Building a 2m antenna out of 450 ohm window line is one of those ham basics, and I'd been needing one of these for the go bag. I looked around on the web for plans, many of which included the unuseful comment "adjust feed point to minimize SWR." Well, it's a pain in the butt to hunt and peck along ladder line to find that exact spot, so instead I just found a site that gives the dimension (thanks!):
http://www.kn9b.us/slim-jim-antennaIn case that site ever goes bye-bye, here's the key diagram giving all of the needed dimensions (in inches, btw):
It's a really nice design, and the only non-integer dimensions come close enough on 1/8" yardstick markings. Very easy to lay out and mark.
That diagram plus some rudimentary soldering skills are all you really need to build this thing, but I'll add a few more notes that may help out.
- Blue painter's masking tape is
the way to control 5' of window line when you're trying to mark precise cut locations. Tape down the ends onto a flat work surface – I used the kitchen floor, but a workbench would've been better. Blue tape is good for marking where cuts, taps, etc. will have to go too. It really made the job a lot easer and more precise.
- Position the 1" gap between the feed stub and the rest of the antenna in the middle of one of the long plastic spacer sections. That way it'll have the most strength for a part that must have one of the wires and some of the plastic cut away. The rest of the cuts will then have to fall where they may, which can be a pain, but at least you won't have that awkward situation where the gap falls half-way into a window space and things are left twisting by one wire.
- A soldering iron tip burns nice holes for cinching down zip ties, or for the hang loop's hole. Here's a picture before I goobied things over with liquid tape:
Notice that I didn't bother to peel off the tape when done. Also notice that I clipped on a little ferrite choke on the feed pigtail, to keep stray signal from creeping back down the outside of the feed line. Don't know if it's big enough to do much good, but maybe it'll bring good luck.
- I used an 8" pigtail with a PL239 plug for the feed. It also needed a barrel adapter to plug into a 25' length of coax. It could just as well have had a 25' length of coax soldered directly on, but I kept it separate to prevent tangles. Either way works, it's just a preference.
- Liquid tape gives the raw ends a nice, somewhat weather tight finish.
- About SWR... I just wanted a "cut'n'hang" design, didn't want to spend a day screwing around with feed points. Slim Jims typically have very broad working bandwidths, so it probably doesn't need measuring, but I checked it anyway with a CB meter (thread on this:
http://thesurvivalpodcast.com/forum/index.php?topic=50271.0). It seemed to be pretty flat over the portion of the 2m band used for FM voice, so I'm calling it good enough and calling it done.
Seems to work pretty well. Hanging 35' from a tree limb, it was able to reach a repeater at 500' 25 miles away on a 5w HT, and got a good signal report from the guy who answered.
Well, there it all is, including some things I figured out along the way. Spent nearly two hours on the entire thing, but I could probably knock one out in a half-hour now.
Finally... I was sorely tempted to get one of these pre-built for a mere $20:
http://www.2wayelectronix.com/N9TAX-Original-VHF-Slim-Jim-antenna-vhf-slim.htm The guy seems to do a nice job, and with MFJ charging $40 for a 2m SWR meter, it is in a way a relative bargain. But there is something satisfying about any DIY project, especially one that costs only a couple of bucks, and this thing probably doesn't need the check. I wouldn't fire up 75 watts on my 2m base with it without checking SWR first, but for a $40 5w Baofeng? Pffft, works great. If you've just got to meter this thing, there's always friends, clubs, or the above-mentioned CB meter trick. Don't hesitate, fire up your soldering iron and jump right in.
