5 to 1.0 between 147.330 and 146.400. On UHF we had the same SWR between 445.8 and 447.0 which makes this antenna an excellent dual band antenna for cross band repeating.
See the SWR/IMP chart below for the results.
Mounted 55 feet up a tree (the elevation at the base of the tree is 305 feet) and connected with 107 feet of LMR-400 I could hear, for the first time, the Mt Michelle Repeater in Ashville NC, that’s 145 miles as the crow flies from Columbia. Pretty impressive.
A receive signal check was performed with the help of K9DBC, Dan Lexington SC. I was listening to Dan on my Diamond X200 mounted in my attic and his signal was S3 signal report = 33, while he was transmitting I switched to this Slim Jim antenna and he was received at S9 signal report =59. Big difference.
Questions on building or tuning will be answered by Jay W4TFX or K4LLE Pete, Jay is our resident expert on this type of antenna as he has built a couple of them and the tuning can be a little squirrelly.
We all know that Amateur Radio is all about experimenting with our equipment. Jay’s eagle eye noticed an unplanned experimentation (that’s a pretty crafty why to say a mistake !) between the field picture above and the construction diagram. After we tried to tune in UHF frequencies we noticed a big SWR problem so it was time to abort the unplanned experimentation and go with what works. Can you spot the issue ?
Here are our SWR/IMP results:
| Freq | SWR | IMP | Freq | SWR | IMP | |
| 147.600 | 1.40 | 38 | 445.100 | 1.06 | 52 | |
| 147.500 | 1.41 | 43 | 445.200 | 1.04 | 51 | |
| 147.400 | 1.36 | 46 | 445.300 | 1.03 | 50 | |
| 147.300 | 1.35 | 48 | 445.400 | 1.03 | 50 | |
| 147.200 | 1.34 | 52 | 445.500 | 1.04 | 50 | |
| 147.100 | 1.32 | 54 | 445.600 | 1.05 | 48 | |
| 147.000 | 1.28 | 56 | 445.700 | 1.07 | 48 | |
| 146.900 | 1.18 | 57 | 445.800 | 1.08 | 47 | |
| 146.800 | 1.13 | 57 | 445.900 | 1.10 | 46 | |
| 146.700 | 1.08 | 54 | 446.000 | 1.12 | 45 | |
| 146.600 | 1.04 | 52 | 446.100 | 1.12 | 45 | |
| 146.500 | 1.01 | 50 | 446.200 | 1.11 | 45 | |
| 146.400 | 1.05 | 50 | 446.300 | 1.11 | 45 | |
| 146.300 | 1.06 | 51 | 446.400 | 1.09 | 46 | |
| 146.200 | 1.06 | 47 | 446.500 | 1.08 | 46 | |
| 146.100 | 1.10 | 53 | 446.600 | 1.08 | 47 | |
| 146.000 | 1.11 | 53 | 446.700 | 1.09 | 49 | |
| 145.900 | 1.15 | 54 | 446.800 | 1.10 | 50 | |
| 145.800 | 1.18 | 53 | 446.900 | 1.09 | 51 | |
| 145.700 | 1.20 | 47 | ||||
| 145.600 | 1.22 | 48 | ||||
| 145.500 | 1.25 | 50 | ||||
| 145.400 | 1.27 | 52 | ||||
| 145.300 | 1.26 | 53 | ||||
| 145.200 | 1.27 | 53 | ||||
| 145.100 | 1.26 | 57 | ||||
| 145.000 | 1.26 | 58 |
7 comments
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Inverted antenna, feed point is 180 out, flipped center wire feed point. 🙂
You are correct. The schematic is the correct way to do it.
What are the spacers between the pipes where it is open?
Charles
KB5ZMN
They are insulators you would use at the end of a long wire antenna. I got them from DX Engineering and then ground them down on my grinder until I got the spacing of 2.375in
Did you get it to tune with the coax on the long (1/2 wave) side of the slim jim? I put the coax on the shorter (1/4 wave) side and I can tune 146 OR 440 1.2:1 or better, but not both at better than 2.9:1.
The coax center wire should be on the long 3/4 wave copper tube, just like the schematic shows, the shield should be connected to the 1/4 wave copper tube, the schematic is correct and the picture is wrong. We did try to tune it up with the shield on the 1/2 wave copper tube but could not get anything below 2.0:1 VHF and then we decided to just re-mount the coax pigtail to the correct end of the antenna and we got the SWR posted above. This is a dual band antenna.
Ok, I am in process to build this with slightly different measurements based on center frequency(s) of 146.525 and 442.733. Frequency based on average of our local repeaters.
Cut lengths are over all (OAL) not center line to center line.
58 7/8″ Long Side (3/4 Wave Length)
37 5/8″ (1/2 Wave Length)
18 3/4″ (1/4 Wave Length)
The over all width will be 2 1/2″.
Pieces are cut, next up will be solder then SWR check in position.
de K7WCH