Base Station VHF Copper Slim Jim Antenna Construction Complete

Tonight I put the finishing touches on the two meter copper tube slim jim (“slim” for its narrow form factor, and “J.I.M.” for “J-type Integrated Matching). Supposed to have about 6db gain over a 5/8 wave or groundplane antenna due to its small take-off angle which gives it extra punch, I guess. Can’t wait to tune it up and try it out. Currently, I’m strictly mobile on 2M. Check it out. This was fun to build:

 

At only 58 inches in total length, you can stick this thing (invisibly) just about anywhere. Here’s a little more detailed view of the tuning mechanism:

The heads of the two outer 1/4×20 carriage bolts 3/4″ long simply rest against the two respective tubes (one is 3/4 wave long, the other 1/4 wave). The wing nuts on these two bolts receive the center conductor and shield respectively of the coax feedline. The center bolt, 1.5″ long, is simply a clamping device to hold this tuning mechanism in place. I’ll use my MFJ259B antenna analyzer to tune for lowest VSWR by sliding this mechanism up and down to find the sweet spot. Then I’ll mark the spot on the antenna where the feedline should be attached and solder in place (removing the temporary tuning device, saving it for the next J-pole-type antenna project. Then up on the roof she’ll go.

But first, I’ll put the darn tuning device on the correct end of the antenna! In looking at the pic above, I just noticed I screwed the darn thing onto the wrong end of the antenna! Oh well, my very first mistake in life had to happen sooner or later!

One pointer I recently picked up is that close-clamping such an antenna even to a non-conductive support (like a PVC or fiberglass mast) is likely to detune the antenna somewhat, so it’s best to stand it off a few inches with a couple of short non-conductive stand-offs (short lengths of PVC, for example, with the ends cut to the contour of the antenna and your mast. Always something to be learned!

73 de Gene K0GKJ dit dit