Tuesday, January 13, 2015
J-Pole & SlimJIM Antenna Tab Clearing
First, the Wikipedia short write-ups: J-pole slim JIM
Don't want to read all that? Here's the short version. Both are variants of a half-wave antenna that has the crucial features of being fed from the bottom end and not needing any kind of ground plane. This makes them physically convenient to mount, either from the lower end atop a mast, or from the upper end hanging from a support. Relevant propagation features of the two are their respective take-off angles, somewhere in the range of 20 and 8 degrees. The former is nearly ideal for skywave insertion, while the latter is well-suited for local line-of-sight paths. The basic design is popular and of practical size starting in the neighborhood of 10 meters at ~24', up into the VHF range, with a 2 meter antenna totaling ~5'. At lower frequencies than 10 meters, their size becomes increasingly impractical. Unless of course, you just want to hang it out the back end of a Zeppelin, the use for which it was originally invented.
Back to the tab clearing. Here are some designs for the 2m band that I've recently constructed and/or used:
- A good basic easy one, built out of 450 Ohm window line. This is ideal as a roll-up go-bag antenna, for outdoor activities or for running from hurricanes. It can take at least 65 Watts input, maybe more.
- A similar design, but built out of 300 Ohm TV twin-lead. Much harder to construct due to the smaller width and relative fragility of the twin-lead. The design page states that it's good for up to 10 Watts.
- The Jurassic Duck, a bike antenna made out of 300 Ohm twin lead and a length of CPVC pipe. Mine's built, and waiting until I can ride a bike again to test. Construction note: leave the radiator end (i.e., the top) about 8" longer than specified at the design page and trim down to adjust SWR. I cut mine to the length in the plans and had to solder on extra sections of wire, a slightly more difficult proposition. No matter, it works in the end and all of the messiness is conveniently hidden in the CPVC pipe.
- Here's a guy who'll build you a very nice roll-up slim JIM out of 450 Ohm line at a reasonable price.
- And here's the guy's answer to putting one of his antennas on a bike, ATV, pack, etc.
- Finally, here's a guy who makes damn nice copper pipe J-poles, slim JIMs, etc. in a variety of configurations for a variety of bands. Mine's still holding up like new, and easily reaches the 25 miles to the Biloxi repeater.
300 or 450 Ohm line? In general, 450's bigger, thicker, tougher, easier to handle, easier to solder, and will carry at least 6.5x the power. Unless you really have an overriding reason to choose 300 – and the Jurassic Duck antenna is the only good reason that I can see – go with the 450. At $0.40 a foot for the 450 stuff, even free 300 isn't worth the hassle.